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Section
V
b
I. Connect other test ohmmeter lead to terminal
with whitelorange wire.
m.Turn412AON.
n. Resistance indicated on test ohmmeter should be I
b
t o 2 megohms.
0.
Turn
41
2A OFF and replace lamp and V102.
Iy
5-37. DEMODULATOR REPLACEMENT.
5-38.
The photoconductors in the Demodulator are not
separately replaceable. If a cell is defective, the
Demodulator assembly should
be
replaced, using the
following procedure:
a. Disconnect instrument from power source and
remove cabinet.
b. Disconnect the three wires
to
Demodulator
assembly. Figure 5-6 shows wire color codes for
both cabinet (4 12A) and rack (4 12AR) models.
c. Remove four screws holding light-beam chopper.
It is not necessary t o disconnect wires.
d. Remove
n u t s from four screws holding
. I
Demodulator assembly and remove assembly from
instrument.
5-39. SWITCH CHECKS.
CLEAN RUBBER GLOVES SHOULD
BE WORN WHEN WORKING ON
THE
RANGE SWITCH. CONTAMINATION
6
5-8
412A
(CABINET MODEL)
412A-8-50160
Model 4 12A
OR FINGERPRINTS MAY KEDUCE
ACCURACY OF INSTRUMENT. IN
ADDITION, LOW ROSIN CONTENT
SOLDER SHOULD BE USED WHEN
REPLACING COMPONENTS. USE A
M I N I M U M
O F
H E A T
W H E N
SOLDERING, AND REMOVE TRACES
OF FLUX FROM CONNECTIONS. IF
NECESSARY, RANGE SWITCH MAY
BE CLEANED USING A SOLUTION OF
MILD DETERGENT AND WARM
WATER. RINSE THOROUGHLY IN
DISTILLED WATER
AND
DRY
IMMEDIATELY.
5 4 0 . I f the malfunction is peculiar t o one range
or
function, trouble may be in the switching circuits. Either a
component or switch contact may be defective. Figures
5 8
through
5-10
show simplified schematics of range and
function switching, and may be used t o isolate the trouble.
For example, if the instrument operates properly on all
voltage ranges except .003; R1, R9, or the associated switch
contacts may be defective. Figure 7-3 identifies the
components mountcd on the rmgc switch.
5-41. ADJUSTMENT
O F
FACTORY SELECTED
COMPONENTS.
542. R34*.
5-43.
The value of R34* is selected t o bring the OHMS ADJ
potentiometer within the proper range, and is dependent
upon the power supply voltage and the value of other
resistors in the ohmmeter circuit. Replacement of R34*
should not be necessary unless V104 has been replaced or
the range switch components have been changed. If the
V I
412AR
(RACK MODEL)
Figure 5 6 Demodulator Assembly
TO
Cl13
TO
C114
AND
R l l 7
TO
RI 1 6
00 1 29-5
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