noise in - americanradiohistory.com...figure 2 complete input circuit showing "shot noise"...

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SYLVANIA NEWS

FEBRUARY 1956 VOL. 23 NO. 2R. A. Humphreys, Technical EditorThis information in Sylvania News is furnished without assuming any obligations

NOISE INTELEVISION RECEIVERS

We have all wondered or have beenasked what causes snow on the screenof a television receiver. You haveundoubtedly thought or heard it saidthat if it were not for the snow thepicture received would be a lot better.

The snow that we see is due to anunwanted signal which is present inthe early stages of the receiver. Forour discussion, we shall call this un-wanted signal "noise". The effects ofnoise are observed in all sensitivereceiving equipment._ In television wesee snow, in radio we hear a fryingsound, on a radar screen it is grassand now with color television we haveconfetti. Regardless of the type of re-ceiver, we have the effects of noise.

Shot Noise

As we know, electrons in a vacuumtube leave the hot cathode, travelthrough the grid and strike the plate.When the electrons strike the platethey produce a noise current. Just asa handful of pebbles produce a noisewhen thrown on a tin roof, electronsproduce a noise when they strike thetube's plate. This noise is called"shot" noise. The magnitude of thisnoise current is given by the equation

i =v 2e I B A2 (Equation No. 1)

By E. H. B o d e n, Advanced Applications Engineer

Where T__ the average noise currentin amperes

e = the electric charge of anelectron (1.59 x 10-19)coulombs

I = the dc plate current inamperes

B =the bandwidth of thetuned circuits in c.p.s.

A2 is a constant which isdependent on the random-ness of the electrons.

Figure 1

Equivalent input circuit of first amplifierof any receiver.

Ra Transformed antenna resistanceRc Resistance due to circuit losses

Re Transit time resistance

stage

The term A2 will need some expla-nation. If the plate attracts everyelectron emitted by the cathode, thearrival of the electrons at the platewill be completely random (in no or-der whatsoever). In this case A2=-_- 1.

If the plate takes only some of theelectrons emitted by the cathode andthere is an electron cloud formedaround the cathode, then the electron..arrival is not random and A2 is ap-proximately 0.05.

Noise in Resistive ElementsIn addition to shot noise there is also"Johnson" noise. "Johnson" noise ispresent in all resistive elements; i.e.,an ordinary carbon resistor, the re-sistance in a wire or the resistance ofthe antenna all produce a noise powerwhen at some temperature other thanabsolute zero. Therefore, if we wereto connect a theoretical, ultra -sensi-tive voltmeter across any of the abovementioned resistances, we would finda voltage. In fact, the average voltageread (e) would be

e= V4k T B R (Equation No. 2)In this equation k is a constant equalto 1.38 x 10-23, T is the temperature(absolute scale) of the resistance, B isthe frequency bandwidth of the volt-meter and R is the ohmic value of theresistance. This will mean more if wehave some numbers. Therefore, let ussay that the voltmeter bandwidth isabout that of a television set, say 5

February 1956 Sylvania News 5

Figure 2 Complete input circuit showing "shot noise" expressed as an equivalent seriesresistance, Rea.

mc. Also, let us say that the voltmeteris connected across the 300 ohm ter-minals of an antenna and that thetemperature is 290° K (just belowroom temperature). Under these con-ditions, an average voltage of 4.9microvolts is measured across the an-tenna terminals. In the same way, a300 ohm VI watt or 100 watt resistorat the same temperature measuredwith the same voltmeter would have4.9 microvolts across its terminals.These then are the two kinds of noisefound in a receiver.

Before going further, let us look atthe input circuit of the first amplifierstage of any receiver. As we see bythe circuit shown in Figure 1, thereare three resistances between gridand cathode. Ra is the transformedantenna resistance and Rc is a resist-ance due to circuit losses. Re is tran-sit time resistance. Because the elec-trons require a definite length oftime to travel from the cathode to thegrid plane and because as the fre-quency of operation is increased theelectron travel time becomes a largerportion of the time for one cycle, aresistive loading is produced betweenthe grid and cathode of the tube. Thisresistance can be measured with suit-able equipment. As the frequency ofmeasurement is increased, the valueóf Re becomes lower and lower.Briefly the formula for Re is:

(Equation No. 3)

Re-k 1

f2

In this equation f is the frequencyof operation. The constant k is de-pendent on the tubes transconduct-ance, time required for electrons totravel from the cathode to the gridplane and/or element voltages. So wesee that Re reduces quite rapidly asfrequency increases. For most tunertypes, Re is approximately 1000ohms at 200 mc. The Type 6AN4 at1000 mc has an Re of about 50 ohms.

Now, returning to equation No. 2,T is room temperature (290° K) forRa and Rc. However, because Re iswithin the tube where the tempera-ture is approximately five times roomtemperature, due to the heated cath-ode, the "Johnson" noise output volt-age is equal to the 5 or 2.4 approx.times that of a resistor at room tem-perature having the same ohmic valueas Re.

At very high frequencies, the tran-sit time resistance (Re) is the mostimportant term. Its value will changewith tube construction. Good circuitdesign can keep circuit losses at aminimum.

Since we have found there to bethree noise -producing resistances inthe input, Ra, Rc and Re, it wouldbe most convenient if we could speak

of "shot" noise in terms of some re-sistance in the tube input. Measure-ments have shown that the equivalentresistance of "shot" noise reflected atthe input is equal to 2.5 divided bythe tube's transconductance. Actu-ally, this is a fictitious resistancecalled Req and is used to more easilyadd in "shot" noise. Figure 2 showsthe complete input circuit.

Measuring NoiseWe now need some convenient meth-od of comparing tubes and circuitsand this we do in this way. If wecould eliminate Rc, Re and Req wewould have a perfect receiver. There-fore, for convenience, circuits andtubes are compared with a theoreticaltube and circuit. If an amplifier isfound to have ten times the noise ofa perfect amplifier, it is said to havea "noise factor" of 10 or 10DB. If itis four times noisier than a perfectamplifier, its noise factor is then four,or 6DB.

To measure the noise figure of areceiver one makes use of a circuitsimilar to that shown in Figure 3.With this circuit and an AC volt-meter connected to the receiver's de-tector output, one may measure thenoise figure of a television receiver(VHF). The 300 ohm resistor (in thecircuit) has replaced the antenna im-pedance. The Sylvania type 5722 isa temperature limited noise diodeand, therefore, has an A2=1 (equa-tion No. 1). To find the noise figureof a receiver, the noise power meas-ured by the AC voltmeter connectedto the detector output is first re-corded. The noise diode is thenturned on and R1 is increased untilthe noise power measured by the ACvoltmeter is doubled; i.e., twice thepower output. Since we now observetwice the noise output, we know thatthe noise diode is just as "noisy" asthe receiver. Therefore, knowing thenoise diode current, we may computethe noise figure of the receiver by

(Equation No. 4)

NF=20 Id Ra

6 Sylvania News February 1956

Where Id = the diode plate currentin amperes

Ra = antenna resistancewhich is 300 ohms in atv receiver

Wa have measured the noise figure ofthe entire receiver. Let us now seewhat the noise figure of a receiver iswith a booster.

Figure 3 Noise

N1=N12-N2-101

Now, by connecting the booster inquestion to the input of a receiverwhose N2 we know, Ni may be deter-mined by equation No. 6.

It is important to take heed to whatequation No. 5 is telling us. If a re-ceiver has a tuner with a high noise

generator for measuring receiver noise figure.

The Effects of Boostersif we the noise figure of the re-ceiver itself, the noise figure of thebooster and the power gain of thebooster the overall noise figure N12will be

(Equation No. 5)

N12=N1+N2-1G1

Where N12 =the noise figure of thebooster and receiver

Ni =the noise figure of thebooster

G1 =the power gain of thebooster

N2 =the noise figure of thereceiver

Equation No. 5 will tell us if we canimprove the picture by adding abooster, providing we know N1 andG. Gi can be measured with a signalgenerator. We can determine thenoise figure of the booster (N1) if weperform an algebraic manipulationon equation No. 5. The new equationbecomes

(Equation No. 6)

figure, say 10 or 20 on Channel 4,and we add a booster whose N1 = 4and G1= 5 the overall noise figurewould then be about 6 (7.8DB). Thenoise figure is one half and the sensi-tivity has been doubled. (Noise figurehas been reduced 3DB or sensitivityhas been increased by 3DB.) SupposeGi of the booster were 100, then N12would be 4, in which case the im-provement would be 3 times (4.8DB).Let us now take another example.This time N1= 4, N2 = 4 and G1= 100. In other words, we have agood set and a good booster. Substi-tuting these numbers in equation No.5, we see we have not improved thesensitivity of the receiver one bit. Thebooster is of no use.

Visible ImprovementThis brings us to a very interestingquestion. How much does the noisefigure have to be improved to pro-duce a visible improvement on thescreen? Experiment has shown thatthe minimum visible improvement innoise figure which can be observedon a tv screen is from 2 to 21/2 DB,

providing the change is made whilewatching the screen. If, however, theeyes are removed from the screenwhile the noise figure is degradedthen 3 DB is about the minimum.This means if you have a snowy pic-ture and equation No. 5 says the im-provement in noise figure is less than3 DB, there may not be visible im-provement in the picture.

Long Transmission LinesWe may make further use of equationNo. 5. This time we have a tv set ofnoise figure N2. For one reason oranother, the antenna is several hun-dred feet from the tv set. The loss(total) in the transmission line be-tween the set and the antennas is L1.The noise figure as seen by the an-tenna is

(Equation No. 7)

N12 =N1 + (N2 -1) L1N12=1+(N2-1)Ll

Where:N1=1N2 = noise figure of the

television receiverL1= transmission line loss

Here we see that if Li is. large be-cause of line length and/or the fre-quency involved, the noise figure ofthe system will be greatly increased.The addition of a booster at the re-ceiver, having a noise figure equal tothat of the receiver, will not improvethe system's noise figure. If, however,we place a booster at the antennawhich has a noise figure equal to thereceiver's and a power gain equal tothe transmission line losses, the dam-aging effects of the transmission linewill be removed.

1. "An Antenna for UHF TV Reception", byJ. S. Allen-SYLVANIA NEWS, March, 1952.

To summarize, the limit of a re-ceiver's sensitivity is its noise figure.A booster can only improve the sensi-tivity if its noise figure is better thanthe receiver's and has sufficient gainto remove the effect of the receiver'snoise figure. And finally, if transmis-sion line losses increase the noise fig-ure, then a booster should be used atthe antenna.

Februory 1956Sylvania News

7

SYLVANIA OSCILLOSCOPE HINTS -MODEL 400A. change can be made in the earliermodel 400 oscilloscopes using theneon lamp in the sweep oscillatorcircuit which will improve the syncstability especially at the lower sweepspeeds.

