Section Vlli
SERVICE GROUP
C
Model
3455A
g.
If all the
above
checks are good and the 34SSA displays approximately 10
V
ac
(with
10
V,
KX)
Hz input), the RMS
convertor
should be
ready
for
calibration.
If the
reading
is incorrect, A10Q3 may
not be
turned
on
or may be defective.
B-C-4.
Preamplifisr and Input Attenuator
Circuitry.
8-C-S. The
waveshape
at TPS appears to be incorrect, try the
following checks
(except
where
noted,
the input
signal
should be a
10 V, 100
Hz
sinewave).
a. Check for the correct
power
supply
voltages
at U6 pins
4
and
7.
Pin
4
should
have
approximate-
ly -IS
V
and pin
7
should
have approximately +
IS
V.
b. Set the
34SSA
on
the 10 V range and short the input terminals. Make sure TPS can reach zero
volts, when adjusting R6S. If unable
to
reach zero, try changing R77 to 412 kO (part number:
0698-
4540). If R77 is a 412 K resistor already, replace U6.
c. If the signal at TPS is riding on a high dc
level,
make sure CR7 and Q20
are
not touching any
shielding. Also make sure
Q19
is not touching the heat
sink
of U6.
Check CR7 and
Q20
for shorts.
d. If the zero reading at TPS is good on the 10
V
and 1000
V
ranges and bad on the I
V
and
100 V
ranges, try the
following
checks.
1.
Check the gain
switching circuitry of
Q16
to
Q19,
and US. Make sure TP7 reads approx-
imately zero volts on the 10 V and 1000 V ranges. On the 1 V and 100
V
ranges TP7 should read
approximately
-15
V.
2. If the gain
switching
is correct,
lift the drain
or
source of
Q19. Check
for a zero reading
at
TPS with the 3455A in the 1 V range. If the reading is good, replace
Q19.
3. If the reading at TPS is still bad, short the
drain
to the source of Q18.
If the reading
is
then
good, replace
Q18.
4.
If
the reading at TPS
is
still bad, short TPS to the junction of R86 and R87. If the reading
is
corrected, troubleshoot the feedback
network.
e.
If the 3455A has a
history
of
bad
QI9’s
replace
Kl,
K3, and QI8.
Q18
may be damaged
if
Q19
has been damaged. The timing of Kl and K3 could be
incorrect, causing
Q19
to be destroyed
by a
I(XK) V input. Check ac calibrator output for any spikes and make
sure the
10'’
V Hz limit
has not
been
violated.
f. If it becomes necessary to
replace
the
matched set of resistors R76, R86, and R91, the
new
set
should be
properly
aged.
Do the following procedure.
1. Set the 34SSA to the 10
V
range and apply a
10 V, 100 Hz signal to the input. Note the
reading on the display.
2. Apply a
1000 V at 100 Hz signal to the input.
Leave
the
1000 V
connected
for about two
minutes.
3.
Remove
the 1000 V signal and reapply the 10 V at 100 Hz signal
to the
input.
After a cool-
ing
down
period (less than 2 minutes), the reading on the display should have
not changed more
than 25 counts from the reading in 1
above.
Replace R76, R86, and R91 if necessary.
g.
If it becomes necessary to replace the matched set of resistor
R91 and R93, they also need to be
aged. Use
the procedure in f above. The only exception to the
procedure is to have the aging done on
the
100 V range rather than the 10
V range.
A 100
V at 100 Hz signal should also be applied in place
of the 10
V
signal.
h.
Other
circuits on
the A15 board may cause preamplifier
malfunctions.
The
preamplifier can
be
isolated from the other circuits by lifting R52
and R64.
If the
preamplifier is working correctly, after
8-60
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