Table of Contents
Advertisement
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 Oattened 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
24
detector (A2 Pin 6 and A2 Pin 7), these should be
o
+j-.OSV dc.
@
Troubleshoot the Meter Differential Amplifier
(A3Q2 through A3Q5), the Meter (M I) and the
Meter Offset Current Reference.
®
Check the detector diodes CA2CR2 I, A2CR22)
and capacitors A2C40*, A2C41 and A2C44.
7ChecktheDetectorAmplifierA2Q24.A2Q25and
@
associated components.
5-78. TROUBLESHOOTING THE POWER SUPPLY.
NOTE
To protect the output capacitors C9 and
CIO (rated 3 V). pull connectors olT of
pins 15 and 16
0
f the A2 board. Ensure
that the dc voltages on pins IS and 16 are
near zero « 500 mY) before
recoll-
necting the wires to C9 and C I O. Avail-
able test points for for power supply
vol tage:
+ 31 V at AI pin 12, A2 pin I, A3 pin 5;
- 26 V at Al pin 13, A2 pin 2, A3 pin 4
WARNING
I
TUR.!'\) OFF POWER BEFORE CON-
N ECTING
OR
DISCONN ECTING
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 vol tages, 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 morti to ring the - 26 V. If - 26 V loads down,
5-22
turn off the power and lift one end of A2R20 and
A2R25. If the - 26 V supply is satisfactory, trou-
bleshoot the Oscillator. If the negative voltage
·
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
A2DSVI.
c. Connect the signal generator with a large (at least
10 microfarad) non-polar capaci tor in series to
A2R38. The capacitor blocks any dc present on
the signal generator output.
- - - - - - N O T E - - - - - -
If a large non-polar capacitor is not
available, use two polarized capacitors in
series. with their
+ ends connected
toget her.
d. Set the signal generator frequency to I kHz and the
output level to about .25V rms (monitor with an
ac voltmeter).
e. Signal trace the ALe loop. Signal levels, anc
l
voltages should correspond with those shown
0
the schematics.
5-81. TROUBLESHOOTING
THE
OSCILLATOR
CI RCUIT.
- - - - - - N O T E - - - - - -
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
CD
a. If the signal at thejunction of A2R22 and A2R23
is twice the normal amplitude then the Peak
Detector is not operating. In particular check
A2Q7 and A2CII.
b. If A2C 12 is close to the A2Q I FET the oscillator
coul d break in to spurious oscilla tions above
100 kHz.
c. If there is no signal at A2 Pin 4 isolate the Peak
Detector by lifting one side of A2CII 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.
Advertisement
Table of Contents