The procedure for making thischange is as follows:

a. Unsolder the red and blackwires from the neon lamp andremove the black wire from thecircuit completely. Note whichterminal strip lug that this wirewas removed from. Remove theneon lamp.

b. Remove resistor R-44 whichis 56,000 ohms and replacewith 47,000 ohms, 1 watt.

c. Install a single lug terminalstrip to the inside screw of the5V4 tube socket mountingbracket.

d. Add a 2700 ohm 1/2 watt re-sistor from the terminal stripinstalledin step c to the termi-

_.nal_ strip lug from which theblack wire was removed instep a.

e. Connect the red wire removedfrom the neon lamp to theopen end of the resistor in -

SERVICE

HINTS

stalled in step d.f. Check sweep speed for the

proper overlap on each posi-tion of the course frequencycontrol and adjust if necessary.

These changes have been made onall 400 models with serial numbersfrom 2700 up.

To obtain flat square wave re-sponse on the model 400, a specialcompensating adjustment, R-99, isprovided.. This control is located onthe chassis between the first andsecond vertical amplifier tubes. Inmaking this adjustment, be sure thatyou have available a perfect 60 cyclesquare wave from a reliable squarewave generator. With this squarewave connected into the vertical in-put terminals of the oscilloscope,set the sweep speed to obtain threestationary square waves on thescreen with about 4" of total deflec-tion. Set the step attenuator to the10:1 position. If the figures observedon the screen are not absolutely flat-topped, carefully adjust R-99 untilthis condition is obtained.

Many of the earlier 400 model'scopes,used a dual 4 mfd capacitor,C -9a and C -9b, one section located

MOM

in the plate circuit of the first verticalamplifies and the other section inplate circuit of the first horizontalamplifier. Any leakage between sec-tions of this capacitor causes a "ver-tical bounce" condition, that is, acontinuous vertical jumping of thepattern. To .correct the condition, re-move the dual capacitor and substi-tute two single 4 mfd capacitors.

Slight leakage in C-29, locatedbetween plate and grid of V -8a, willalso cause vertical bounce. More than.1 volt drop across the associatedresistor, R-55, will indicate excessiveleakage in C-29. After replacingC-29, adjust R-100 so that exactly1.6 volts appears at the grid of V -8A.(Meásurments should be made witha VTVM.)

Test Equipment Service Dept.Williamsport, Penna.

CORRECT{ON

Figure 2 appearing on Page 6 of theNovember issue should have shownthe following electrode potentials:Plate Voltage 40 VoltsGrids amber 2 and 4,30 Volts

EDITOR'S NOTE: Sylvania offers $10.00 in Advertising Material Certificates for all technical hints that it believes useful to the service -dealer readers of SYLVANIA NEWS. Sylvania is not obligated to return any material submitted for publication, whether or notpublished.

USE FOR BATTERY SNAP FASTENERS

Remove the snap fasteners beforediscarding small 45 and 671/2 voltbatteries. They make excellent con-nectors, especially for speakers. Be-cause of their sturdy constructionand large contact surface, they cansafely carry large currents.

Hyman HermanFlushing, New York

CONTROL REPAIR

With the wide variety of television ,receiver panel controls employed inmodern sets, the serviceman is oftenforced to hold a set until an exactreplacement control can be obtained.Sometimes, however, a temporaryrepair can be made to restore service.

Rough operation is often caused byfatigue of the area directly under thesliding contact. The contact shouldbe bent slightly so that it rides on theunworn surface. Such _a repair, al-though temporary, has proven verysatisfactory.

J. F. KollerNew York, New York

8 February 1956 Sylvania News

JSYLVANIA NEWS

n

MARCH 1956 VOL. 23 NO. 3R. A. Humphreys, Technical EditorThis information in Sylvania News is furnished without assuming a

TRIODE SYNCSEPARATOR -AMPLIFIER

CIRCUITS By C. A. PETERSON and

A previous issue of the SylvaniaNews (November 1955 Vol. 22 No.9) described the operation of the Syl-vania Types 6CS6 and 3CS6 in atypical sync separator -clipper -ampli-fier stage of a modern TV receiver.This article will describe anotherpopular circuit utilizing two triodesto accomplish the functions of syncseparation and amplification. Varia-tions of the circuitry described canbe made to include noise immunityand other special features. The basiccircuit, however, enjoys widespreadpopularity and can be found in agreat many TV receivers, both oldand new.

Basic Circuit RequirementsFigure 1 shows a typical circuit whichuses a Type 12AU7 dual triode tubeand functions as a sync separator andamplifier. The circuit consists basi-cally of two cascaded resistance cou-pled amplifier stages; however, thesetwo stages are so designed as to beable to select the desired portion ofthe input signal, separate it from theremainder and amplify the selectedportion for use in following stages.The input signal is in the form ofcomposite video, which includes pic-ture information, blanking pulsesand the vertical and horizontal syn-chronizing pulses.

March 1956

R. A. HUMPHREYS

The polarity of the input, i.e., syncpositive or sync negative, must betaken into account. If the verticaloutput pulses from the sync ampli-fier are to be used to trigger a con-ventional blocking oscillator, theymust be positive. Since we have twobasic amplifier stages in series, Fig-ure 1, each of which inverts the sig-nal, it follows that to obtain positivesync output we must have a videosignal input which has positive goingsync. In order to have the circuitoperate effectively we also require asignal of appreciable amplitude, ap-proximately 60 volts peak to peak.

These two requirements of the in-put signal are easily met in almost allpresent day receivers. Since the videosignal to the picture tube is generally

applied to the cathode, it must havepositive going sync and must alsohave an amplitude of approximately60 volts. Hence, we take the signalvoltage for the sync separator fromthe plate circuit of the video ampli-fier, or looking at it another way,from the cathode circuit of the pic-ture tube. This signal should resem-ble that shown in Figure 2.

Sync SeparationThe only part of the video signalthat is useful in synchronizing thesweep oscillators is that portioncontained in the blacker -than -blackregion represented between the twohorizontal lines in the top portion ofFigure 2. Furthermore, it can be saidthat we must have nothing but this

C

.047

R

10K

FROMVIDEO AMP

PLATE CIRCUIT

SYNC ,SEP

12AU7

R30.5

.22

22OK 1"Il

R2

22 M ,

C2

470ypf

R4330K

125

C3

VOLTAGES READ UNDERNO SIGNAL CONDITIONS

TO VERY. INTEGRATOR

SYNC AMP+182 f 12AU7

R6 R722K 2.2K

BOAAI

R86.8K

R8

3.3K

+210

.001

-1r-TO HOR.

PHASE OFT.

Figure 1 Circuit diagram of a typical Double triode type sync separator -amplifier.

Sylvania News 5

Figure 2 . Composite video signal applied to sync

portion for proper operation. Thefunction of the sync separator sec-tion of the circuit in Figure 1 is toremove the video from the signaland pass the remaining portionthrough the tube to the sync ampli-fier section.

The signal is taken from the videoamplifier plate circuit through aseries circuit consisting of R1 and Cl.Capacitor C1 is used to AC couplethe signal to the sync separator gridcircuit and also stores the developedbias voltage when the circuit is inoperation. Resistance R1 is used pri-marily to prevent excessive loadingon the video amplifier plate and sub-sequent deterioration of the picture.

The grid circuit consists of thehigh value grid resistor R2 and theparallel combination of R3 and C2.The time constant of R3 and C2 main-tains the proper clipping level andalso accentuates and preserves thesteep wavefronts of the sync pulses,so necessary in maintaining perfecttriggering and good interlace.

It will be noted that the sync sep-arator plate voltage is extremely lowand that the input signal of approxi-mately 60 volts peak to peak is muchtoo large for the tube to pass com-pletely. The large input signal causesthe grid to be driven into conductionand the resulting grid current flowthrough the grid resistance R2 re-sults in a DC bias being developed.This becomes the operating biasupon which the signal rides, withonly the tips of the sync pulses driv-ing the grid to conduction. Onlyenough grid current is drawn tomaintain this bias, consequently, onlyminor clipping occurs on the positivepeaks in the grid circuit.

separator.

With the low plate voltage present,the negative grid voltage required tostop plate current flow is very small.Consequently, as the signal swings ina negative direction, plate currentceases and the portion of the signalbelow this cut-off point is not passedthrough the tube to the plate circuit.The plate voltage, signal amplitude,and grid circuit resistance and capac-itances are so chosen that only a por-tion of the available sync is passedthrough the tube. This assures asafety factor to compensate for in-coming signal strength and a shift incomponents or tubes. Figure 3 willserve to demonstrate the action of thegrid circuit in removing the syncfrom the video signal.

As stated previously, the sync sep-arator is basically an amplifier and,although its primary purpose is toremove the video from the signal, italso amplifies the sync. In the processof amplification, however, the posi-tive tips of the sync pulses are alsoclipped somewhat. Because of a pe-culiarity of triodes operated at low

plate supply voltages, the lowest platevoltage swing can be achieved beforethe maximum grid voltage is reached.Consequently, a point is reachedwhere increasing the grid voltage willgive no .change in plate voltage. Theresult is plate circuit clipping. Thisclipping action takes place when thegrid signal approaches the grid cur-rent point. Additional clipping is us-ually required, however, and isaccomplished in the grid circuit of thesync amplifier. The waveform of theseparated sync appearing at the plateof the sync separator is shown inFigure 4.

Sync AmplificationThe separated sync signal is appliedto the grid of the sync amplifierthrough coupling capacitor C3, Fig-ure 1. The sync amplifier grid is re-turned to a source of positive voltagevia R. The purpose of this is to makethe grid conduct harder on the posi-tive portion of the applied signal,thus causing the positive going signalto be clipped. This is necessary toremove any irregularities which mayhave been passed by the sync separa-tor. Because the grid draws current,a negative voltage is developed atthe grid. As with the sync separatorstage, the signal (approximately 60volts p/p) is too large to be amplifiedcompletely by the tube. The negativeportion once again drives the gridinto the plate current cut-off region,thereby clipping off that portion of

60 VOLTSPtoP

GRID CLIPPINGLEVEL ZERO BIAS

APPROX

RECOVEREDSYNC

f

A

BELOWCUTOFF

GRIDCUTOFF

Figure 3. Line presentation illustrating separation and clipping action of sync separator stage.

6 Sylvania News March 1956

Figure 4. Sync signals at plate of sync separator. Note that the sync has been separated,clipped and amplified. (Vertical)

Figure 5. Sync signal appearing at plate of sync amplifier. The sync signals have again been clippedin both the positive and negative direction and amplified. (Vertical)

the signal below the cut-off point.This action tends to clip off noisespikes which may be present on theincoming video signal, especiallywhen the signals are of lower thannormal value.

