1V1VUi;;1 UJ L,f\
5-55. The recommended signal generator is a 652A or
651 B (which has 50 ohms output impedance and a
frequency range of 1 0 Hz to 10 MHz). If this is not
available, any oscillator with low output impedance and
capable of driving up to 3 V rms into approximately 200
ohms will suffice. In most cases it will not be essential to
use the generator at frequencies over 1 kHz.
NOTE
Owing to the physical makeup of the FRE
QUENCY RANGE switch (SI), it is not
practical to isolate the components connected
between A 2 Pin 10 and Section SIC (F) of the
range switch; depending on the position of the
switch, there will always be an impedance in
parallel with the signal generator. On the X 1M
range this could be as low as 225 ohms.
a. Turn off the power to the 652A.
b. Disconnect the Power Amplifier by disconnecting A2
Pin 5.
c. Disconnect the Peak Detector by lifting one side of
A2Cl I and the emitter lead of A2Q7.
d. Disconnect the tuner capacitor (C 1 ) by unscrewing
the green wire connected to the underneath of C I .
e . Disable the RC tuning network o f the Wien Bridge by
disconnecting A2 Pin 16.
f.
Connect the signal generator with a 1 0 microfarad
non polar capacitor in series to the green wire disconnected
from C l , and to the 652A chassis ground. The capacitor
blocks any dc present on the signal generator.
NOTE
If a large non polar capacitJr is not available,
connect two polarized capacitors in series with
their + ends connected together.
g. Turn the 652A power on. Set the signal generator
frequency to I kHz and the output to approxllilately 2.3 V
rms (monitor with an ac voltmeter on the green lead).
Monitor the output signal with an oscilloscope on A2 Pin 5.
I . The amplifier has a voltage gain of approxllilately
2, therefore, the output should be a sine wave of
between 10 and 1 2 volts peak-to-peak.
2. If the signal does not appear at A2 Pin 5, then
troubleshoot the amplifier (A2QI through A2Q6
and associated circuitry) using normal trouble�
shooting techniques for an amplifier. A2Q5 and
A2Q6 should be replaced as a p air if either has to
be replaced.
NOTE
In some instruments A 2Q2and A2Q3 will be
different types from those listed in the Parts
List (Table 7-1) of this manual. In this case, if
either A2Q2 or A2Q3 has to be replaced, then
replace both together with the new part
number given in this manual. Also, the value of
SICI3', SICI5*, A 2C21 *, A2R16* and
A2R24* may have to be changed in accor
dance with the procedure outlined in the
Frequency Calibration Procedure of Paragraph
5-27 and following.
3. When the amplifier is operating correctly, check
the frequency response, which should remain
essentially flat between l O Hz and
lO
MHz, by
sweeping the signal generator over that range
while maintaining a constant input voltage on the
green lead.
4. Reconnect A2C l l and A2Q7 to ensure that the
Peak Detector does not disable the amplifier (if it
does, troubleshoot the Peak Detector). If the
signal is still present at A2 Pin 5, then the
probable cause of trouble in the oscillator circuit
is the FREQUENCY RANGE switch or associated
componen ts.
5-56.
Troubleshooting the Power Amplifier.
a. Disconnect the lead (whiteiorangejgreen) from A2
Pin 4 and connect the signal generator (recommended in
Paragraph 5-55) to A2 Pin 4 (the dc blocking capacitor is
not necessary).
b. Observe the output, if any, at A2 Pin 20, the
amplifier should have a voltage gain of approximately 1 .3.
Check the frequency response over the frequency range of
1 0 Hz to 1 0 MHz. This should remain essentially c onstant.
c. If there is not output at A2 Pin 20, then troubleshoot
the amplifier (A2Q8 through A2Ql2) using normal trouble
shooting techniques for an amplifier. If A2C l 9 is shorted,
then both A2Q I l and A2Q I 2 will be destroyed.
5-57.
Troubleshooting the Attenuator.
a. If the output is radically out of specification in one
or more attenuator positions, then check the attenuator
resistors.
b. If only slightly out of specification in the lower
positions, " 40 dBm and below, then suspect the attenua
tor grounding. Tighten all shield screws, output jacks and
the main nut on the attenuator assembly. Make sure that
the internal metal shields are visible on the outside metal
cover of the attenuator. Tap firmly on the attenuator shield
with a screwdriver handle to improve grounding.
5·19