Troubleshooting; Front Panel Indications; Power Supply And Amplifier Checks; Modulator Clieck - HP 412A Operating And Service Manual

Section V
Check Point
b. Adjust rear panel OHMS ADS (R36) for meter
indication of aD (corresponding to
1.0
on upper
scale).
Voltage
525. TROUBLESHOOTING.
5-26. When the Model 412A/AR operates improperly, first
adjust and calibrate it according to the procedures outlined
in Paragraph 5-15. If calibration
is
not possible, procecd
with the troubleshooting steps.
C O N T A M I N A T I O N O R
FINGERPRINTS ON THE RANGE
SWITCH O R THE COMPONENTS
MOUNTED ON THE RANGE SWITCH
MAY REDUCE ACCURACY OF
INSTRUMENT.
5-27. FRONT PANEL INDICATIONS.
5-28.
If the malfunction is common to all ranges and
functions, the trouble may be in the amplifier or power
supply areas. If the instrument operates improperly on only
one range or function, the trouble
is
probably in the
switching section or input cables.
5-29. POWER SUPPLY AND AMPLIFIER CHECKS.
5-30. Check power supply voltages at points listed in Table
5-4.
Voltages are measured with respect
to
circuit common
(A).
Figure 7-2 shows the location of components.
5 3 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
Note I 1 in Paragraph 7-2. Allow
3
tolerance of 210%.
5-6
Model
4
12A
Table 5-5. Tube Iteplacement
Tube Iteplaced
VI01 (12AX7) Amplifier
V102 (6AU8) Amplifier/
Cathode Follower
V103 (6x4) Rectifier
V104 (OA2) Regulator
VI05 (OB2) Regulator
5-33. MODULATOR CHECK.
Adjustment
Huni
Bal.
Par. 5-20
Gain Cal. Par. 5-23
Bias Adj. Par. 5-21
Cain Cal. Par. 5-23
None
Ohms Adj. Par. 5-24
- 7 1
a. An oscilloscope and a
10
Maresistor are required
for this test.
b. Disconnect 412A from power source and remove
cabinet.
c. Remove VlOl and reconnect 412A to power
source.
d. Connect a clip lead from the center arm of R116
(rear panel BIAS ADJ) t o the termination of the
VOLTS input cable (terminal 4 in Figure 5-7).
e. Set 412A FUNCTION t o VOLTS, RANGE to
.001.
f. Set oscilloscope input to
DC.
g. Connect a 10
M f i resistor in series with
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.
I
Figure 5 4 .
Input
Circuit Board
00 1 29-5