.
.
,
.
. -.
.
,
.
.
._...
.
..
-
-
..
.,
.
. --*w
”-
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
to 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 to disconnect wires.
d.
Remove nuts 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
MINIMUM OF HEAT WHEN
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.
540.If
the malfunction is peculiar to one range
or
function, trouble may be in the switching circuits. Either a
component
or
switch contact may be defective. Figures
58
through
5-10
show simplified schematics of range and
function switching, and may be used to 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
OF
FACTORY
SELECTED
COMPONENTS.
542. R34*.
5-43.
The value of
R34*
is selected to 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
VI
412AR
(RACK
MODEL)
Figure
56
Demodulator Assembly
TO
Cl13
TO
C114
AND
Rll7
TO
RI
16
00
1
29-5
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