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Section V
Model 654A
b. AC waveshape should have the same wave shape as
shown on Schematic No. 2. If you have a flatness
problem, check the wave shape for symmetry and
see if output capacitor C9 or CIO are leaking.
5-90. The following checks will determine if the trouble is
in the Control Integrator or in the Meter Differential
Amplifier.
NOTE
Disconnecting the outputs of A2CR21
and CR22 will cause the voltage readings
to be incorrect.
a. If A2CR21 is not zero ± 20 mV, the trouble is in
the Meter Differential Amplifier.
b. If A2CR22 is not zero ± 20 mV, the trouble is in
the Control Integrator.
5-91. TROUBLESHOOTING
THE AMPLITUDE
CON-
TROL INTEGRATOR.
5-92.
The Amplitude Control Integrator consists of A3Q6
thru A3Q9. The output is + 5 V to + 7 V dc. The output
feeds back into the Photocell Module of the Buffer
Amplifier. Check the resistance of the filament in the lamp.
It should be approximately 40 SI The photocell should
have approximately 1 kn to 2 kft resistance when a + 5 V
to + 7 V dc is applied to the filament.
5-93. TROUBLESHOOTING THE ALC LOOP.
5-94. lf the trouble is known to be in the ALC loop
(Buffer and Balanced Amplifier and the Automatic Leveling
Circuits) and cannot be isolated to a unit by the preceding
paragraphs, then this method can be used to break open the
loop. The Buffer Amplifier is then driven by an external
signal generator (0.25 V rms at 1 kHz into 2.5 ka) and it
can be checked as any amplifier.
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 liF) 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 1 kHz and
the output level to about .25 V mis (monitor with
an ac voltmeter).
e. Signal trace the ALC loop. Signal levels, and
voltages should correspond with those shown on
the schematics.
5-95. TROUBLESHOOTING THE METER
DIFFEREN-
TIAL AMPLIFIER.
5-96. The Meter Differential Amplifier is used for the
meter in all functions. Use voltages on the schematic for
locating the trouble. If the base of A3Q2 is not zero, the
trouble could be in the offset current reference circuit.
5-97.
TROUBLESHOOTING THE ATTENUATORS.
5-98. If either side of the attenuator is shorted to ground,
remove the cover and make a physical check. The signal
path is probably touching the metal case or cover. The
contacts of the switches should be kept clean and lubri-
cated (refer to CAUTION on Page 5-13 and Service Note
M45B).
For proper lubricant, use Electrolube 2A or
Electrical Contact Lubricant, - hp-Part No. 6040-0300.
NOTE
In order for the attenuator to make
specifications and ensure proper ground-
ing, all mechanical connections must be
very tight.
5-99. SERVICING ETCHED CIRCUIT BOARDS.
5-100. The
Model 654A contains four plated-through,
double-sided, etched circuit boards. When working on these
boards, observe the following rules to prevent damage to
the circuit board or components:
a. Use a low-heat (25 to 50 watts) soldering iron with
a small tip.
b. To remove a component, clip a heat sink (long
nose pliers, commercial heat sink tweezers, etc.)
on the component lead as close to the component
as possible. Place the soldering iron directly on the
component lead, and pull up on the lead. If a
component is obviously damaged or faulty, clip
the leads close to the component, and remove the
leads from the board.
EXCESSIVE OR PROLONGED HEAT
CAN LIFT THE CIRCUIT FOIL FROM
THE BOARD OR CAUSE DAMAGE TO
COMPONENTS.
c. Clean the component lead holes by heating the
solder in the hole, quickly removing the soldering
iron, and inserting a pointed, non-metallic object
such as a toothpick.
5-24
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