Thus it can be seen that the syncamplifier effectively clips both thepositive and negative extremes of thesync signals fed to it from the syncseparator. This amplifier sectionoperates at considerably higher platevoltage and the output is conse-quently higher than that of the sepa-rator section alone. An output ofapproximately 90 volts peak to peakis readily available at the plate. Thisis somewhat lowered by the fact thatthe tube is also used as a phase in-verter where an appreciable cathoderesistance is used, R7. The output atthe plate of the sync amplifier takesthe form shown in Figure 5.

The sync pulses at the plate of thesync amplifier are in the positive di-rection and are coupled directly tothe vertical integrator network wherethe wider vertical pulses are inte-grated to form the synchronizingpulse fortriggering the vertical oscil-lator.

The phase inverter characteristics

of the circuit are used to provide thetwo out -of -phase signals required forthe typical double -diode heizontalphase detector. Positive pulses areobtained from the plate circuit andnegative pulses from the.cathode. Toobtain pulses of equal amplitudes, avoltage divider is used in the platecircuit, Rs and R9. Figure 6 showsthese pulses.

Circuit Variations and FeaturesThe simplicity of the circuitry in-volved in the double triode type ofsync separator amplifier system hasmade it one of the more popular inuse today. Although it has somemeasure of "built in" noise suppres-sion, additional immunity can begained by refinements to the cir-cuitry. Some manufacturers add anoise gate, which is usually a biaseddiode set to clip signals above a pre-determined level. Others reduce theinfluence of noise by adding anotherstage of clipping, which operates inthe same manner as that portion ofthe circuit described herein.

Servicing

The problems involved in servicingthis type of circuit can best be solvedby knowing how the circuit operates

and using this knowledge in isolatingthe trouble. An oscilloscope, alongwith a good vacuum tube voltmeterare indispensable aids. In TV re-ceivers using a phase detector type ofhorizontal control, troubles in thesync circuits resulting from low out-put alone will usually be detected inthe vertical system first, since theamplitude of the vertical sync pulsedetermines directly the vertical oscil-lator sync stability. The horizontalphase detector, on the other hand, ismore prone to act up when irregu-larities are present. Hence, if thehorizontal appears stable while thevertical is touchy, it is a good bet thatthe output has dropped off some-where along the line from the videoinput to the sync amplifier output.

If the horizontal is touchy and thevertical appears stable, look forsomething getting through the syncseparator or amplifier that shouldn't,i.e., hum, noise or video. A good ex-ample of this is the horizontal pullingso often experienced when video getsthrough along with the sync. Anotherexample of pulling occurs when 60or 120 cycle hum gets into the sync.These troubles are not always gen-erated in the sync circuits, however.Whenever they do occur, you . canusually isolate them with the aid ofyour oscilloscope.

Although these circuits are notgenerally considered to be critical,they dó require that the voltages ap-plied to them are reasonably closeto those stated by the manufactureron his circuit diagram. If they aretoo far out of line, the circuit maynot operate well under all conditions.

27 VOLTSPtoP

27 VOLTSPtoP

Figure 6. Horizontal sync pulses A. Junction ofRe and R9 B. Cathode of sync amplifier

March 1956 Sylvania News

SERVICE

HINTS

TIE POINT. ON DAMPER TUBE SOCKETI was called in to service a set with-a burned out fuse. Thumping thedamper tube produced internal arc-ing, so after replacing the fuse anddamper, I went on my way. Nextday, I was called back for the sametrouble. Therefore, I decided thechassis should go to the shop for acheck.

In the shop, I noticed internal arc-ing in the new damper tube. Check-ing the damper tube socket with anohmmeter, I found only 100 ohmsbetween pins 2 and 3, which weregone on removal of the tube.

This particular model used pin 2as a grounded tie point. Apparentlythe internaLarcing between the cath-ode (pin 3) and the ground formeda carbon path of 100 ohms. Remov-ing the tie point soldering cured thetrouble.

John D. Hand, Jr.Shreveport, La.

EDITOR'S NOTE:

Tube manufacturers caution that unusedpins on damper tubes should not be usedas tie points. For example, data on SylvaniaType 6AU4GT states: Caution: Note: Pre-cautions should be taken on terminals 1, 2,4 and 6 when used as tie points, due to highvoltage breakdown.

MOTOROLA TV MODEL 2118A

AND SIMILAR SERIES

The symptoms were intermittent pic-ture brightness, narrow raster andsound always present.

This set would work fine most ofthe time. The only indication offaulty operation was an occasionalreduction in width. When it finallydid this on the service bench allvoltages were ok, except the filamentvoltage which was only 4.5 volts. It

was discovered that the filamentground through a rivet to the chassismeasured approximately 400 ohms.The rivet is a lug on a stand-off in-sulator which holds the filament fuse(a piece of #28 wire). To cure thetrouble, solder a heavy wire to aspare soldering lug and also, fordouble protection, solder the rivet tothe chassis.

J. NolterMahanoy City, Pennsylvania

CONTROL REPAIR

With the wide variety of televisionreceive Panel controls employed inmoderNets, the serviceman is oftenforced to hold a set until an exactreplacement control can be obtained.Sometimes, however, a temporaryrepair can be made to restore service.Rough operation is often caused byfatigue of the area directly under thesliding contact. The contact shouldbe bent slightly so that it rides on theunworn surface. Such a repair, al-though temporary, has proven verysatisfactory.

J. F. KollerNew York, New York

TV PICTURE WITH PULLING

ON LEFT

An Airline Model 15 WG 3046Chad vertical pulses getting into thehorizontal. The symptoms picturewise: a curved pix (not an S curvebut one that looked more like a C)and very poor horizontal sync. Ver-tical sync not too bad. The troublewas traced to C77, a filter condenserwhich is more or less common to theB supplies to the two sections (ver-tical and horizontal).

George's Radio ServiceSanta Ana, California

SCREW ALIGNMENT POINTER

Adjustment of trimmers and con-trols of tv sets can result in quitea bit of additional work if the troubleis found elsewhere and it is neces-sary to return the adjustments to theoriginal positions. It is easy to losetrack of where they had been before,especially in the case of recessed ad-jústments.

To simplify this I use commonbobby pins and solder the open endby wrapping around some fine tinnedwire and soldering. This makes apointer with good tension that canbe slipped on the shaft of an align-ment tool and can be moved up ordown the shaft for best position.

Duringsert tuning tool into adjustment screwand then move pin down shaft ofalignment tool close to chassis andmark chassis to coincide with end ofpin which is the reference point.

By observing movement of pointerit is easy to see whether your adjust-ment is IA turn etc. and right or left.

Adams Radio ServiceWilliamsport, Pennsylvania

COVER SWITCHES ON BATTERY

PORTABLES

The switches which open and closethe A and B circuits of personal typeportables are usually operated byopening and closing the cover. Manytimes the switch is not pushed downfar enough «to cut off the circuit,thereby running down the battery. Iadd a bead of solder to the exposedpart of the switch on sets which havethis trouble. The longer lever willinsure that the switch opens everytime.

Hyman HermanFlushing, New York

8 Sylvania News March 1956

SYLVANIA NEWS

MAY -JUNE 1956 VOL. 2

R. A. Humphreys, Technical EdiThis information in Sylvania News is 1

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SYLVANIA'S NEW8 -INCH CHECK

TUBE, TYPE8XP4

y Thomas A. LeshEDITOR'S NOTE: Sylvania's new type8XP4 rectangular glass check tube joinsSylvania's popular 5AXP4 round glasscheck tube in the Sylvania line of picturetubes for testing purposes. The type 8XP4is packaged in a long-lasting easy -to -carrywhite protective carton with top handles-permitting safe and easy carrying forhome service calls. Complete applicationinstructions are enclosed with each unit.

A new TV -receiver check tube withan 8 -inch rectangular screen hasbeen designed by Sylvania ElectricProducts Inc. This tube, the 8XP4,can be used with nearly any receiveras a temporary substitute for the pic-ture tube. It has great value as aservicing aid because of its compact-ness and adaptability. Although the8XP4 is small, it is capable of dis-playing almost the entire raster pro-duced by a 90 -degree deflection sys-tem. See Fig. lA. This feature is verydesirable because it permits the tech-nician to detect troubles that affectthe edges of the pictures and to makeadjustments of size and linearitywhile the check tube is being used.Until now, checks and adjustmentslike these could not be adequatelyperformed on a receiver having 90-degree deflection unless a full-sizedpicture tube of the proper type wereconnected to the chassis.

When the 8XP4 tube is used ina 70- or 53 -degree deflection system,it will usually be found that the pic-ture does not completely fill thescreen. In addition, some pincushioneffect may be visible at the edges ofthe picture. These effects will detractonly slightly from the usefulness ofthe tube in normal testing applica-

May-June 1956 Sylvania News5

(Fig. 1.) Test Patterns Produced on the Screen of the 8XP4 Check Tube.(A) By a 90 -Degree Deflection System.

(B) By a 70 -Degree Deflection System.

tions. A test pattern that was pro-duced on the 8XP4 tube by a receiverhaving 70 -degree deflection is shownin Fig. 1B.

The check tube has many featureswhich simplify the handling of thetube on the service bench. The 8XP4is able to reproduce a good pictureunder a wide variety of operatingconditions, and connections betweenthe check tube and a receiver can bemade rapidly and easily. All the tech-nician has to do when hooking up

A

B

the 8XP4 tube is to slip the yokearound the neck of the tube and toattach the tube socket and the anodelead. Since electrostatic self -focus-ing is employed, no external focus-ing device is required. In addition,the electron gun is constructed tooperate without an ion trap magnet.It may develop an ion spot duringuse, but the spot will not impair thetube's usefulness. Centering magnetswill, in general, be unnecessary. The8XP4 voltage ratings are high

enough to permit its use in any con-ventional receiver. Potentials as highas those encountered in any conven-tional receiver may be applied to theanode.

The results of using the 8XP4 intesting picture tubes by substitutionare conclusive. If a picture tube issuspected of being defective, thecheck tube should be tried in itsplace. A normal picture will bedeveloped on the check tube if thereceiver circuits are operating prop-erly, and any suspicion that the pic-ture tube might have been causingtrouble will then be confirmed. Insome cases, the picture on the checktube may be better than that on theold picture tube but still not satisfac-tory. This indicates that there maybe defects in the circuits as well asin the picture tube. The use of thecheck tube allows the technician toisolate defects to the chassis or tothe picture tube so that troubleshooting can be done more effi-ciently.

Even if the picture tube of areceiver might appear to be in goodcondition, the technician may wish toreplace it with an 8XP4 during serv-icing for the simple reason that thecheck tube is much less bulky thanthe picture tube. This use of thecheck tube for the sake of conven-ience in handling is especially impor-tant in view of the current trendtoward the cabinet mounting ofpicture tubes. Many of the newreceivers having vertical chassis arefeaturing this style of tube mounting,and an increasing percentage of thereceivers with horizontal chassis arealso using cabinet mounting.

