Sect. IV Page 4 Model 412A .
4) Adjust Gain Cal. (R119), for zero on test volt- the trouble to a few components. For example,
meter. For location of R119 see Figure 4-10. if the instrument operates properly on all voltage
A 10-mv reading on test voltmeter indicates 1% ranges but .003, only R1, R9, or associated switch
error in gain calibration. contacts and wiring can be faulty. Fora simplified
breakdown of the switching sequence, see Figures
5) Adjust Meter Cal. (R46), to set meter pointer 4-5, 4-6 and 4-7. Figures 4-11 and 4-12 identify
on 1.0. parts mounted on the RANGE, FUNCTION and
POLARITY switches.
6) Disconnect test voltmeter.
NOTE
7) Check 412A reading versus Voltmeter Calibra- BE CAREFUL when working with the RANGE switch.
tion Generator output on each range. See note in Avoid touching the switch :wafers. A dirty switch
step 5 of paragraph 4-3A. If any reading is in
error more than 1% of full scale, readjust Gain can degrade instrument performance. When solder-
Cal. (R119), to bring all readings within 1%. ing to the switch, use a minimum of heat. Exces-
sive heat will melt the wafer material.
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E. OHMMETER ADJUSTMENT Should trouble occur in the amplifier/power supply
1) Set FUNCTION selector to OHMS and RANGE section, it will be common to all ranges and all
switch to X1K. Re sure MA/OHMS and COM leads functions. Voltages are indicated at various points
on the schematic diagram; these are typical volt-
are not connected through some external resistance. ages and may vary somewhat from instrument to
instrument.
2) Adjust OHMS ADJ. (R36), to set meter pointer
on 00, which corresponds to 1.0 on upper voltage
scale. R36 is located on instrument rear. The 412A is a sensitive instrument. If it has an
offset from zero or gives unexpected readings on
its lowest voltage ranges, it may be measuring
thermoelectric voltages, etc., in addition to the ex-
4-6 TUBE REPLACEMENT pected voltage (see paragraph 2-1). When indoubt,
Tubes will sometimes cause trouble; check them check instrument performance (paragraph 4-3) be-
by substitution and replace only those which are fore trouble shooting.
defective. Following the replacement of a particu-
lar tube, make the adjustment indicated: If the instrument fails to operate at all, be sure
power is supplied to it, the POWER switch is on, and
fuse FlOl is good before checking inside.
Tube Replaced Adjustment
VlOl (12AX7) Amplifier Hum balance; check
gain calibration 4-8 MODULATOR CHECK
V102 (6AU8) Amplifier/ Cathode follower bias; 1) Unplug instrument from power source and re-
move cabinet.
Cathode Follower check gain calibration
2) Remove shield from left side of chassis to ex-
V103 (6x4) Rectifier None
pose terminals of C103 (see Figure 4-10).
V104 (OA2) Regulator Ohmmeter
3) Connect 1.5 volt battery acrossC103. Anyother
source of dc voltage between 1 and 5 volts will do
V105 (OB2) Regulator None as well.
4) Remove V101.
4-7 TROUBLE SHOOTING
5) Connect one lead of 22M resistor to signalcon-
When isolating trouble, consider the 412A as having nector of oscilloscope. With jumper wire, connect
two sections: switching and amplifier/power supply other resistor lead to pin 2 of VlOl socket (grid
sections. Front-panel indications should indicate connector of VlOlA). Connect common lead of
the section in which a trouble is locatedand, in the oscilloscope to 412A COM lead. The 22M resistor
case of switching-section trouble, should isolate reduces circuit loading by oscilloscope. |
ACEC
