HP 654A - Page 42

Section V Model 654A
Ensure that the problem is not in the cables before
attempting to troubleshoot the attenuators.
Check with an oscilloscope, the ac signal from the
detector amplifier at the collector of A2Q25. This
should be a flattened sine- wave ( see schematic 2)
of about . 8V peak- to- peak. Note that there are
two possible paths for the FAIL mode.
Check the dc voltages at the outputs of the
detector ( A2 Pin 6 and A2 Pin 7), these should be
O +/-. 05V dc.
25 Troubleshoot the Meter Differential Amplifier
(A3Q2 through A3Q5), the Meter ( M1) and the
Meter Offset Current Reference.
6- i• Check the detector diodes ( A2CR21, A2CR22)
•= 1 and capacitors A2C40*, A2C41 and A2C44.
21 Check the Detector Amplifier A2Q24, A2Q25 and
associated components.
5-78. TROUBLESHOOTING THE POWER SUPPLY.
NOTE
To protect the output capacitors C9 and
C10 ( rated 3 V), pull connectors off of
pins 15 and 16 of the A2 board. Ensure
that the dc voltages on pins 15 and 16 are
near zero (< 500 mV) before recon-
necting the wires to C9 and C10. Avail-
able test points for for power supply
voltage:
+ 31 V at Al pin 12, A2 pin 1, A3 pin 5;
- 26 V at Al pin 13, A2 pin 2, A3 pin 4
WARNING
TURN OFF POWER BEFORE CON-
NECTING OR DISCONNECTING
POWER SUPPLY LEADS.
5-79. Disconnect the power supply from the A2 and A3
boards. The pins are given in the preceding note. If the
power supply voltages are still incorrect and cannot be
adjusted, troubleshoot the power supply.
a. After the power supply has proper voltages, turn
off power and reconnect the power supply leads to
A3, one at a time while monitoring the - 26 V. If
the negative voltage loads down, then troubleshoot
that specific board.
b. Last, connect the power supply to the A2 board
while monitoring the - 26 V. If - 26 V loads down,
turn off the power and lift one end of A21120 and
A2R25. If the - 26 V supply is satisfactory, trou-
bleshoot the Oscillator. If the negative voltage is
still loaded, troubleshoot the Buffer Amplifier,
Balance Amplifier and Average Detector.
5-80. PROCEDURE.
a. Lift one side of A2R33 to isolate the Wien Bridge
Oscillator from the circuit.
b. Lift the side of A2R38 which is connected to
A2DSV1.
c. Connect the signal generator with a large ( at least
10 microfarad) non- polar capacitor in series to
A2R38. The capacitor blocks any dc present on
the signal generator output.
NOTE
If a large non- polar capacitor is not
available, use two polarized capacitors in
series, with their + ends connected
together.
d. Set the signal generator frequency to IkHz and the
output level to about . 25V rms ( monitor with an
ac voltmeter).
e. Signal trace the ALC loop. Signal levels, and
voltages should correspond with those shown on
the schematics.
5-81. TROUBLESHOOTING THE OSCILLATOR
CIRCUIT.
NOTE
This procedure assumes that the front
panel checks ( Table 5-6) have been made
and that the Troubleshooting Tree has
been followed ( together with the
information in Paragraph 5-77) to branch
a. If the signal at the junction of A2R22 and A2R23
is twice the normal amplitude then the Peak
Detector is not operating. In particular check
A2Q7 and A2C11.
b. If A2C12 is close to the A2Q1 FET the oscillator
could break into spurious oscillations above
100 kHz.
c. If there is no signal at A2 Pin 4 isolate the Peak
Detector by lifting one side of A2C11 and the
emitter of A2Q7. If the oscillator comes on, check
the Peak Detector components. If there is no
apparent fault in the Peak Detector it is possible
that A2R3 may be out of adjustment. To check
this reconnect the peak detector, and perform the
adjustment outlined in Paragraph 5-50.
5-22