When a receiver must be taken tothe shop for servicing, the chancesare that the chassis will have to beremoved from the cabinet sooner orlater. In the case of a receiver thathas a cabinet -mounted picture tube,it is a sensible practice to remove thechassis in the customer's home. Thechassis should then be taken to theshop where it can be serviced withthe aid of the check tube, and the

6 Sylvania News May -June 1956

(Fig. 2.) Technician Repairing a Cold -Solder Joint on a Vertical Chassis in Which the 8XP4 Is Installed.

(Fig. 3.) Technician Using an Ion -Trap Bracket as a Mounting Clamp for the 8XP4.

cabinet and picture tube should beleft behind. This practice saves thetechnician the effort of hauling theentire receiver to the shop, and itminimizes the risk that the picturetube might be broken or that thecabinet might be scratched. The pic-ture tube should be taken to the shoponly if it is suspected of being defec-tive.

The yoke assembly does not ordi-narily have to be removed from thepicture tube if a satisfactory substi-

tute yoke is available at the shop, butthere are certain cases in which theyoke will be taken along with thechassis. The yoke leads of somereceivers are soldered directly to var-ious terminals on the chassis, andthe technician has the choice ofunsoldering these leads or of remov-ing the yoke from the picture tube.He should follow whichever coursehe considers to be the easier.

One practical way in which the8XP4 can be connected to a vertical

chássis is demonstrated. in Fig. 2.The yoke is placed in its normal oper-ating position, and the neck of thetube is inserted into it. All connec-tions to the 8XP4 can then be madewithout the use of extension cablesin spite of the fact that the yoke andsecond -anode leads of most verticalchassis are very short.

Notice that the check tube doesnot interfere with the accessibility ofparts on the wiring side of the verti-cal chassis. Nearly all servicing oper-ations can be carried on while thetube is installed in the position shownin Fig. 2.

More often than not, a yoke is at-tached to a vertical chassis only by itsleads. Even if the yoke is held inplace on the chassis by a mountingdevice, this device will seldom berigid enough to provide a firm sup-port for the 8XP4. Some kind of propshould therefore be placed under thecheck tube. The U-shaped supportwhich is shown in Fig. 2 is veryhandy for this purpose. This shouldbe placed under the front of the tubebecause the weight of the tube is con-centrated near its face. Since the tubeis supported at two points, it is heldsteady and does not tend to rockfrom side to side as it might if it wereset upon a flat surface.

A better picture will be obtainedand the tube will be held in placemore firmly if the yoke is pressedtightly against the bell of the tube. Aclamp which will accomplish this canbe made out of a discarded ion trap(with magnet removed) or a center-ing -magnet assembly. Remove themagnets from whichever is used, andslide the clamp forward on the neckof the tube until it rests against theyoke. The positioning of this clampis shown in Fig. 3. Once in place, theclamp grips the neck and preventsthe tube from sliding out of position.

A special rack in which the 8XP4can be rested is a useful accessory forthe service bench. Fig. 4 shows a de-vice of this kind being used duringthe servicing of a horizontal chassis.Some receivers, like the one shown

May -June 1956Sylvania News

7

(Fig. 1.) Test Patterns Produced on the Screen of the 8XP4 Check Tube.

(A) By a 90 -Degree Deflection System.


tions. A test pattern that was pro-duced on the 8XP4 tube by a receiverhaving 70 -degree deflection is shownin Fig. 1B.

The check tube has many featureswhich simplify the handling of thetube on the service bench. The 8XP4is able to reproduce a good pictureunder a wide variety of operatingconditions, and connections betweenthe check tube and a receiver can bemade rapidly and easily. All the tech-nician has to do when hooking up

A

B

the 8XP4 tube is to slip the yokearound the neck of the tube and toattach the tube socket and the anodelead. Since electrostatic self -focus-ing is employed, no external focus-ing device is required. In addition,the electron gun is constructed tooperate without an ion trap magnet.It may develop an ion spot duringuse, but the spot will not impair thetube's usefulness. Centering magnetswill, in general, be unnecessary. The8XP4 voltage ratings are high

enough to permit its use in any con-ventional receiver. Potentials as highas those encountered in any conven-tional receiver may be applied to theanode.

The results of using the 8XP4 intesting picture tubes by substitutionare conclusive. If a picture tube issuspected of being defective, thecheck tube should be tried in itsplace. A normal picture will bedeveloped on the check tube if thereceiver circuits are operating prop-erly, and any suspicion that the pic-ture tube might have been causingtrouble will then be confirmed. Insome cases, the picture on the checktube may be better than that on theold picture tube but still not satisfac-tory. This indicates that there maybe defects in the circuits as well asin the picture tube. The use of thecheck tube allows the technician toisolate defects to the chassis or tothe picture tube so that troubleshooting can be done more effi-ciently.

Even if the picture tube of areceiver might appear to be in goodcondition, the technician may wish toreplace it with an 8XP4 during serv-icing for the simple reason that thecheck tube is much less bulky thanthe picture tube. This use of thecheck tube for the sake of conven-ience in handling is especially impor-tant in view of the current trendtoward the cabinet mounting ofpicture tubes. Many of the newreceivers having vertical chassis arefeaturing this style of tube mounting,and an increasing percentage of thereceivers with horizontal chassis arealso using cabinet mounting.

When a receiver must be taken tothe shop for servicing, the chancesare that the chassis will have to beremoved from the cabinet sooner orlater. In the case of a receiver thathas a cabinet -mounted picture tube,it is a sensible practice to remove thechassis in the customer's home. Thechassis should then be taken to theshop where it can be serviced withthe aid of the check tube, and the


7111.110"---

SYLVANIA TYPE 8XP4

TELEVISION RECEIVER CHECK TUBE

8" Direct ViewedRectangular Glass TypeGray Filter Glass

Magnetic DeflectionSelf Focusing (Electrostatic)No Ion Trap Required

12-S

CHARACTERISTICSGENERAL DATA

Focusing Method Self Focusing (Electrostatic)Deflecting Method MagneticDeflecting Angle (approx.)Vertical.. 68 DegreesHorizontal .. .. .. .. .. .. .. .. .. .. .. 85 DegreesDiagonal 90 DegreesPhosphor. P4Fluorescence WhitePersistence Short to MediumFaceplate Gray Filter GlassLight Transmittance (approx.) 80 Percent

ELECTRICAL DATAHeater Voltage 6.3 VoltsHeater Current 0.6 AmpereDirect Interelectrode Capacitances

Cathode to All Other Electrodes. 5 µµfGrid No. 1 to All Other Electrodos 6 µµf

MECHANICAL DATAOverall Length

11ó{3 ± b -i InchesMinimum Useful Screen Dimensions 7%3 x 5% InchesBulb Contact (Recessed Small Cavity Cap.) J1-21Base (Small Shell Duodecal 5 -Pin) B5-57Basing12S

RATINGSMAXIMUM RATINGS (Absolute Maximum Values)

Anode Voltage22,000 Volts d cGrid No. 2 (and Grid No. 4) Voltage 550 Volts d cGrid No. 1 Voltage

Negative Bias Value 155 Volts d cNegative Peak Value. 220 VoltsPositive Bias Value 0 Volts d ePositive Peak Value 2 VoltsPeak Heater -Cathode VoltageHeater Negative with Respect to Cathode

During Warm-up Not to Exceed 15 Seconds 450 VoltsAfter Equipment Warm-up 200 VoltsHeater Positive with Respect to Cathode 200 VoltsTYPICAL OPERATING CONDITIONS

Anode Voltage16,000 Volts d cGrid No. 2 (and Grid No. 4) Voltage 300 Volts d cGrid No. 1 Voltage Required for Cutoffs -28 to -72 Volts d c

CIRCUIT VALUESGrid No. 1 Circuit Resistance....

1.5 Megohms Max.

SYLVANIA PICTURE TUBESIssued as a supplement to the manual in Sylvania News for May -June 1956

chassis is demonstrated. in Fig. 2.The yoke is placed in its normal oper-ating position, and the neck of thetube is inserted into it. All connec-tions to the 8XP4 can then be madewithout the use of extension cablesin spite of the fact that the yoke andsecond -anode leads of most verticalchassis are very short.


More often than not, a yoke is at-tached to a vertical chassis only by itsleads. Even if the yoke is held inplace on the chassis by a mountingdevice, this device will seldom berigid enough to provide a firm sup-port for the 8XP4. Some kind of propshould therefore be placed under thecheck tube. The U-shaped supportwhich is shown in Fig. 2 is veryhandy for this purpose. This shouldbe placed under the front of the tubebecause the weight of the tube is con-centrated near its face. Since the tubeis supported at two points, it is heldsteady and does not tend to rockfrom side to side as it might if it wereset upon a flat surface.

A better picture will be obtainedand the tube will be held in placemore firmly if the yoke is pressedtightly against the bell of the tube. Aclamp which will accomplish this canbe made out of a discarded ion trap(with magnet removed) or a center-ing -magnet assembly. Remove themagnets from whichever is used, andslide the clamp forward on the neckof the tube until it rests against theyoke. The positioning of this clampis shown in Fig. 3. Once in place, theclamp grips the neck and prevents

of the tube from sliding out of position.A special rack in which the 8XP4

can be rested is a useful accessory forthe service bench. Fig. 4 shows a de-vice of this kind being used duringthe servicing of a horizontal chassis.Some receivers, like the one shown

May -.Tune 1956Sylvania News

7

(Fig. 1.) Test Patterns Produced on the Screen of the(A) By a 90 -Degree Deflection System.


tions. A test pattern that was pro-duced on the 8XP4 tube by a receiverhaving 70 -degree deflection is shownin Fíg. 1B.

The check tube has many featureswhich simplify the handling of thetube on the service bench. The 8XP4 th,

ís able to reproduce a good pictureunder a wide variety of operatingconditions, and connections betweenthe check tube and a receiver can bemade rapidly and easily. All the tech-nician has to do when hooking up

enough to permit its use in any con-ventional receiver. Potentials as highas those encountered in any conven-

SYLVANIA ELECTRONIC TUBES


(Fig. 2.) Technician Repairing a Cold -Solder Joint on a Vertical Chassis in Which the 8XP4 Is Installed.

(Fig. 3.) Technician Using an Ion -Trap Bracket as a Mounting Clamp for the 8XP4.

cabinet and picture tube should beleft behind. This practice saves thetechnician the effort of hauling theentire receiver to the shop, and itminimizes the risk that the picturetube might be broken or that thecabinet might be scratched. The pic-ture tube should be taken to the shoponly if it is suspected of being defec-tive.

The yoke assembly does not ordi-narily have to be removed from thepicture tube if a satisfactory substi-

tute yoke is available at the shop, butthere are certain cases in which theyoke will be taken along with thechassis. The yoke leads of somereceivers are soldered directly to var-ious terminals on the chassis, andthe technician has the choice ofunsoldering these leads or of remov-ing the yoke from the picture tube.He should follow whichever coursehe considers to be the easier.

