Section V Model 4 12A
b. Adjust rear panel OHMS ADS (R36) for meter Table 5-5. Tube Iteplacement
indication of aD (corresponding to 1.0 on upper
scale). Tube Iteplaced Adjustment
525. TROUBLESHOOTING. VI01 (12AX7) Amplifier Huni Bal. Par. 5-20
Gain Cal. Par. 5-23
5-26. When the Model 412A/AR operates improperly, first
adjust and calibrate it according to the procedures outlined V102 (6AU8) Amplifier/ Bias Adj. Par. 5-21
in Paragraph 5-15. If calibration is not possible, procecd Cathode Follower Cain Cal. Par. 5-23
with the troubleshooting steps.
V103 (6x4) Rectifier None
V104 (OA2) Regulator Ohms Adj. Par. 5-24
CONTAMINATION OR
FINGERPRINTS ON THE RANGE VI05 (OB2) Regulator -71
SWITCH OR THE COMPONENTS
MOUNTED ON THE RANGE SWITCH 5-33. MODULATOR CHECK.
MAY REDUCE ACCURACY OF
INSTRUMENT. a. An oscilloscope and a 10 Maresistor are required
for this test.
5-27. FRONT PANEL INDICATIONS.
b. Disconnect 412A from power source and remove
5-28. If the malfunction is common to all ranges and cabinet.
functions, the trouble may be in the amplifier or power
c. Remove VlOl and reconnect 412A to power
supply areas. If the instrument operates improperly on only source.
one range or function, the trouble is probably in the
switching section or input cables.
d. Connect a clip lead from the center arm of R116
(rear panel BIAS ADJ) to the termination of the
5-29. POWER SUPPLY AND AMPLIFIER CHECKS.
VOLTS input cable (terminal 4 in Figure 5-7).
5-30. Check power supply voltages at points listed in Table e. Set 412A FUNCTION to VOLTS, RANGE to
5-4. Voltages are measured with respect to circuit common
.001.
(A). Figure 7-2 shows the location of components.
f. Set oscilloscope input to DC.
g. Connect a 10 Mfi resistor in series with
Check Point Voltage oscilloscope vertical input. Using a clip lead,
connect resistor to junction of RlOl and ClOl
(input circuit board, Figure 54). Connect
oscilloscope common to 412A COM lead. Set
Vertical Sensitivity to 0.2 V/cm.
53 1. Tubes are a coinmon source of trouble and are most
easily checked by substitution. Following replacement of a
particular tube, iiiake the adjustrneiits indicated in Table
5-5.
5-32. Additional checks of amplifier operation niay be
made by cliecking voltages shown on the amplifier section
of tlic sclrwutic diagram. Be sure to observe conditions in
I
Note I1 in Paragraph 7-2. Allow 3 tolerance of 210%. Figure 54. Input Circuit Board
5-6 00 1 29-5 |
ACEC
