HP 6259B Operating And Service Manual page 45

Table 5-4. Feedback Loop Isolation (Continued)
STEP
ACTION
RESPONSE
PROBABLE CAUSE
2
Inspect OVERVOLTAGE lamp
on front panel.
a. On.
b. Off; output voltage high.
c. Off; output voltage low.
a. Check setting of OVERVOLTAGE
ADJUST (A5R125). Check A4CR110 for
short. Proceed to step 3.
b. Check OVERVOLTAGE ADJUST
(A5R125). Check A4CR110 for open. Also
check Q91 and Q92. Proceed to step 3.
c. Check OVERVOLTAGE ADJUST
(A5R125). Check A4CR110 for open. Also
check Q20, Q91, and Q92. Proceed to step 3.
3
Isolate fault to series regulator
or preregulator by proceeding
as follows:
(1) Open the gate lead of
triac A2CR1 by disconnecting
a. Output voltage normal
(variable from 0 volts to
about 9 volts).
a. Check each series regulator transistor
(A4Q103 through A4Q106, A4Q108, or
A4Q110) for open. Then check preregulator
by disconnecting external source and proceed¬
ing to Table 5-7.
one end of R88 (TP87 or TP88).
(2) Observing correct
polarity, connect a small dc
b. Output voltage high.
Varying controls has
little or no effect.
b. High voltage condition in series regulator.
Proceed to Table 5-5. Leave external source
connected.
power supply across input
capacitor (Cl 01). A 0-10V,
2A supply is sufficient.
(3) Set external supply to
ten volts.
(4) Vary front panel
voltage controls.
c. Output voltage low.
Varying controls has
little or no effect.
c. Low voltage condition in series regulator
loop. Proceed to Table 5-6. Leave external
source connected.
Table 5-5. Series Regulator Troubleshooting, High Output Voltage
STEP
ACTION
RESPONSE
PROBABLE CAUSE
Make these tests with external source connected as described in step 3 of Table 5-4. Leave the coarse voltage
control set to the center of its range while making these tests.
Check turn-off of series
regulator transistors A4Q103
through A4Q106, A4Q108, or
A4Q110 by momentarily
shorting base (TP101) to emitter
(TP103).
Check turn-off of driver
A4Q102 by momentarily
shorting base (TP100) to
emitter (TP101).
a. Output voltage remains
high.
b. Output voltage
decreases.
a. One or more of A4Q103 through A4Q106,
A4Q108, or A4Q110 shorted or A4CR105
shorted. Check A4R150-A4R165 (as
applicable).
b. Proceed to step 2.
Check turn-on of error
amplifier A4G101 by
momentarily shorting Q42
emitter (TP46) to collector
(TP68).
a. Output voltage remains
high.
b. Output voltage
decreases.
a. Output voltage
high.
b. Output voltage
decreases.
a. A4Q102 shorted.
b. Proceed to step 3.
a. A4Q101 open.
b. Proceed to step 4.
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