Section
Vlll SERVICE GROUP
C
Model 3455A
e. Unable to
adjust 100 V at 40 kHz,
within
limits:
Moving
the
wire
connecting the R92, C29, and
C32,
C34 modes away from the shield, may raise the
reading.
f. The 100
V and 1000 V ranges inaccurate: The R92 and R93 resistor
divider
may
have changed
value.
K1
-
K3 contacts may be resistive.
g.
Overload
indication
with
a 1000
V
at
1 kHz
to
10 kHz
sinewave
applied to the input: AlOKl,
K2, or KS may
have developed leakage. The leakage can be isolated by
removing
the
orange jumper
from
the froni/rear switch connected to K1 and KS. If the
overload condition disappears, KI or K5
may be defective. If the
overload
condition
remains, remove the jumper from K2 and connect the
jumper directly to the input of the
AIS
board. If
the overload condition disappears, K2 may be defec-
tive.
h. 1
V
and 10
V
ranges
inaccurate and out of calibration; A1SK3 may be shorted.
i. Full scale
readings
go
high
with
an increase in temperature: A1SQ9 or
Ql 1
may be
defective.
j.
1500 counts error on the
higher
ac
ranges: Connect guard to
low.
k. Differences in high frequency readings
between
the
front and rear input terminals: Short the
rear
terminals’ guard to
low.
l.
The ac convertor should be
calibrated with the guard
cover
in place.
8 C-17.
Average Responding AC
Convertor (Schematic
2).
8-C-18. Due to the
simplicity of the
average
responding ac
convertors, only a few pertinent
troubleshooting hints
are
given.
a. Component
location and layout may be critical to the
convenor’s freqency response.
Capacitors,
especially in the input circuit, should not be too
close or too far from (he PC board.
Make sure
(he relays are not touching the
ground plane.
b. The ac
convertor should be calibrated with the inguard
cover
in place.
c.
A13Q15, U4, US, and associated circuitry may
occasionally fail.
d. To help flatten the
frequency response of the
convenor,
especially at
10 kHz, C2S is usually
padded with a
33
pF capacitor (pad
list in the parts list). If unable to bring the
level down at 10 kHz, a
28
pF capacitor
may be used.
e. To help in
troubleshooting the ac convertor, the
following checks can be made.
1
.
Apply a I V, 100 Hz
sinewave
to
the input of the 345SA,
with
the
instrument
set to the
1 V
range.
2. US pin 6 should
have a
1
V peak to peak
sinewave
and TPl
should have a
S
V peak to peak
sinewave.
3.
A
6.67 V dc voltage should be read at
TP2. If this voltage is good and there is an
incorrect
reading on the display, A10Q3 may
not be turned on or may be
defective.
8-64
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