Regulator - HP 8110A Service Manual

HP 811 OA Service G
u
i
d
e
4- 12 T
r
o
u
bles
h
ooti
n
g
If
e ither f an draws in excess of
a bout 150%ο of normal requirement
then Q410 i s switched ON through the gating of U403 . This
indicates that one or
b
oth
o
f the
f
ans
m ay be blocked or faulty.
If α regulator output is short-circuited the signal
S
HORT s witches
Q10 ON .
If you s uspect α
p
ossible
f
ault in
t
he
F an Control/ Overload circuit,
and want to i solate i t from the rest of t he circuitry,
p
roceed as
follows :
1 . M ake α s hort-circuit across pins 1 and 2 of
o
pto-coupler CP401 .
(you may
a lso need to short pin6 to pin 4 on
U
401 to start the
oscillator)
2 . Ensure that the only load is the 100 Ω resistor on the -5 .2 V
regulator .
3 . Set the Scope to 500 V DC
4 . Connect the Scope to CR407 anode (see Figure 4-4
f
or connexion
point) .
5 . Check t hat the observed waveforms are similar to those shown in
Figure 4-6
6 . Switch off m ains
p
ower and remove the short across the
optocoupler .
If
t
he oscillator i s willing operate with the opto-coupler pins
shorted, then t here is α
f ault in the fan control
a
nd short-circuit
detection circuitry.
If the w aveforms are
p resent and correct and it is known t hat
regulator outputs are unloaded (except
f or the -5.2 V regulator), then
suspect the Fan Control/ Overload circuit .
R
eg
u
latorS
Two of t he regulators are of greatest importance in the
P
ower
Supply : -5 .2 V
a
nd +32 V . The -5 .2
V
regulator must
b e working, as
failure of the control signals CONTROL+ and CONTROL- causes
U401 (the PWM oscillator) to shut down. The +32 V regulator is
important b ecause it provides the gate supply voltage for the other
regulators .
-5 .2
V
Regulator
1 .
U
sing the Scope,
m easure that there i s α waveform across
T
401D
2 .
U
sing α Voltmeter,
m easure voltage between L417 and
L
418 . This
should be approximately 6 V DC
3 . If this DC v oltage is OK, check L418,
L
422,
L
417, L421 and
associated components
4 . If this DC v oltage is
n ot OK, check C475, C476 and C477