One practical way in which the8XP4 can be connected to a vertical

chassis is deinonstrated. in Fig. 2.The yoke is placed in its normal oper-ating position, and the neck of thetube is inserted into it. All connec-tions to the 8XP4 can then be madewithout the use of extension cablesin spite of the fact that the yoke andsecond -anode leads of most verticalchassis are very short.


More often than not, a yoke is at-tached to a vertical chassis only by itsleads. Even if the yoke is held inplace on the chassis by a mountingdevice, this device will seldom berigid enough to provide a firm sup-port for the 8XP4. Some kind of propshould therefore be placed under thecheck tube. The U-shaped supportwhich is shown in Fig. 2 is veryhandy for this purpose. This shouldbe placed under the front of the tubebecause the weight of the tube is con-centrated near its face. Since the tubeis supported at two points, it is heldsteady and does not tend to rockfrom side to side as it might if it wereset upon a fiat surface.

A better picture will be obtainedand the tube will be held in placemore firmly if the yoke is pressedtightly against the bell of the tube. Aclamp which will accomplish this canbe made out of a discarded ion trap(with magnet removed) or a center-ing -magnet assembly. Remove themagnets from whichever is used, andslide the clamp forward on the neckof the tube until it rests against theyoke. The positioning of this clampis shown in Fig. 3. Once in place, theclamp grips the neck and preventsthe tube from sliding out of position.

A special rack in which the 8XP4can be rested is a useful accessory forthe service bench. Fig. 4 shows a de-vice of this kind being used duringthe servicing of a horizontal chassis.Some receivers, like the one shown

May -June 1956Sylvania News

7

(Fig..4.) Technician Using the 8XP4 in a Rack.

in Fig. 4, are already equipped withconnecting leads that are longenough to reach the check tube with-out difficulty. The picture -tube leadsof some other receivers are so shortthat a. direct connection is awkwardor impossible to make. In the lattercase, extension cables must be em-ployed. The rack can be used withboth horizontal and vertical chassis.The check tube is protected againstrough handling when it is mountedin this manner, and it can be easilymoved around on the bench. Therack is also useful as a storage placefor the tube.

Construction of such a mounting rack is a simple task. The one in Fig.4 was made from several small piecesof wood, a strip of sponge rubbersalvaged from the rim of an old pic-ture tube, and an elastic band ob-tained at a dime store. A strip ofwood and two small triangular blocksare fastened to one end of a woodbase. These serve to hold the bell ofthe tube. A large block of wood witha notch cut in its top is attached byscrews to the other end of the base,and this block supports the neck ofthe tube. Strips of rubber are used ascushions for the neck of the tube andfor the rim of the faceplate. The

elastic band is fastened to the sidesof the triangular blocks; and when itis slipped over the bell of the tube,it exerts a forward pressure and seatsthe tube firmly in its mounting. Thetension of the elastic is only moder-ate, and the base of the tube caneasily be lifted so that a yoke can beplaced around the neck.

More elaborate mountings couldalso be devised for the 8XP4. Forexample, the tube might be perma-nently enclosed in a box which couldbe used in the shop or carried in theservice truck. Why would a techni-cian wish to carry an 8XP4 in theservice truck? The answer is that asuitably mounted check tube wouldbe useful on home calls for the test-ing of picture tubes by substitution.If the technician specializes in servic-ing receivers which all have approxi-mately the same yoke inductance, hemay build a self-contained unit thatincludes a yoke and a set of exten-sion cables as well as a check tube.He should keep in mind the fact thatno single yoke will work properlywith all receivers. For general use,such a box should be designed so thatvarious yokes can be temporarilyslipped over the neck of the tube.The mounting brackets should be

kept clear of the neck. This could bedone if the bell of the tube were heldin place by straps fastened to thefront of the box.

This discussion of mounting meth-ods for the 8XP4 has dealt mostlywith receivers that have cabinet -mounted picture tubes. Receivers inwhich the picture tube is mounted onthe chassis can usually be servicedconveniently with the picture tube inplace. There would be some advan-tage in using a check tube in placeof the picture tube if a chassis wereundergoing extensive repairs. Theimplosion hazard would be greatlyreduced, and the chassis would bemuch lighter. The chassis couldtherefore be moved around on thebench with considerable freedomduring a long series of servicing op-erations.

The 8XP4 was designed so thatversatility would be its most out-standing feature. It is adaptable tonearly any situation in which a con-venient substitute for a picture tubeis needed.


SYLVANIA NEWS

SEPTEMBER 1956 VOL. 23 NO. 8R. A. Humphreys, Technical EditorThis information in Sylvania News is furnished without assuming any obligations

FACTORS

AFFECTING THELIFE OF

HORIZONTALOUTPUT TUBESBy G. L. Quint - Field Engineer,

The continual advancement in TVreceiver circuits has produced a needfor many new tube types. Notableamong these are horizontal deflectiontubes. The TV service technicianhas demanded that the tube industrysupply more reliable and rugged re-placement tubes. These factors havecaused the electron tube manufac-turers to upgrade their early andsometimes overworked types. Theinformation and know-how gainedthrough making these improvementsis applied in the design and manu-facture of new tube types.

Most Frequently Serviced CircuitThe horizontal deflection tube is

one of the hardest working tubes ina television receiver. The energydelivers to the horizontal deflectionsystem must perform many functions,such as providing horizontal scan,high voltage to the picture tubesecond anode, focusing voltage forsome types of picture tubes, filamentvoltage for the high voltage and focusvoltage rectifiers, keying pulses forkeyed and amplified a g c systems, akeying pulse for the burst amplifierin color receivers, a feedback timingpulse for some types of horizontaloscillator control systems, and other

Radio Tube Division

functions which may be germinatingin the minds of TV receiver designengineers.

It can be seen that the horizontaldeflection system is being monitorednot only by its ability to providesufficient scan and high voltage butalso by other circuits whose operationdepend upon signals derived from thehorizontal deflection system. Theability of these other circuits tofunction properly as the horizontaldeflection tube characteristics declineon life will determine how soon andhow much the picture will be affected.Since the eye is much more criticalthan the ear, small amounts of picturedistortion or degradation are moreirritating to the viewer than a com-parable distortion of the sound. Thus,the TV service technician is calledupon to service the horizontal de-flection system more often than theother circuits in the receiver.

Design and Environmental FactorsSome other factors which must

be considered since they affect thefailure rate of the tubes are the oper-ating conditions imposed upon thetube. These factors may be brokendown into two groups-design andenvironment.

, _._,,._.e ._......Y._..q_<_b-.

__t-

The design factors are those whichdetermine the plate and screen dis-sipations, peak and average plate,screen and cathode currents, the highamplitude pulse (or peak positiveplate voltage) across the tube duringhorizontal retrace time, and the d cplate supply voltage (d c plus boostedB+). Normally, the adjustment ofthe drive, linearity, and width con-trols are also design considerations.Depending on the amount of "knobtwisting" done by the layman, theymay be "environmental factors."

Some of the environmental factorsare the line voltage and ambienttemperature at which the tubesoperate.

Some of the operating conditionsdescribed above also affect the initialperformance of the tube and the TVservice technician should make theproper width, linearity and driveadjustments when replacing the hori-zontal output tube.

Design Improvements and FeaturesThe new Sylvania Types 6BG6GA,

6BQ6GTA and 6CD6GA are a fewexamples of the changes that havebeen made to improve the reliabilityand performance of the horizontal

(Continued on page 6)

Sylvania News September 19565

(Continued from page 5)deflection tubes. We shall discussthree types since they reflect theimprovements made over their pro-totypes 6BG6G, 6CD6G and the6BQ6GT. Bear in mind that thenewer deflection tubes such as theSylvania Types 6DN6, 6CU6, 6DQ6and 6DQ6A, incorporate parallel im-provements plus other features asdictated by the particular require-ments that these newer types mustsatisfy.

One of the first steps taken duringthe redesign of the deflection tubeswas to alleviate the destructive effectscaused by the high temperature andvoltages encountered in horizontaldeflection tube operation. Heatspeeds up the liberation of gas fromglass, metal, and mica parts. Theheat and high voltages combine tocause chemical decomposition of theglass, known as glass electrolysis.The part of the tube most susceptibleto glass electrolysis is the stem, orthat part which holds the leads that

provide the electrical (and mechani-cal) connection between the tubeelements and the base pins.

Wafer Construction-Stem Electrolysis Minimized

Figure 1, right, shows the old-style,flat -press stem construction. Note thatthe leads are small in diameter andclosely spaced. Figure 1, left, shows thenew wafer stem construction utilizedextensively by Sylvania not only ondeflection tubes but on upgradedversions of practically all old GT typetubes. The circular pin arrangementof the wafer stem allows the use oflarger diameter lead wires with greaterspacing between the wires. Thesefeatures are also clearly shown inFigure 2. The larger lead wires pro-vide for greater heat conduction awayfrom the tube elements while thewider lead spacing reduces the voltagegradient between leads. These twoadvantages of the wafer stem haveeliminated the problem of stem elec-trolysis.

FIGURE 1-SHORTER, MORE RUGGED MOUNT. Added support, shorter construction,and larger diameter leads are clearly visible in the mount employing a wafer stem, left, ascompared to the press stem mount shown on the right.

Less Susceptible to VibrationThe wafer stem construction also

provides a rugged three-point supportfor the tube mount which makes itless susceptible to vibration, as com-pared to the weaker two point supportof the flat press construction. Thus,the wafer stem construction providesmuch needed electrical and mechani-cal tube improvements.

Reduced Internal ArcingWith the wafer stem construction

the mount may be placed lower in thebulb. This provides for greater spac-ing between the plate pigtail lead,going to the top of the bulb, and therest of the mount. This allows agreater safety factor for peak platevoltages and has substantially reducedinternal arcing in the tube.

New Screen Grid ConstructionOne of the greatest sources of

trouble with the deflection tubes hasbeen screen grid dissipation. Recentchanges in screen grid structure, coat-ing materials and advanced processingtechniques . have eliminated "wild"tube characteristics which occurwhen the screen grid becomes sohot that its mechanical dimensionsbecome distorted.

It must be appreciated that theperformance requirements for thevarious horizontal deflection tubessometimes change as new receiverdesigns come on the market. Thiscan temporarily cause some difficultyin the replacement field. There willbe a time delay until the tube manu-facturer can ascertain what improve-ments are needed; the given tube'sdesign is upgraded; and the upgradedproduct is placed in the field. Syl-vania keeps a watchful eye on thereplacement market to detect anysuch potential difficulties so that thetime lag in getting the upgradedversion of the tube type on theserviceman's shelf will be minimized.

