Advertisement
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
Advertisement
