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Many VCR technicians ask how to usetheir Waveform Analyzer to set the head-switching signals in VCRs. Another namefor this adjustment is the PG Shiftercontrol. This Tech Tip explains theadjustment in detail.

We will start by explaining how the head-switching adjustment affects VCR

performance. We will then explain twoways to adjust the circuits using yourWaveform Analyzer. The first method isbased on using the CRT in a conventionalmanner and manually counting syncpulses. The second method uses theDelta Time function to convert themeasurement to a digital reading. TheDelta Time method can also be used forany other VCR adjustment needing a timedelay between two signals, such as thetracking-fix (sometimes called trackingpreset) adjustment and the timing of thehi-fi heads in VHS tape decks.

Why Head Switching NeedsAdjustment

Before we explain how to set the head-switching signal, lets be sure youunderstand what it does. Every VCR usesa pair of video heads when playing a tapeat normal speed. Even decks with 3, 4, or5 video heads use the heads two at atime. A 30 Hz square wave from the servo

circuits controls an electronic switch atthe head amplifier output. The switchselects the amplifier for the head which isin contact with the tape and turns off thechannel for the head which is on theopposite side of the drum. If the secondhead was not turned off, it would addnoise to the playback signal.

Noise appears in the video signal whenthe switching happens. You can see thisnoise by adjusting the vertical holdcontrol to display the sync interval on a

TV connected to the VCR. The switchingnoise is a horizontal tear in the picture afew horizontal lines above the black syncbar.

The head-switching circuits change thetiming of the switching signal compared

to vertical sync. Switching should takeplace a few lines before vertical blanking,to place the noise in the bottom 3 lines ofthe picture. Since most TVs areoverscanned (the vertical deflection isslightly larger than the CRT screen),switching is invisible, because it happenswhile the electron beam is below thescreen. If the circuits switch too early, thenoise moves up into the visible part of thepicture. If the circuits switch too late, thenoise occurs during the sync pulse,

causing poor vertical stability.

Now that you understand how adjustment affects the circuits, should have a better understanding

Setting VCR Head Switching With The Waveform Analyzer

# 1 0 8 8

Fig: 1: The head switcher uses a 30 Hz square wave from the servo circuits to turn o

ampli fiers of the head which is not contacting the tape.

Fig. 2: Noise appears in the picture a

point where the VCR switches from video head to the other. The switch

adjustment keeps this noise close tobot tom of the screen, so that is

annoying.

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why the timing must be correct. Thisunderstanding should also helpunderstand the alignment procedures.Now, lets see how to adjust the pulseming. We will start with the conventional

CRT-based method.

How To Adjust With The CRT

The first thing you need to do is locate theest points and the controls which affecthe head switching. The service literatureor the VCR you are servicing is the bestsource of this information. The serviceterature also tells you how many

adjustments the VCR contains.

Fig. 3: This is a typical manufacturers head-

w i t ch ing ins t ruc t ion . Use the serv i ce

iterature to determine the test points and

adjustments to use for the adjustment.

Depending on the VCR, it may have one,wo, or three adjustments. Most 2-headedVCRs have only one control. VCRs withour (or more) video heads may have twoplayback adjustments. If so, you will needa test tape recorded at the fastest tapespeed (SP or Beta l) to adjust one control

and a tape recorded at the slowest speedEP or Beta 111) to adjust the other.Some early VCRs also have a thirdadjustment in the recording circuits.

The instructions will usually tell you toadjust the control until the switchingsquare wave is 6.5 horizontal lines aheadof vertical sync. If you are counting pulseso make this adjustment, remember thatyou must count every other pulse throughhe blanking interval if your test tape has

interlaced sync. This happens because thevertical blanking pulses contain equalizingpulses at twice the rate of the horizontalsync pulses. If your tape has non-interlaced sync, it may not containequalizing pulses, so you must countevery pulse. You can avoid the question ofwhether to skip pulses by rememberingthat the blanking interval is always threehorizontal lines wide. Count 3.5 horizontal

lines from the start of blanking instead of6.5 lines from sync. This lets you use thesame procedure, whether or not yoursignal contains equalizing pulses.

Setup For The Wavefor m Analyzer

Refer to the manufacturers serviceliterature to find the needed test pointsand adjustment locations. Then, use thefollowing procedures to make each head-switching adjustment with the CRT:

. Connect the Channel A probe to theVCR video output and the Channel Bprobe to the test point with the head-switching square wave signal.2. Set the TRIGGER SOURCE switch to theCH B position (to trigger from thesquare wave) and the TRIGGER MODE

switch toAUTO. (The TRIGGERPOLARITY switch lets you select therising or falling transition, depending onwhich you want to use. )

Set the TIMEBASE-FREQ switch to the1 msec position (press the HORIZPOSITION control to confirm that its inthe in position for a non-expandedtrace).

4. Press the A&B (dual trace) selectorbutton (below the CRT) and adjust theinputs and triggering circuits until the twotraces are locked onto the CRT.

Place the VCR into the record orplayback mode, depending on themanufacturers alignment instructions.

left side of the CRT, adjust the horizontalvernier control (the small knob in thecenter of the TIMEBASE-FREQ control)until you see two vertical sync pulses onthe channel A traceone at the left edgeand the second one near the right edge ofthe CRT. (Channel B should show asquare wave transition near the secondsync pulse. )

Fig. 4: Follow these steps to display the dead-switching signal on the Waveform Analyzer.

