MUHIMETER MAINTENANCFAND CALIBRATION

Ohmmeter functionohmmeter operates on two sets of batteries. Two pen light uM-3 (1.5 vi

batteries are connected in serie-s-to give a 3 votts supply to lower rJng.r if , Xf 0,andX'f {. The highest range X10K is operated by a s;ngie

'g volts uatt"rf (006p}. A dead

g.t-l in any of"the 1.5 V batteries will render the l6wer ranges inoperative. Similarlyif 9 V battery is dead range Xl0K will not work.Some amount of current is drained every time the ohmineter is used. After

several tests have been done the battery gradually loses iis power. Weak nrtirri*result in inaccurate performance of the olimmeter. Such eondition,is indicated bythe inability of zero ohm adiust control to set the,pointer to zero on the ohmmeterscale. ln this case there is a need to replace the old batteries with nr* on*.

Misapplicaticin of an ohmmeter to measure AC of 100 volts otro* results inthe burning of an internal resistor and it renders the ohmil;ft;"1v "*rr**tr.' lf the ohmmeter cannot measure accurately at low ranges this is an indicationof defective shunt resistors. A quick way to deteimine a damaged shunt resistor is'tohold the tips o-f the prods with your hands separately, set the selector.to R X 1 andthen to R x 10 and to R X 1 K. lf there is no defleciion in the first two ranges andvery little deflection in the third range the shunt resistorc "rr

g""i. w6ui.ru mid-scale.deflection at low ranges indicates a defective shunt resisior. tts reptacementwill restore normal operation.

igure C-17, Hands holding test prods.

Another possible troubte that can occur is a dead AC voltmeter while DC volt-meter is good. The voltmeter does not operate on any of the AC ranges. The prinresuspects in this case are the two diodes.

25

(t\dodel YX-360 TR)

DCV {ZAKohms/v) DC mA ACl/ (8 Kohrns/v)

250 50 10 2.5 0.5

t0a0

R4

output

R8 R7 R6 R56)

b(o

+

T(s

q

3:,(e.5 2s a25Al xt xto xtk x,ok w 59 e!0 qa}

FI-H#"l"l llUlR{ilJ ln

b\J

:D,

b

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7I

IH

Figure C-18. Schematic diagram'

Sometimes the AC voltmeter works but DC voltmeter does not functiorr. The

*urp*.t could be shunt resistor R3-tSKohms) for .5 V DC selector tsee schennatic

diagram). Also chebk the contact poinJs of the selector switch for proper contact.

It tte first two checks do not reveal the causeof thetroubie,acomponenttocom-ponent check is necessary. Pay speciai attenticn to the resistors whlch are in series

io the shunt resistors.:- The multitester is a precisioll instnrment and it should be handled with care.

Its accidental drop can dislocate the rnoving coil and the glass window of the scale

may be shattered into pieces. A dislocateO coil p!'events the narrnal movennent of the

Bointer resr-llting in inaecurate measurernentsAfter ro*r years of use the indicateir pcinter^rnay not be resting on zero on the

left side of the scaie. This error can be corrected by indicator zero adjustment.

Some mgltirneters are protected by fuse. When the ohrnmeter is misa plied or

mistakenly used as a voltmeter the fuse blows. All nneter functions becorne inopera-

tive. The darnaged fuse should be replaced by another one with an exact rating so

that its prctection properties would be retained. - .

The rnost. eommon cause of a rnultimetgr failure'to operate in all functions is

,n up*h in proCs wires. A quick pheck can be done by connecting the negative and

posiiive terminals of the meter by a piece of wir:e. After that turn the selector torny rroge of ihe ohmmeter. lf the pointer deflects to zero ohm it means an open,-oJ

deiective-test prod. Replacement of the prods will restore normal operatiqr+-of'tlremultimeter. A ciose inspection of the test prods and wires ca.n lead Vou fo the open

oi nrnnen wire connection. Resoldering it can save you from tiuVil$ a new one.

26

' a':-' ::5.:5-

,.,.-' Parts list

ls::-[n** o,

n. J.

Ri.YXR 02 fiesistor {240 ohms}, series R2-YXR 03 R3YXR 04 Resistor {40k ohrns), Z.bV DC multiplier R4

.YXR 05 Hrrirto, (150k 1,"r. R5

"YXR 06 Fesistor {800k ohms), 50v DC multiplier R6.YXR 07 I%l

.YXR 08 I

R-Iultipiier R8

YXR Og Fiebistor, (3k ohms), series R9.YXR 1O Resistor (92 ohms), Z.5mA DC shunt R 10.YXR 11 Resistor (9 ohnrs), 2bmA DC shunt R i1-YXR 12 Resistor (1 ohms), 0.2SA De shunt R 12YXR 13 Reslstor_g3k otry!, 10v Ac muttiptier R 13YXR 14 Resislor (320k ohrns), EOV AC multiplier R 14YXR 15 Resistgr {1.9M ohms}, 2b0V 4G rnqttiplier R 15YXR 16 Resistor (6M ohms), 1000V AC multintier R 16YXR 17 Resistor (2k - 4k ohms), ACV sensitivity calibration R17YXR 18 Resislor (32k ohms}, shunt R 18YXR 19 Resistor (2k ohmij, diode series

Resistor (44k ohms), ohm series

R 19.YXR 20 R20YXR 21 Besistor i19 ohms), ohms Xl shunt R21YXR 22 un, R22YXR 23 Reslsrgl 113.3k ohms), ohms Xl k shunt R23YXR 24 Resistor (194k ohms). ohrns XlOk.series R24YXR 25 Resistor (101< ohms),0 ohms adjuster R25YXR 26 Resistor {16k ohms}, shunt R26fiF 01

c049Rectifier D1, D2

c1c050 Capacitot' c2voql8001

Varister VDry cell UlVl 3'(1,5V),2 required

- B18005 Dry cell 006P (9V) B2

Meter movement @4 Al2kl MP016

PA17 Panel dial (YX - 3G0Tt) |Ix01 0

IIlI

1I

Rear case (YX - 3O0TR)

1.*4.+-z_

i

i

L

fr

tI

27

-\

Part No" Description R.S. -'

MBl O Meter movement base

T001 Terminal jack (2@1,3 r:equired

SWl5 Ranqe seleetor switch

K015 Range selectol' knob

K007 Oohms'adjuster knob

Narne ptate {YX - 360T11)

L002 Test leads pair (201

v005 Rear case bolt (40)

R.S. = Reference SYmbol

*