Troubleshooting The Oscillator Circuit - HP 652A Operating And Service Manual

®
The normal signal at A2 Pin 20, when the 652A
output voltage is 3, will be a sine wave of about
1 6 V peak·to·peak.
@
A procedure for checking the Power Amplifier is
in
given Paragraph 5·56.
@
Check S2Rl through Sl Rl2 if there is no output
at either the 50 ohm or 600 ohm connector.
Check S2Rl3 if the output is missing only at the
600 ohm connector. If attenuator is out of
specification, tighten mounting screws before
checking resistors. If intermittent or noisy, check
or
for poor dirty switch contacts.
®
When the 6S2A output voltage is 3 , the normal de
voltage applied at Al Pin 1 6 is approximately
+ 2.7 V. The voltage at AI Pin 1 7 should be about
. 1 4 V less than that at AI Pin 1 6 ; if not , M l may
be faulty.
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The normal signal at Al Pin 14, when the 6S2A
output voltage is 3, will be a sine wave of 5.8 V
rms.
®
Check the lead to A2 Pin 3, A2R41 and A2C2S*.
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When the 6S2A output voltage is 3 , the ac signal
applied to the rectifier from the collector of
Al Q9 is approximately .75 V rms.
®
Check the de voltages on Al Q8 and Al Q9 at the
points listed below:
Emitter Al QS
. . .
Collector A I Q9
. . . . . . . . . . • . . . . . . .
Base A1Q9 . . . . . . . . . . . . . . . . . . . . . . . . · 9.S
Emitter Al Q9
. . . . . . . . . . . . • • .
Also check AI C I 1 , A1CRI O and the rectifier
diodes A1CRS and A1 CR9.
®
Check the rectifier diodes Al CRS through
A I CR I O, and capacitors A I C l 2 and A I C I 3 .
Also, check A I C I I and A I CR I I .
®
Set the MONITOR to EXPAND, and vary the
REF SET controls from minimum to maximum.
The voltage at A3 Pin 9 must vary from 0 to at
least · . 1 5 V dc.
Disconnect M I from the circuit, and apply a
current of 1 .5 mA to check for full scale deflec·
tion. Decrease the current, and the pointer deflec­
tion of M l should decrease proportionally.
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The normal dc voltage at A3TPI is · 9 V ± 0.2 V,
when the 6S2A output voltage is 3 . When the
AMPLITUDE controls are varied from minimum
to maximum, the voltage at A3TPI should cover a
range of approximately · I .S V to · 10 V.
5· 1 8
�t:tinn V
. . + I .S V
. . . . .
. . . . . . . . .
+ 2.5 V
10.5
.
. . . . .
®
Check for .72 V ac at the junction of A3C R I and
A3CR2 and check A3Ql , A3CRl , and A3CR2.
®
Vary the REF SET controls from minimum to
maximum; voltage at A3TP2 must vary from
· 1 6.4 V to · 1 8.8 V.
Troubleshoot the Variable Reference Supply in
the same way as any power supply. Check
A3CR6, A3Q7, and A3Q8. If A3Q8 is shorted,
A3Q7 will probably also need replacement.
Check the current source, A3Q4 and then check
A3Q2, A3Q3, A3QS, and A3Q6. If these compo·
nents are good, check A3CRS, A3CR7 and
A3CR8.
5-53.

Troubleshooting the Oscillator Circuit.

NOTE
This procedure assumes that the front panel
checks (Table 5· 8 ) have been made and that
the Troubleshooting Tree has been followed
(together with the infonnation in Paragraph
5·52) to branch 6.
a. If there is no signal at A2 Pin 5 with the RANGE at
Xl K and the dial at I , then switch to X I OOK. If a signal
now appears, then check A2Q 1 .
b . If the signal a t A2 Pin 5 i s twice the normal
amplitude then fhe Peak Detector is not operating.
particular check A2Q7 and A2C I I .
NOTE
In older instruments (refer to Appendix C,
Manual Backdating
A2CR5.
c. If there is no signal at A2 Pin 5 , isolate the Peak
Detector by lifting one side of A2C l i 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 A2Rl 6 may need to be read·
justed. This is likely to occur if only components (such as
A2CR6, A2CR&, A2Q2, and A2Q3) have been replaced. To
check this reconnect the peak detector and perform Fre­
quency dial and Xl K Range Adjustments (Paragraphs 5·29
and 5·30.
d. If the oscillator still cannot be made to operate,
perform the procedure of the following paragraph.
In
5-54. the following procedure the oscillator circuit is
driven by an external signal generator and the Wien Bridge
is disabled. luis allows the circuit to be checked out using
the normal troubleshooting techniques for an amplifier.
In
Changes) also check
Model 652A