HP Integral Personal Computer Service Manual page 109

a..a
TroubI ............. DI_oetIoe
TIl .... 1-4. Po..,.luppl, R .... lr (ContInued,
l,.nptom 3.
High-t1Oltage rectification is good, but the system does not start.
Note. All resistance measurements should be made with your meter set to the 20-ld} scale.
Always tum off the
power
before moving a test lead or disconnecting anything. Leave the
power
supply test tool connec:ted during all tests to provide an appropriate load.
s ....
t.
1\un on the
power
and monitor the dc volt-
age across
CRt using an
oscilloscope. Be
s~rt to use an isolation transformer on
the
oscilloscope or power SU"iy when making this
measurement.
2. Use an oscilloscope to monitor the dc volt-
age at power-on between
+
5V and GND
on
the power
supply
test
tool.
I
............ ActIon
If the voltage builds up to about 40V and
quickly
discharges, the start circuit is operating.
(Refer to waveforms A and B in
figure
12-11,
the
power
supply schematic.) Go to step 2.
If the voltage does not build up to 40V or does
not discharge quickly, the start circuit is not op-
erating. Measure the resistance between pin 8
and pin 4 of U2
(+
to -). If the resistance is
less than 4 kO, replace in order VR9, U2, and
U4, checking the resistance after each. Check
the continuity of CRt, CR2, VR3, Ql, and Q2;
replace as
required.
If the start circuit still is not
operating, replace the power supply peA.
If the voltage is present briefly and then decays
sharply, it is likely that the over-voltage protec-
tion circuit is being triggered. Go to step 3.
If the voltage is
not present, go to symptom 2,
-Secondary
Circuit, - and do steps 7 through 10.
3. Turn R49 fully counter-clockwise. Then turn When the output voltage
exceeds 6.2V, the pro-
R49 dockwise while monitoring +5V with tective circuitry should trigger, and the voltage
an oscilloscope. should fall to zero. If the protective
circuitry
triggers at 6.2V" go on to step 4.
If the protective circuitry
is triggering when the
output voltage is less than 6.2V, check VR32,
Q7, and Q8 for functionality. Repair as needed
and go on to step 4.
4. Adjust R49 for an output voltage of
+
5.1 V.· U the adjustment cannot be made, replace R49
and
UIO. If the adjustment still cannot be made,
replace the power
supply
PCA.
• The +5.0 Vdc power supply output should be set at +5.1 Vdc to overcome voltage drops
in
the power cable hamess.