Thorough TestingTo support and insure continuation

of the improvements which havebeen built into the deflection tubes,Sylvania has imposed more stringentspecifications and developed new testprocedures to pick out as many latentdefects as possible in the tubes whichmight show up in the field as shortlife.

6 September 1956 Sylvania News

FIGURE 2-REDUCED GLASS ELECTROLYSIS AND ARCING. Wafer stem constructionis shown on the right, the old press stem construction on the left. It is readily seen that thewafer stem leads are farther apart which (coupled with cooler operation resulting from heavier,shorter leads) reduces glass electrolysis and arcing.

Also, in order to further study andimprove the performance capabilitiesof these deflection tubes, as well asmany other types in the line, Sylvaniahas established an Application QualityLaboratory which continually evalu-ates tube production for performance,Figure 3. Samples of all productionlots of tubes are tested in receivers.

Sylvania feels that the ApplicationQuality tests are very important sincemere reliance on the data from statictest conditions, Figure 4, is no assur-ance of satisfactory tube performancein actual applications. Once theinitial operating capabilities of thetubes have been ascertained, informa-tion as to how they will perform

FIGURE 3-APPLICATION QUALITY LAB. Corner of the Application Quality Lab.Samples of all production lots of tubes are evaluated for performance in TV receivers.

during months and months of setoperation is also very necessary.

TV Life TestSometime ago, Sylvania instituted

the life testing of tubes in TV re-ceivers representing many differentmakes and models, Figure 5. The lifetest is performed with the receiversoperating at high a c line voltage.The receivers are cycled on and offeach hour for approximately 1500hours (approximately one year of setoperation) . The tests are run at highline voltage to accelerate anytendency for the tubes to fail. Theperformance of the receiver is con-tinually monitored during the lifetest period and any failure, whethertube or component, is carefully noted.These tests have shown that operatinga TV receiver at 130 volt line willcause the failure rate of the entirereceiver to increase by a factor of2.37 times as compared to the failurerate at 117 volt line*. What is theline voltage in your area?

Static Life TestA static life test is performed in

addition to the TV life test. The staticlife test operates the tube under d ecircuit conditions, at or near maxi-mum plate and screen dissipationsand d c cathode current. The tubesare read periodically for electricalcharacteristics. When the character-istics which must be controlled onlife to insure satisfactory tube opera-tion read above or below specifiedvalues, the tube is considered to have


FIGURE 4-PORTION OF TEST set up forchecking major electrical characteristics ofhorizontal deflection tubes. Although verynecessary in determining quality level,Sylvania feels that this test alone does notassure satisfactory performance in actualapplication.

Sylvania News September 19567

FIGURE 5-TV LIFE TEST. Tubes are life tested in TV receivers representing many differentmakes and models, for a period approximately equivalent to one year of set operation.

(Continued from page 7)

reached the end of its life as far asthis test is concerned. It must beremembered that the specified life testend points cannot possibly includevalues for all of the different circuitconditions which the many receivermanufacturers may impose upon thetubes in their various receivers.These limits must be chosen withcertain compromises and with theidea that the tubes will give satis-factory performance in the greatmajority of receivers.

Dynamic Life Test

In the dynamic life test, Figure 6,tubes are operated in horizontal sweepsystems which are adjusted so thateach tube type is operated underconditions more severe than thoseencountered in a properly designedhorizontal deflection system in a TVreceiver. The operating conditionsof the dynamic life test can be closelycontrolled. Thus, this test providesstandardized information as to tubeperformance, whereas the TV receiverlife test provides information abouttube performance under the differentconditions found in various makesand models of TV receivers.

The Final Product and Field Problems

The product is released for use inthe field only when all efforts of thetube manufacturer have been ex-pended to the point where reliableperformance has been ascertained bythe various characteristic and life

tests. It is at this point, especially inthe replacement tube business, thatthe manufacturer usually loses allcontrol on how his product will beused. True, the tubes are intendedto replace a defective tube of thesame type in some equipment, thedesign of which was, or should havebeen, such that the tubes are operat-ing in a reliable environment to givesatisfactory life and performance.However, the gradual deterioration ofall the components in a TV receivermay change the operating conditionsof the tubes to the point where certainmaximum ratings are approached or

exceeded. Adjustment by the laymanmay change operating conditions tothe point where tube failures areaccelerated. The receiver may be ina locality where the line voltage ishigh, or fluctuates during the day asvarious industries increase or de-crease their demands, thus affectingthe- set failure rate as discussed pre-viously.

Since no manufacturer of equip-ment, especially home entertainmentdevices, can possibly know or controlthe local conditions under which hisproduct must operate, he is in thesame position as the componentmanufacturer in that he mustdetermine under what average orperhaps extreme conditions hisproduct must operate satisfactorily.

After due consideration of all thefactors, technical and economic, theresulting decisions are of necessity acompromise of all the importantfactors. Sometimes an accumulationof compromises produces a high per-centage of field headaches, but thefact that there are millions of tubesoperating in millions of receiversproves that the electronics industryhas been very successful in providinghome entertainment devices of re-liable and satisfactory performance,even in these highly competitive days.

*Effect of Environment on Tube Life-Elec-tron Tube Life and Reliability-Chapter IVby M. A. Acheson.

FIGURE 6-DYNAMIC LIFE TEST. In the dynamic life test tubes are operated in horizontalsweep systems which are adjusted so that each tube type is operated under conditions moresevere than those encountered in properly designed receiver deflection systems.

8 September 1956 Sylvania News

SYLVANIA NEWS

OCTOBER 1956 VOL. 23, NO. 9R. A. Humphreys, Technical EditorThis information in Sylvania News is furnished without assuming any obligations

UNDERSTANDING FLYBACK CIRCUITSBy R. F. Bergdahl

Modern television receivers haveshown vast improvements in circuits,components and tubes, giving us per-formance unthought of a few yearsback. This article is intended to givea better understanding of a frequentlyencountered modification of the usualflyback circuit. The knowledge ofboth types is necessary for effectiveand efficient troubleshooting.

BASIC CIRCUIT DIFFERENCES

The two types of flyback circuits,which are most often encountered,may be classified by the type of fly-back transformer employed. The first

is the conventional flyback trans-former which is easily identified bythe separate windings for the damperand deflection yoke, Figure 1. Thesecond is the direct drive or auto typeflyback transformer, Figure 2. Theprinciple of operation is very similarfor both types. A few special notes,however, will clarify the basic circuitdifferences. (1) The conventionalflyback transformer applies a negative

) pulse to the plate of the damper andthe boosted B+ voltage is taken fromthe cathode. (2) The direct drivetype of flyback transformer (auto)applies a positive pulse to the cathode

HOR DEFAMP'

DRwEi-

TBOOST

RV TOPIX TUBE

ANODE

HOR DEFYOKE

FIGURE 1-Flyback Circuit Employing Separate Winding Type Transformer.

of the damper while the plate goes toB+ and the boosted B+ voltage istaken from the low side of the flybacktransformer winding.

CIRCUIT OPERATION

Now let us consider the operationof a typical horizontal deflectioncircuit employing an auto type fly-back transformer, Figure 2. Thewaveshape of the driving voltage isthe familiar sawtooth, common to theapplication, Figure 3A. Operatingbias is developed by the tube; that is,the input signal drives the grid slight-ly positive, thus causing current toflow from the cathode to the grid andthrough the grid resistor, R1. Theresulting voltage drop across R1charges the coupling capacitor, C1,in a manner such that the d c operat-ing level of the grid is negative withrespect to the cathode.

It should be noted that the cathodeof the horizontal deflection amplifiershown is grounded directly, Figure 2.In the absence of drive, in such a case,the tube would be operating withoutbias, resulting in excessive plate andscreen current. In many circuits aprotective bias voltage is providedby the incorporation of a cathoderesistor. This arrangement limits thecurrent flow to a safe value shouldgrid excitation not be present.

Now that we have established themethod used to obtain bias, let's seewhat basically happens in the rest ofthe circuit for the period of one drive


Sylvania News October 1956 5

IIcIDRIVE+t-iF-

BOOST

FIGURE 2-Flyback Circuit Employing an Auto Transformer.

(Continued from page 5)cycle. Referring to Figure 3A, timeT1, we find that the tube is cutoff.At time T2 the drive voltage hasincreased to a value which is equal tothe cutoff voltage of the tube. At thispoint the tube begins to draw currentand continues to do so, increasingsteadily, until time T3. At this pointthe tube is drawing its maximumcathode current. Figure 3B showsthe waveshape of the cathode currentthrough the tube, and thereforethrough the flyback transformer. Thisincreasing current through thecircuit, between times T2 and T3,produces a corresponding and pro-portionate increase in the magneticlines of force in the flyback trans-former and deflection yoke, thusproviding the energy required forone-half the scanning cycle (to bediscussed later).

Returning to Figure 3A, it will benoted that the driving voltage, uponreaching time T3, suddenly drops toits most negative value, T4. Thisnaturally causes the tube to be cutoffsharply and stops the current flow inthe flyback transformer. Since thereis no longer any current flow throughthe flyback transformer the developedflux rapidly decays and in so doingproduces a large positive voltagepulse, Figure 3C (1), of short timeduration. Now, as this positive pulsedecays it induces a magnetic field inthe flyback transformer which is ofthe opposite polarity. Collapse ofthis newly created flux produces a

HV

+

HV TOt PIX TUBE

ANODE

HOR DEFYOKE

negative pulse, voltage in the flybacktransformer windings, but of a lowermagnitude than the positive pulse,Figure 3C (2) . These pulse voltageshave somewhat of a sinewave appear-ance and will continue "ringing" or

back and forth" until theenergy required to continue theaction is expended in transformerlosses. This ringing effect can becompared to a musical instrument inwhich the plucking of a string willcause the string to vibrate back andforth until it gradually comes to rest.

DAMPING ACTION

Since a smooth, linear scan isrequired on the picture tube, and themagnetic flux required to bend theelectrons in the picture tube isdirectly proportional to the currentin the yoke, the yoke must not havethe current fluctuations that resultfrom the ringing action just described.To eliminate this difficulty a dampingdiode is placed in the ringing circuitso that every time the ringing voltageswings negative the tube will conductand absorb the energy required tocontinue the ringing. Nothing needbe done to the positive pulse voltage,Figure 3D, (first swing of the flybacktransformer) since it occurs at a timewhen the picture tube is cutoff, i.e.,during the horizontal retrace time.

