HP Lab Series - Page 41

c. Turn VOLTAGE control fully clockwise.
d. Set METER switch to highest cwrent range
and turn on supply.
e. Adjust CURREN.T controls until ,P-.nel metez;
reads e:>tactly the maximum rated output current.
f, Read. and recoid voltage indicated on dif-
ferential voltmeter.
g. Short out load resistor by closing Sl.
h. If reading on differential· voltmeter differs
by more than l.26mV from the reading in Step (f).
replace Zener diode.
5-61 REGULATING LOOP TROUBLES
5-62 If the voltages in Table 5-2 have been
checked to eliminate the reference ~md.bias cucu.its
as a source of trouble; the malfunction is caused
by either the series regulator or preregulator feed-
back loop. Because the interaction between these
two loops make logical troubleshooting difficult,
the follow,tng steps are proVided to temporarily sep-
arate the action of the two feedba,ck loops. Once
the loops are separated, the operation of either
loop can be analyzed independently. This method
should be followed whenever a high or low output
voltage condition exists and when troubleshooting
gets beyond the "quick and easy stage".
a. Disconnect the gate leads from SCR CR3 5.
b. Connect a small de power supply across
the input capacitor Cl4. A 0-20 Volt, 0 •. 2. Ampere,
supply is adequate.
c. Turn on the power supply and, with no
load connected, set the external voltage source to
100 Volts. To verify the operation of the series
regulator loop, vary the front panel voltage control.
The output voltage of the supply should be variable
between 0 and about 90 Volts (approximately lOV ls
dropped across the series regulator). If this is the
case, the series regulatOr feedback loop is opera-
ting normally and the trouble condition is probably
due to a defect in the preregulator feedback loop
(refer to Table 5-6). If the o~put voltage remains
in the low or high stage, and varying the voltage
control has little or no effect, then the trouble is
probably in the series regulator feedback loop.
Refer to Table 5-4 _(htgh.outPut) or Table·5-5 ·uow
output).
5-63 After the trouble has b.een isolated to one of
.the feedback loops, troubleshooting can proceed as
described in Tables 5-4, S-5, or 5-6. After both
loops are operating in a satisfactory manner, the
eictemal power supply should be removed and the
SCR gate lead reQC>nnecte~.
5-64 Series Regulating Feedback Loop. When
troubleshooting the series regulating loop, it ls
useful to open the loop since measurements made
anywhere within a closed loop may appear abnormal.-
With a loop closed, it is very difficult to separate
cause from effect. As described in Tables 5-4 and
5-5, the condu~on or cutoff capability of each
stage is checked by shorting or opening a previous
stage, as follows:
. a. Shorting the emitter to collector of a tran-
sistor simulates saturation, or the full ON condition.
b. Shorting the emitter to base of a transistor
cuts it off, and simulates an open circuit between
emitter and collector.
5-65 Although a logical first choice might be ·to
break the loop somewhere near its mid-point, and
then perfonn successive subdividing tests, it is
•
more useful to trace the loop from the series regula- •
tor backwards a stage at a time, since loop failures
occur more -often at the higher power levels.
5-66 Preregulator Feedback Loop. The preregulator
feedback loop (SCR control circuit) can be conven-
.iently checked by observing the waveforms given on
the schematic at the rear of the manual. As indi-
cated in Table 5-6, the control circuit is checked by
starting with the eutput waveform and tracing back-
wards from this point:
Table 5-2. Referenee Circuit Troubleshooting
METER METER NORMAL NORMAL
Il' INDICATION ABNORMAL,
STEP
COMMON
POSITIVE INDICATION RIPPLE (P-P)
TAKE THIS ACTION
1
+s
33
6, 2 :I: 0.3Vdc
2mV Check 12 .4 Volt bias or VRl.
(Refer
to Paragraph 5-60.)
2
31
+S
6. 2 :I: 0.3Vdc
2mV
Check 12.4 Volt bias or VR2.
(Refer to Paragraph 5-60.)
3
+S
37 12.4 :1: l.OVdc
4mV
Check QB~ Q9, CR22, CR23, ClO,
T3.
•
s-10