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7. Adjust the HORIZ POSITION controluntil the right-hand vertical sync pulse(and square wave transition) is in thecenter of the CRT. Pull the HORIZPOSITION control to its out~position toexpand the waveforms by ten times.

Carefully watch the trace as you adjustthe Head Switch (PG Shifter) control inthe VCR. Start by adjusting the timing

until the square wave just touches thevertical sync pulse. Then, move thetransition to the beginning of verticalblanking. Finally, move the transitions 3.5horizontal lines before blanking (which isthe same as 6.5 lines ahead of verticalsync).

Some people prefer to ADD channel A tochannel B by pressing the CHAN A andCHAN B CRT buttons simultaneously. Thismakes it easier to compare the timing ofthe two signals. When added, the squarewave causes a step to appear in the videowaveform. Adjust the head-switchingcontrol until the step is 3.5 horizontallines ahead of the vertical blanking. (SeeFigure 5.)

Fig. 5: Pressing the CHAN A and the

CHAN B buttons at the same tim e adds thetwo signals, causing a jog at the point where

the heads switch.

Using Delta Time ToAdjust Digitall y

You can use the Delta Time function toeliminate the need to count pulses. Youpre-set the Delta Begin and Delta Endcontrols until the digital readout showsthe correct time and then adjust the head-switch control until the sync pulsetouches the highlighted area of thewaveform .

To use the Delta Time function, you needto know how many microseconds to leavebetween the square wave and the syncpulse. Simply multiply the lines specified

by the time for one horizontal line: 63.5microseconds. Your servicing instructionsmay use one of three delays: 6, 6.5, or 7horizontal lines. Figure 6 shows the calcu-lated values for each delay.

the waveforms onto the CRT byfollowing the previous steps 1 through 7.Then, choose the Delta Time function andset the intensified Delta Time bar until its

beginning just touches the square wavetransition to the left of the sync pulse.Next, adjust the Delta Bar to extend to theright of the transition until the digitalreadout shows the correct time. Finally,adjust the head-switch control until thevertical sync pulse just touches the end ofthe intensified Delta Bar.

LINES MICROSECONDS

6 3816.5 4137 444

Fig. 6: The number of mi croseconds thatcorrespond to typical head-swi tchi ng

specifications.

The Delta Bar lets you pre-set the timedifference between signals, because it hasthe same starting point and the samelength in both CRT channels. Since theDelta Bar is referenced to a high accuracycrystal, the digital display gives a very

accurate measurement. You are using theintensified area like a tape measure topre-measure the desired delay beforematching the signals to it.

use the Delta Time function to set

head switching:

Follow steps 1 through 7 from aboveto display the two signals on the CRT.

Press the Delta Time DIGITALREADOUT button and adjust theINTENSITY control until you can see theintensified area. NOTE: You may wish toreturn the sweep to the non-expandedmode (push the HORIZ POSITION controlto its in position) until you have locatedthe beginning and end of the Delta Bar.Expand the trace after the Delta Bar isnear the sync interval.

Adjust the DELTA BEGIN control untilthe beginning of the Delta Bar justtouches the square wave transition in

channel B.

4. Adjust the DELTA END control untdigital readout shows the correct amof time (for example, 413 uS for a 6.5delay). Dont be too fussy in this setsince the circuits only need to be adjuwithin 30 microseconds of the iamount .

5. Adjust the VCR head-switch Shifter) control until the beginning o

vertical sync pulse just touches the ethe intensified Delta Bar.

Fig. 7: Use the Delta Time function to

set the time needed between signals,

then adjust the circuits unti l the sigtouch the intensified area of the waveform.

Setting Other VCR Controls

You can use this same procedure time you need to set a time delay betwthe signals at two test points. Sim

Fig. 8: Tracking f ix adjustments usrequire that you adjust a signal for the c

delay from the head-switching square wave

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mark the beginning, adjust the length forhe correct delay, and then adjust thecircuit until the second signal touches thentensified zone. Two common VCRadjustments needing a fixed delay are theracking-fix and the hi-fi head switchingme delays.

The instructions for setting the tracking-

x adjustment are shown in Figure 8.This example calls for a delay of 360 to440 microseconds between the head-switching square wave and a servo pulse.Figure 9 shows how the display will lookwhen making this adjustment.

Figure 10 shows the procedure for settinghe switching signal for the hi-fi heads.Notice that the instructions give twosteps: 1) Setting for a time delay, and 2)Checking for a no dropout in the FM hi-fi

signal. Use the Waveform Analyzer to setthe 5.5 millisecond delay. Then, move oneof the probes to the FM test point toconfirm there is no dropout. The DeltaTime function lets you confirm the timingis correct, without counting ormultiplying. The solid sync simplifies thetest when confirming minimum dropout.

Form 4966 Printed In U.S.A.

# 1 0 8#10

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Fig. 10: This service literature shows that the timing of the hi-fi heads needs two steps: 1) Adjusting for a 5.5 mS delay, and

2) Confirmi ng there is no dropout in the FM signal.

Fig. 9: Adjust the Delta Bar until it bridgesbetween the two t racking f i x s ignals to

determine the delay between them.

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