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Model, 654A
Section V
d. if the oscillator still cannot be made to operate
perform
the
procedure
of
the
following
paragraphs.
5-82. In the following procedure the oscillator circuit is
driven by an external signal generator and the Wien Bridge
is disabled. This allows the circuit to be checked out using
the normal troubleshooting techniques for an amplifier.
5-83. The recommended signal generator is a 652A or
651B (which has 50 ohms output impedance and a
frequency range of 10Hz to 10MHz). If this is not available
any oscillator with low output impedance and capable of
driving up to 3V rms open circuit will suffice. In most cases
it will not be essential to use the generator at frequencies
over IkHz.
a. Turn off the power to the 654A.
b. Disconnect
the Buffer Amplifier and Counter
Emitter-Follower by lifting the negative side of
A2C12.
c. Disconnect the Peak Detector by lifting one side
of A2C11 and the emitter lead of A2Q7.
d. Disable the RC tuning network of the Wien Bridge
by disconnecting A2 Pins 8, 10 and 11.
e. Connect the signal generator through a large
capacitor (as described in Paragraph 5-80c and the
note) to A2 Pin 10. Connect the ground side of
the generator to A2 Pin 8.
f. Turn the 654A power on. Set the signal generator
frequency
to
lkHz
and
the
output
to
approximately
1V rms (monitor with an ac
voltmeter on A2 Pin 10). Monitor the output
signal with an oscilloscope on A2C12.
1) The
amplifier
has
a voltage
gain
of
approximately 2, therefore the output should
be
a sine
wave
of between
5 and
6V
peak-to-peak.
2) If the signal does not appear at the output then
troubleshoot
the
amplifier (A2Q1
through
A2Q6 and associated circuitry) using normal
troubleshooting techniques for an amplifier.
A2Q5 and A2Q6 should be replaced as a pair if
either has to be replaced.
3) When the amplifier is operating correctly check
the frequency response, which should remain
essentially flat between 10Hz and 10MHz, by
sweeping the signal generator over that range
while maintaining a constant input voltage on
the green lead.
4) Reconnect A2C11 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 components.
5-84. TROUBLESHOOTING THE BUFFER AMPLIFIER.
NOTE
AC output will be erroneous if the
control voltage to DSV1 is the wrong
value. This should be approximately 5 to
7 V dc.
5-85. The ac gain of the Buffer Amplifier should be 0.6 or
less than 1. The dc voltage at A2R44 should be 0 V
± 150 mV and the ac voltage should be between 3 and 4 V
PT-
5- 86. TROUBLESHOOTING THE BALANCED
AMPLI-
FIER.
5-87. The balanced amplifier should have an ac gain of
approximately two. The dc voltage at both sides of A2C32
should be near zero (± 300 mV) with one side positive and
the other negative. The ac waveforms at each side of A2C32
should be equal in amplitude but 180
°
out of phase. The
amplitude should be 6 to 7 V p-p.
a. If the ac voltages are near equal and the dc
voltages are of opposite polarity but not near zero
and the amplitude control has little effect, then
the trouble is probably in the feedback loop
A2Q17, Q22 or Q23. This circuit holds A2C32
near zero.
b. If the balanced amplifier output is clipped or
distorted,
replace A2Q17 with approximately
500 el resistor between emitter and collector,
using pc board holes. If the balanced amplifier's
output becomes a good sine wave, this indicates
the Q17, 022, Q23 loop is faulty. If not, the
balanced amplifier differential pairs (Q11 thru
021) are at fault.
c. If A2C32 ac voltages are not approximately equal,
the fault is probably in the differential pairs Q 11
thru Q21.
d. If the instrument will not pass its balance specifi-
cations check to see that A2R74 and R75 are the
same value.
5-88. TROUBLE ISOLATION IN THE REMAINDER OF
THE LEVELING LOOP.
5-89. The following check will determine if the trouble is
in the Average Detector.
a. The input of A2C40 should be 4 to 5.5 V dc and
the ac signal should be 0.8 V p-p to 1.0 V p-p.
5-23
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