To understand how the dampingdiode absorbs power from the negativepulse voltage, let us look at the simu-lated circuit shown in Figure 4 andoperate a rectifier tube with the

cathode a e fed. Under these con-ditions the tube will conduct whenthe a e voltage on the cathode swingsnegative with respect to the plate.When conduction occurs, capacitorC3 becomes charged and the positiverectified voltage then appears at thecathode. Since the plate of therectifier tube is already at B+ poten-tial, effectively there are two suppliesconnected in series. Thus, the sumof both the rectified a c voltage andthe battery B+ voltage will be presentacross the load resistor, RL. In theactual flyback circuit, Figure 2, thenegative portion of the ringing sine -

wave voltage is rectified by the damp-ing diode and appears as a d c voltageacross the boost capacitor (C2) . Thisvoltage is called the boost voltage andthe sum of the boost voltage and B+(boosted B+) is used in many makesof television receivers to (1) supplythe first accelerating anode voltage tothe picture tube, (2) supply thevertical and/or horizontal oscillatorplate voltages, (3) supply the verticaldeflection amplifier plate voltage, aswell as supply a higher voltage to theplate of the horizontal deflection tube.

HORIZONTAL SCAN

To continue with the basicfunctions of the horizontal flybackcircuit, let us examine the deflectionyoke. The horizontal deflection yokeconsists of two separate windings.(These coils are usually connected inseries but depending on the im-pedance required may be connectedin parallel.) With a normal operatingreceiver, let us assume no currentflow in the horizontal coils of theyoke. This would leave a verticalwhite line at the approximate centerof the picture tube screen. There-fore, to produce scan on the left sideof the picture tube screen, currentmust flow in one direction, which wewill call the negative direction, and toproduce scan on the right side of thepicture tube screen, current mustflow through the yoke in the oppositedirection, which we will call thepositive direction.

To scan the left side of the picturetube, the negative current required isproduced when the damper drawscurrent during the negative portionof the flyback ringing. To understand


6 October 1956 Sylvania News

DRIVE VOLTAGE AT GRID OF (A)HORIZONTAL AMPLIFIER

HORIZONTAL AMPLIFIERCATHODE CURRENT

HORIZONTAL AMPLIFIERPLATE VOLTAGE

(B)

(C)

DAMPER CATHODE VOLTAGE (D)

HIGH VOLTAGE RECTIFIERPLATE (E)

YOKE CURRENT (F)

RETRACETIME

(I)

Ti

(2)

I. -GRID CURRENT DRAWNHERE. 0 BIAS (APPROX)

T3

- CUTOFF

T4

L

PICTURE TRACEImo- 53.3 .0 SEC .I I

k --SCAN TIME = 63.5 J.t SEC

FIGURE 3-Waveforms Encountered in Typical Flyback Circuits.

HOR AMP

-0 CURRENT

DAMPER

VOLTAGE SUPPLYTRANS

R

C3

C4

r (rtn) c-N

I

RECT I

T

FIGURE 4-Simulated Boost Circuit.

(Continued from page 6)how the damper draws currentthrough the yoke in a negative direc-tion it must be understood that theyoke and flyback transformer are tiedin parallel and together represent atuned circuit. Therefore, at the timeof the negative pulse (Figure 3C-2)power from both the flyback trans-former and yoke supply the negativepulse which is rectified by the damper.This resulting "negative" current isrepresented by Il, Figure 5.

I2, Figure 5, represents the "posi-tive" current required to scan theright half of the picture tube and isproduced during the conductionperiod of the horizontal deflectionamplifier tube.

Currents Il and I2 are plottedagainst time in Figure 6. Whencurrent Ii, from the damper -yokecircuit dies, the trace returns to thecenter of the screen. By this time,however, the driving waveform hasreached the point where the hori-zontal deflection amplifier starts toconduct (Figure 3A, T2) thus begin-ning the right hand portion of thesweep cycle. The dotted line inFigure 6 shows the resulting yokecurrent waveform when the Il and I2currents are combined and, beinglinear, produce a linear flux in theyoke and a resulting linear _trace onthe picture tube.

DEVELOPMENT OF HIGH VOLTAGE

The flyback transformer has stillanother important requirement in thetelevision receiver. It supplies thehigh voltage to the picture tubeanode. We previously mentionedthat a high voltage pulse (Figure 3E)is generated by the flyback trans-former during the retrace time. Thispulse, when stepped up and rectified,is the source of high voltage for thepicture tube. Step up of the pulse isaccomplished by an additional autotransformer winding which is placedon the flyback transformer in serieswith the horizontal amplifier platewinding, Figure 2. The filamentsupply for the high voltage rectifieris ordinarily obtained by a single turnof wire wrapped around the core ofthe flyback transformer. Since thepower requirements of the filamentare usually low, this arrangementdoes not affect the operation of theflyback transformer.

Sylvania News October 19567

UNDERSTANDING FLYBACK CIRCUITS(Continued from page 71

TIME63.5 Ja SEC

I2

FLYBACKTRANS

B+

FIGURE 5-Deflection Current Paths.

FIGURE 6-Yoke Current For One Scan Line.

YOKE

EDITOR'S NOTE: Sylvania offers $10.00 in Advertising Material Certificates for all technical hints that it believes usefulto the service -dealer readers of SYLVANIA NEWS. Sylvania is not obligated to return any material submitted forpublication, whether or not published.

HOLDER FOR SUBSTITUTE PARTS

The photo shows two test prodsequipped with binding posts (small

size) which are utilized for substitut-

ing capacitors and resistors in , de-fective circuits. Parallel connectionscan be made to obtain desired valuesand a wire connected between theterminals enables the prods to beused for shorting purposes.

Fred W. BrownLos Angeles, California

5AXP4 MOUNTINGThe 5AXP4 TV test tube can be

firmly held in place by putting a handtype vacuum cleaner belt around theneck of the tube. By clipping the beltclose up to the deflection coil, youwill find that it will hold the tubefirmly enough even when chassis islying on its side.

Alex Radio & Television ServiceWoodhaven, New York

PICKUP HEAD PROTECTIVE SHIELD

The sponge rubber liners fromdefective vibrators have proven use-ful to the serviceman in a multitudeof ways. We would like to add tothis list its use as a pickup -head pro-tective sleeve when transportingphono units.

Stein's ShopLos Angeles, California

REMOVING TV CABINET FACEPLATESTwo small suction cups, equipped

with handles made from largewashers, can be a big help in removingglass faceplates from TV sets forcleaning.

The Radio ShopRockland, Maine

8October 1956 Sylvania News

+ i 11 -H44

SYLVANIA NEWS

NOVEMBER DECEMBER' 1956 VOL. 23 NO.10_R. A. Humphreys, Technical EditorThis information in Sylvania News is furnished without assuming any, obligations

ELECTRON TUBE OPERATING

CONDITIONS IN HORIZONTAL

DEFLECTION SYSTEMSBy G. L. Quint - Field Engineer - Radio Tube Division

The automobile has been with usfor about a half century. We are allquite familiar with this mechanicalmarvel. In the early days there wasgreat difficulty in keeping the "Horse -less Carriage" running, but, throughthe years, design improvements, im-proved testing techniques, improvedequipment, and improved understand-ing of the operation of the automobilenow permits the technicians whoservice these machines to analyzedifficulties and correct them on veryshort notice. Indeed, we are verymuch put out if a garage mechaniccannot promptly analyze and lastinglycorrect an operating difficulty.

The engine in your automobile isan interesting device indeed. It is acentral source of power which drivesthe wheels, but it does other jobs too.It drives the electrical generatorwhich provides the lights, runs theradio and operates the heater. Itrequires protective devices such asthe radiator, the fan, the water pump,air cleaner, etc., and has adjustmentsto permit it to be tuned up for opti-mum performance and efficiency. Ithas a gas pedal which regulates howhard it will operate.

What happens when somethinggoes wrong with this complex system?Suppose the radiator clogged up, itcould conceivably cause the motor

Sylvania News

to burn up. True, we will have toreplace the engine, but will it workany better if we don't correct theradiator trouble? This same holdstrue for any of the other protectiveequipment as briefly discussed above.

What happens when the controlsget out of adjustment? The enginemust work harder to do the same joband it does not have its customary"zip". * It also tends to wear outfaster. The owner becomes veryunhappy. Would we now change themotor and leave the adjustments asthey were before? Indeed not. Wewould attempt to have it readjustedfor optimum performance.

The things which we discussedabove seem quite obvious. We havecome to know this from over fiftyyears of experience in this field. Someof the situations discussed can hedirectly related to television servicing.

For example, the horizontal de-flection tube is a power source.It is coupled via a transmission(transformer) to its load (the hori-zontal deflection yoke.) It rendersauxiliary services such as providinghigh voltage and boost voltage toother parts of the receiver. It hasprotective devices surrounding it inthe form of a screen resistor, cathoderesistor and other passive circuitelements. It has a drive control and,

November - December 1956

-----i ii _4_

in most cases, has other adjustmentsto permit optimum performance.Included would be width and linearitycontrols.

If a component associated with ahorizontal deflection tube becomesdefective and causes it to fail, simplyreplacing the tube may make thesystem work for a short time, but,we will very soon have a call-back.If we replace the tube in a circuitwhich has many hours of operationand has components which have sub-stantially deteriorated, we will againhave a call-back on our hands. Ifthe horizontal deflection circuit is notadjusted for optimum operation, theset will lack zip and the auxiliaryservices will suffer.

Now, let us actually explore theeffects of improper circuit adjustment,deteriorated auxiliary componentsand/or "old age" and excessive linevoltage on the operating conditionsof tubes in the horizontal circuit. Inother words, just how hard does ourengine have to work when operatingunder adverse conditions and whatcan we expect from it in terms of life.

We now come to the point of select-ing a receiver for our investigation.Because there are so many variationsin circuit designs and components, itis difficult, if not impossible, to choose


5

(Continued from page 5)

a receiver and say that it is typical.Consequently, the data and measure-ments taken on a given receiver arenot necessarily typical. Therefore,receivers were selected at random andmeasurements taken on the circuitswith the tubes in the sets which weresupplied by the manufacturer. Wemust also have a reference conditionand this was logically chosen to be ata line voltage of 117 volts with the setadjusted for normal viewing. Theoperating conditions were limited tothe worst probable to provide a real -is tic evaluation.

EFFECTS OF HIGH LINE VOLTAGENow, let's see what happens to the

operating conditions of the horizontaldeflection amplifier, damper and highvoltage rectifier as the line voltage isincreased. At a line voltage of 130,increases in operation conditions forthe three tubes in the deflectionsystem ranged from 7.4% to 34.6%,Table I, O.C. 2. Note that the 34.6%increase is for the screen dissipationof the horizontal deflection amplifier.Plate currents of the horizontal de-flection amplifier and damper alsoshow an appreciable increase, as doesthe heater voltage of the high voltagerectifier, 12.2%.

Let's now readjust our test receiverfor a normal picture and see how theoperating conditions of the tubes inquestion are affected, still maintaininga line voltage of 130 volts. The resultsare shown in Table I, O.C. 3. At firstglance, some decrease will be notedfor the majority of measured charac-teristics; but, note that the peakpositive plate voltage of the horizontaldeflection amplifier has further in-creased, 23.7%, as has the heatervoltage of the high voltage rectifier,20.4%, and the peak plate current ofthe damper, 15.5%. The screen dis-sipation of the horizontal deflectionamplifier is still 34.5% above the valuerecorded when the set is operating at117 volts line.

