HP A Series - Page 24

Table 5-3. Low Output Voltage Troubleshooting
STEP
ACTION
RESPONSE
PROBABLE CAUSE
1
Check input voitage to
reference supply; volt¬
age between test points
B (+) and N should read
30 ± 6Vdc.
a. Normal voltage
b. Abnormal
a. Proceed to Step 2.
b. CR1 or CR2 open or shorted.
TCI, T1 open.
2
Check +12.4V reference.
Voltage between test
points P (+) and +S
should be 12.4±1.3Vdc
a. Normal voltage
b. Abnormal
a. Proceed to Step 3.
b. Q1 or R1 open.
3
Check +6.2V and ~7.5V
references. Voltage be¬
tween test points R or
N and +S should be
6.2 ±0.7Vdc, or
7.5 ±0.8Vdc, respec¬
tively.
a. Normal voltage
b. Abnormal
a. Proceed to Step 4.
b. Check applicable zener diode,
VR1 or VR2.
4 Check raw dc input to
main power supply.
Voltage between test
point F and +S are
different for each
model and are given
in a table opposite
the appropriate schematic.
a. Normal voitage
b. Abnormal voltage
(beyond ±20%)
a. Proceed to Step 5.
b. CR3-CR6 (or U4) open. C9
shorted or T1 open
5
To eliminate the cur¬
rent limit circuit as
a possible source of
the trouble, measure
voltage drop across
applicable OR-gate
diode (CR14), Test
points C to G.
a. T.P. C negative
with respect to G
(-1.8V).
b. T.P. C positive
with respect to G
(0.7V).
a. Unit in Constant Voltage mode,
proceed to Step 6.
b. Unit in current limiting mode,
proceed to current limit trouble¬
shooting, Table 5-5.
-^CAUTlON--—
Do not perform Steps 6 and 7. if
unit is in the current limit mode.
If the output is shorted, performing
Steps 6 and 7 will damage the series
regulator.
6
Check conduction
capability of series
regulator by shorting
driver transistor Q3,
emitter-to-col!ector.
a. Output voltage
increases
b. Output voltage
remains low.
a. Proceed to Step 7.
b. Series regulator transistor(s)
Q4 (Q5) open. Wiring to heat sink
open.
5-8