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HP 8110Α Se
r
vice Gui
d
e
as the F ETs Q404 are
m
ounted close to Τ 401, they may prove
difficult to remove. In this case, remove the complete subassembly
o
f
Q404, Q405 and the h eatsink by unsoldering
i
t
f
rom the
P
C
B
oard.
Caut
i
on
When changing devices
m
ounted on h eatsinks, take care
n
ot to
0
damage the insulator between
t
he device and the h eatsink .
When replacing devices
m
ounted on heatsinks, do n ot
f
orget to check
t
he insulator
f
or damage, then replace the spring clamp.
LE
D Indicators
When t he LED indicators are illuminated the oscillator
i
s not
operating .
DS401, Tempe
r
atu
r
e
F
ail
The operation
o
f this circuit is governed by t hermistor RT403
( ΜΟΧΙΕ) . M easure the voltage on the catkode of
C
R405 . This
should be in the region
o
f +12 V. If
t
he voltage is considerably
higher than t his (about +20 V) then Ζ401 can be considered
f
aulty
(open-circuit) . Α voltage
b
elow
a
bout +9 V indicates that RT403 is
operating, and LED DS401 would
b
e illuminated .
If
t
his v oltage
i
s
b
elow 8.5 V suspect α
f
ault in the surrounding
circuitry. You can apply α quick test by touching RT403
b
riefly with
α source of heat (the barrel
o
f α soldering iron,
f
or example, but
MAKE SURE t hat the soldering iron is
p
owered by an i solating
transformer) .
The v oltage on the cathode of
C
R405 should
f
all if the RT403 is
operating . In cases of overheating,
y
ou can get the oscillator going
again quickly, by spraying α temperature-reducing aerosol on the
thermistor .
DS402,
F
an Down/Overloa
d
When this LED
o
perates, it remains on for 1 minute due to Q408
holding current . The holding current will
o
nly fall below its l ower
limit when the Power Supply is switched OFF
a
nd switched on again
a
fter waiting about 1 minute. The oscillator cannot be re-started
until t his time i s over.
A s α quick test, if you want to restart the oscillator, without
w aiting the full minute, short
U
401, pin 6, to pin 4,
b
riefly . While
this connection is
m
ade the oscillator will not run, but will start
again immediately on release of the short (as
l
ong as nothing else is
faulty!) .
D
S402 i s driven from
t
hyristor Q408 and optocoupler CP401 . If
o
ne
or
b
oth fans i s disconnected, Q409 base goes high
a
nd Q409 switches
Q410 ON,
o
perating opto-coupler CP401, which switches thyristor
Q408 ON, t hus switching the oscillator OFF through LED DS402.
T
r
oubles
h
ooting 4-1 1
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