IMPROPER ADJUSTMENT OF DRIVENext, let's return to the 117 volt

line condition and see what happenswhen the drive is at maximum. It isinteresting to note that no drive lineswere evident at the maximum settingof the drive control with this particu-lar receiver. The effects of operation

6

under this condition are shown inTable I, O.C. 4. With reference tothe horizontal deflection amplifier,the d c plate current and d c cathodecurrent have increased by 3L5% and28.7%, respectively.

This increase in d c cathode currentexceeds the rating for the tube byapproximately 26%. The d c cathodecurrent of the damper also shows asizable increase (33.4%) under con-ditions of maximum drive. It shouldbe remembered that the picture beingviewed appeared to be normal and didnot indicate excessive abnormal driveby the presence of drive bars.

LINEARITY AND WIDTH ADJUSTMENTSWith some receivers, improper

linearity and width adjustment mayalso radically affect tube operatingconditions, as to adversely affecttube life.

For example, some receivers havebeen encountered in which adjust-ment of the linearity control at the130 volt line condition caused theheater -cathode voltage of the damperto increase to such an extent thatrapid failures resulted from heaterto cathode arcs.

WHY CALL-BACKS

How does all this relate to call-backs? First of all, our experimenthas shown that there are manyvoltages and currents contributing tothe operation of the horizontal de-flection system. Many of thesecharacteristics are not ordinarilymeasured in normal servicing tech-niques but may be contributing topoor tube life or performance.Secondly, we have seen that althoughthe picture may appear normal,operating conditions of the tubes inthe horizontal system may be farfrom normal.

A light foot on the gas pedal of anautomobile will normally result inlong engine life. On the other hand,abnormally hard driving may drastic-ally shorten engine life. The sameholds true in the case of the horizon-tal deflection system. Increases inelectrode dissipations, resulting fromimproper set adjustment, componentdeterioration, or high line voltage,accelerates the liberation of gas andincreases bulb and stem temperatures,thus decreasing tube life. Excessive

November - December 1956

cathode current may cause loss ofemission early in life. Abnormalheater voltage, as evidenced in thecase of the high voltage rectifier, willseriously impair the life of this tube.

DIFFERENCES BETWEEN RECEIVERS

To illustrate the wide differenceswhich may be encountered betweenvarious receiver designs, two otherreceivers were selected and comparedfrom the standpoint of variations intube operating conditions.

As would be expected, increases inmeasured characteristics are evidentfor both receivers at increased linevoltage, Table III, O.C. 2. The inter-esting point, however, is that whilereceiver C shows an increase indeflection tube screen dissipation of24.7%, receiver B shows an increaseof over twice this value, 56.5%. Thus,the variations between makes andmodels of receivers are immediatelyevident. It is also interesting to notethat when receiver B was readjustedto obtain a normal picture, still main-taining the 130 volt line condition,the plate dissipation of the horizontaldeflection tube increased by 84%(almost twice normal value).

The effects of improper drive upontube operating conditions also showwide variations between differentreceivers. For example, in a certain21" receiver, when operated at the117 volt line condition with the drivecontrol adjusted for minimum drive,the plate dissipation of the horizontaloutput tube exceeded the maximumrated value of plate dissipation bysome 100 per cent. At the same linevoltage but with the drive control atmaximum, the plate dissipation waswell within the rated value for the type.

In some cases, a slump in the hori-zontal oscillator output may make itimpossible to obtain sufficient drive,thus causing the horizontal outputtube to fail in a short time.

The wide differences in tube operat-ing conditions between receivers,whether caused by improper setadjustment or high line voltage, maywell be responsible for many call-backs.

"Rule of thumb" servicing tech-niques are not adequate if long tubelife is to be insured. Each receiver


Sylvania News

TABLE I

EFFECTS OF CIRCUIT ADJUSTMENT AND LINE VOLTAGE

ON TUBE OPERATING CONDITIONS

TubeType

CircuitCharacteristic

Measured

1

(Reference)

Operating Conditions(% Change)

2 3 4

6CD6GA Ebboost V DC 515 14.6 16.5 9.70Ec2 V DC 98 14.3 11.2 -4.09Ik Ma DC 108 15.8 14.8 28.7ik Ma Pk 360 20.8 11.1Ic2 Ma DC 11.2 18.2 21.4 6.25Ib Ma DC 96.8 15.5 14.1 31.5Pc2 Watts 1.1 34.6 34.5 -ebpp Kv 4.45 16.9 23.7 5.61

1B3GT Ef V RF 0.98 12.2 20.4EH.V. Kv DC 13.0 7.7 15.4 -2.3P.I.V. Kv 14.08 7.4 15.4 -2.28

6AX4GT Ib Ma DC 96 16.7 14.6 33.4ib Ma Pk 390 12.8 15.5 -5.3Ehk Kv Pk 3.03 17.1 21.3 5.62P.I.V. Kv 2.78 17.6 21.6 6.5

Operating Condition (O.C.)

1-Eline =

2-Eline =3-Elíne =4-Eline =

117 V. Width, drive and linearity adjusted fornormal picture.130 V. No readjustments from O.C. 1.130 V. Set adjusted for normal picture.117 V. Normal picture, drive at maximum. Note:The maximum setting of the drive control wasobtained with no drive line appearing in the picture.

TABLE II

DEFINITIONS OF SYMBOLS

Ik = DC Cathode Current

ik = Peak Cathode Current

Ic2 = DC Screen Grid Current

Ib = DC Plate Current

Pp = Plate Dissipation

Pc2 = Screen Grid Dissipation

P.I.V. = Peak Inverse PlateVoltage

ib = Peak Plate Current

Ehk = Heater -to -CathodeVoltage (DC + Peak)

Eb = DC Supply Voltage atStage Measured

Ebboost = Boosted B+Voltage

ebpp = Peak Positive PlateVoltage

Ec2 = DC Screen Voltage

Ef (RF) = Filament Voltagefor High Voltage Rectifierat 15,750 cps

EH.V. = Picture Tube SecondAnode Voltage Measured atthe Picture Tube SecondAnode

Ecl = DC Grid No. 1 Voltage

% Change = Percent by whichmeasured characteristicvaried from the referencecondition.

Sylvania News

TABLE III

VARIATIONS IN TUBE OPERATING CONDITIONS

BETWEEN RECEIVERS

OPERATING CONDITIONS (O.C.)

Type 12DQ6 Rec. B Type 6CD6GA Rec. CCircuit

Characteristic 1 2 1 2

Measured (Reference) % (Reference)Change Change

Ebboost V DC 540 10.6 580 12.1Ec2 V DC 140 12.8 140 10.0Ecl V DC - 36 5.55 - 37 16.2

Ik Ma DC 105.8 6.1 118 12.7ik Ma Pk 340.0 0 335 19.4Ic2 Ma DC 9.5 37 14.1 13.5Ib Ma DC 96.3 2.8 103.9 12.6Pc2 Watts 1.33 56.5 1.98 24.7ebpp Kv 4.0 5.0 4.5 13.4eel V Pk -125 16.0 -105 13.3Pp Watts 6.18 23.7

O.C. 1 = Eline = 117 V. AC. Picture and scan normal.O.C. 2 = Eline = 130 V. AC. Picture and scan as adjusted at

117 volt line condition.

November - December 1956 7

Electron Tube OperatingConditions In HorizontalDeflection System(Continued from page 6)

presents a different problem andrequires special attention.

VARIATIONS DUE TO OTHER

CIRCUIT COMPONENTS

Variations in circuit componentsalso influence tube operating con-ditions and consequently tube life.For example, a decrease in the valueof the screen grid dropping resistor ofthe horizontal deflection amplifier willtend to increase the screen grid volt-age and dissipation. Depending uponexact conditions, the life of the tubemay be radically affected. In such a

case, replacing the tube withoutreplacing the defective componentonly leads to a call-back.

An increase in the value of thescreen -grid resistor will cause thescreen voltage to be lowered. Withthis lowered screen voltage, the origi-nal and replacement tubes may giveshort scan. Real circuit difficultycannot be corrected by a new tube.

Defective components in the hori-zontal circuit, as well as impropercircuit adjustment, may influence thelife of tubes and components of othercircuits. An increase in screenvoltage of the horizontal deflectionamplifier due to a defective screengrid resistor may increase the boostvoltage, thus increasing the electrodevoltage, current, and dissipations of

other circuits, such as the verticaldeflection system, which derive theirB supply from this source.

In conclusion, there are manyfactors affecting the life of tubes inthe horizontal deflection system, aswell as other associated tubes and/orcircuits. From our investigations wecan see the disastrous result of im-proper circuit adjustment, excessiveline voltage and defective componentson tube life. If normal tube life anda minimum of call-backs is expected,each of these items must be carefullyconsidered by the service technician.

EDITOR'S NOTE: Sylvania offers $10.00 in Advertising Material Certificates for all technical hints that it believes useful to the service -dealer readers of SYLVANIA NEWS. Sylvania is not obligated to return any materiál submitted for publication, whether or not published.

SECTIONAL BENCH

WITH ROTATING TOP

Intermittent sets often remain onthe bench for a long time, thus re-ducing available bench space.

This problem can be avoided bymaking part of the bench into a move-able section supported by furniturecasters.

In addition, the chassis rest rotateson casters, making it easy to revolvethe set to any position without liftingor dragging it about, see photo.

Henry MayoSarasota, Florida

MODIFIED INVERTER

For those radio shops that arerepairing auto radios and don't havean inverter or at least whose inverterdoesn't have an ammeter, the additionof an ammeter in series with thebattery or inverter of the 20 or 50ampere variety can greatly simplifythe locatization of power supplytroubles. The meter need not be veryaccurate, even an old car ammeter willsuffice. Of course, the meter shouldbe fused at or below its full scalevalue.

With a little experience one soonbecomes aware of the current con-sumptions of the various types of carradios. If current is only half of the

normal value chances are the B -plussupply is dead. Either the vibrator isinoperative or the rectifier is defec-tive. (Open power transformers arenot very common.) If the currentconsumption is half again to twice thenormal value, the B -plus supply maybe shorted. If the ammeter showsthis abnormally high current immedi-ately upon switching on the receiver,chances are the buffer condenser isshorted. (Shorted rectifiers andshorted transformers are not toocommon.) If, however, the ammeterreaches this abnormally high valueafter the rectifier has warmed up, theB -plus is probably shorted after therectifier.

If the ammeter indicates a near -

dead short, or at least several timesnormal value, chances are the vibra-tor contacts are sticking, indicating adefective vibrator.

Willis W. WipfFreeman, South Dakota

8 November - December 1956 Sylvania News

noise in - americanradiohistory.com...figure 2 complete input circuit showing "shot noise"...

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