HP 3455A - Page 148

Section
VIII
SERVICE GROUP B
Model 34S5A
b.
Noisy
on
all ranges.
1. Check the + 10
V
reference
voltage
at A10TP8.
2. Check all
inguard power supplies for oscillations. Clock ringing bn the supplies are nor-
mal and
should
be
ignored. A defective AIOU36 may be noisy.
c.
Noise on
positive
input
voltages
only:
Check
the
-10 V reference voltage at A10TP7 for noise.
The 34SSA should be in
HOLD/MANUAL and with Auto-Cal off. The noise should not be greater
than the
+ 10 V reference noise measured at TPS. If the
-10 V reference
is too
noisy, replace
U7.
d. Readings at 1/10 scale noisy and
several
counts
low
on any
range: A14C2 may be
defective.
e. Noise on the 100
mV
range; Short the
input
of
the 3455A
with
Auto-Cal off. Measure the
voltage level
at AlOTPl. It the
voltage is noisy, try replacing R69, R71, U18, or Q37. If TPl is not
noisy, measure with a DVM across TPS and TP6. The
low
input
of
the
DVM should be connected to
TP6 and the high input to TPS. If
excessive
noise is
measured, replace
U3.
f. Noise and S counts to
10 counts turnover on the 1
V
range: Replace
A10R41
to
R43.
g.
1000 V dc noisy; AlOKS, K6, or R63 may be arcing inside.
If KS
or
K6 are replaced, replace
both
of the relays.
h. Various other possible noise repairs.
1. AI0Q7 or
Q8
may occasionally oscillate. Care
should
be
taken when measuring with an
oscilloscope. A probe connected to the
output of
U2
or the emitter of
Q7
may cause oscillation.
2. A10U2 or
QiS
may also cause
noise.
3. Clean the front/rear input
switch (SI).
8-58