Section VIll SERVICE GROUP C Model
3455A
short Cl 3.
The
squaring amp
circuit
should
now
act like
an
emitter
follower with
a gain of one. Short
the input of the 345SA and if the display indicates zero, Q9A or
Q1
lA may be defective. If the
instru-
ment is still in
overload,
short the cathode of CR3 to TP6. If the
overload
condition disappears, the
square root amp (U2 and associated circuits) or QllB may be defective. If the
overload
condition re-
mains. troubleshoot the integrator (Ul and associated circuits).
h. If
Q9
or QM are replaced, R6 may need padding. With a 1 V, 100 Hz sinewave applies
to the in-
put
of
the 3455A, adjust RIS for a 1
V
reading on the display. If RI7 is out of range, R6 needs pad-
ding. If the reading
is
low, increase
the
value of R6. If
the reading is high, decrease the value of R6. A
change of 4 K ohms should change the reading about .5^t (padding list
is in the
parts
list).
8 C-10. AC/DC Qparation.
8-C-l I . When
dc coupling is used
with
an ac input signal, the specifications of the True RMS conver-
tor become wider. Make
sure the instrument meets specifications before troubleshooting the ac con-
vertor
in dc coupling. Some troubleshooting
hints
for the
ac/dc
operation are given below.
a. Large differences between
an ac and dc
input
signal
with
the 34S5A set for ac/dc operation;
-
Match the betas of
Q9
and Qll.
To further help the performance of the ac
convertor
in dc coupling,
add C40 (2.2 capacitor, part number: 0160-0128).
A short
procedure for the addition of
C40
is as
follows.
1.
Remove
CR3 and install pins into the eyelet holes
where
CR3 has been
removed.
2. Solder both CR3 and C40
to
the pins. Make
sure
CR3
is
mounted
in
the correct
direction.
b.
Difficulty
in
balancing
ac and dc input
signals with the
3453A set for ac/dc operation: Try
changing R5S.
c. Large differences
with
the 34SSA set for ac and ac/dc operation: change R95 to 806 ohms (part
number
0698-3557).
8-C-l
2.
AC
Noisa (Sehamatic
31.
8-C-13.
Before
checking for ac noise,
verify
that the dc readings are good. Troubleshooting for any
dc noise must be done before troubleshooting for any
ac noise.
8-C-14.
Most noise
on the ac
convertor board
can
be isolated into
certain areas of the board. The
following procedure
may be
used
to
isolate
those
areas.
a. Apply a 10
V, 100
Hz
sinewave
to the input of the 34S5A. Set the instrument to the 10 V range
in the ac normal function.
b.
With
a
DVM,
measure the ac signal at TPS. It may also be possible to check the signal of TPS
with
an oscilloscope, if the noise is great enough. If the signal is noisy, troubleshoot the input and
preamplifier circuits of the ac convertor.
NOTE
Someiimes
it is
possible
to check
noise
at TPS
with
a D
VM
in the dc
mode. The input
of
the 3455A has to be shorted with the DVMconnected
to TPS. There should be very little deviation noted.
c.
With
the 10
V,
100 Hz
sinewave
applied to the input terminals, measure the voltage at TP5 with
a
DVM
in the ac mode. Again, it may be possible to measure the
voltage with
an oscilloscope. If the
signal
is
noisy,
troubleshoot the
absolute amplifier circuits.
NOTE
It is also
possible to
check
for
noise at TP5
with
a
DVM
in the dc mode.
8-62
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