HP 202A Operating And Service Manual
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Sect.
IV
Page 1
SECTION
IV
MAINTENANCE
4-1
GENERAL
4-2
POWER SUPPLY
All dc voltages are measured with respect to B- and
not with respect to the chassis. The B- points in
the instrument are connected with
black
hook-up
wire.
Most of the following analyzing and adjustment pro-
After power
supply parts replacements or adjust-
cedures require the measurement of dc voltages or
ments, a final check of regulated voltages should
the observation of waveforms. To obtain accurate
be
made. See Power Supply Regulator Adjustments
results, use a voltmeter with an input resistance of
in paragraph 4-9.
CAUTION
100 megohms or more. The @Model 410B Vacuum
Tube Voltmeter
is
recommended.
Isolate all test equipment from the main chassis or
gromd Otherwise, both
B-
and one side
of
the output
may be connected to the main chassis through the
test equipment.
I€
this happens, one cathode resistor
in output stage V20 will be shorted and the tube will
be
severely damaged.
--------------
TABLE 4-1
Whenever pssible the instrument frequency should
be set to approximately 50
cycles/sec. to permit
the use of a capacitor in series with the ac voltmeter
or oscilloscope to eliminate the dc component.
Interaction between most of the circuits of the Model
202A makes a fairly definite procedure for trouble
shooting necessary. For example,
a
fault in the
oscillator section may easily cause considerable
voltage deviations in the output system. Therefore,
it
is
more desirable to divide the instrument into
five sections as follows and consider each in turn.
4-2 Power Supply
4-3 Function Generator
4-4
Sine Synthesizer and Function Selector
4- 5 Output Amplifier
4-6 Sync Out
SYMPTOM
Instrkent inoperative
(Indicator lamp won't
light, no output volt-
age).
Instrument inoperative
(Indicator lamp lights,
no output voltage).
Instrument inoperative
(normalvoltage
atV21).
(Extremely low or no
voltage between Y5,
pin 5 and B-).
Instrument inoperative
(normal
+
37W reg-
ulated) (+225V reg-
ulated, off voltage).
(+
7W regulated, off
voltage)
Instrument inoperative
(No
+
225 regulated
+
75 regulated voltages,
V5
and/or V6 not
ionized).
1
CAUSE AND/OR
REMEDY
Blown fuse,
F1.
Measure resistance
from V21 socket
(pins 2 or
8)
to B-.
55,000 ohms or
more replace
V21.
If
less than 55,000
ohms clear short
circuit in filter or
regulator circuits
then replace V21.
Defective
6AU5
tubes (V22, V23).
Capacitor C6 short
circuited.
Defective OA2 tube
(V5).
Defective OA3
tube
(V6).
Open circuit in R62,
R84, R85,
R91, or
R92.