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Model
654A
,f A2R20 and
factory,
trou-
ive vol tage
.
er Amplifier,
tor.
e
Wien Bridge
;onnected
to
arge
(at least
in
series
to
.
c presen t on
not
rs in
cted
IkHz and the
nitor
with
an
I levels,
se shown
SCILLATOR
~
ont
lade
has
the
nch
~
and
A2R23
~n
the Peak
icular check
the oscillator
tions
above
ate the Peak
=:11
and the
les on, check
there is
no
it is possible
It.
To
Model 654A
d. If the oscillator still
cannot
be made to operate
perform
the
procedure
of
the
following
paragraphs.
5-82.
In the
following
procedure the oscillator
circuit
is
driven by
an
external signal
generator
and the Wien Bridge
is disabled. This allows the circuit to be checked
out
using
the normal troubleshooting techniques
for an
amplifier.
5.83. The
recommended
signal generator
is
a 652A or
651 B (which has 50 ohms
output impedance
and a
frequency range
of
10Hz to I OMHz). If this is not available
any oscillator with low
output
impedance
and
capable
of
driving up to
3V
rms
open
circuit will
suffice.
In most cases
it will not
be essential
to use the
generator at
frequencies
over I kHz .
a.
Turn
off
the power to the 654A.
b. Disconnect the Buffer Amplifier
and
Counter
Emitter-Follower by lifting the
negative side of
A2C12
.
c. Disconnect the Peak Detector by lifting
one
side
of
A2CII and the emitter lead
of
A2Q7.
d. Disable the RC tuning network of the Wien Bridge
by disconnecting A2 Pins
8,10
and II.
e. Connect
the
Signal generator
through a large
capacitor (as
described in Paragraph
5-80c and
the
note) to A2 Pin 10. Connect the
ground side
of
the
generator
to A2 Pin
8.
f.
Turn the
654A
power
on. Set
the
signal generator
frequency
to
1kHz
and
the
output
to
approximately
I V fins (monitor with an
ac
voltmeter
on
A2 Pin 10)
.
Monitor the
output
signal with
an
oscilloscope
on A2C 12.
I)
The
amplifier
has
a
voltage
gain
of
approximately 2,
therefore the
output should
be
a sine
wave
of
between
5
and
6V
peak -to-peak
.
2) If the
signal
does not
appear
at the
output
then
troubleshoot
the amplifier (A2QI
through
A2Q6 and
associated circuitry)
using normal
troubleshooting techniques for
an
amplifier.
A2Q5
and
A2Q6
should
be replaced
as a
pair if
either
has to be replaced.
3)
When the
amplifier
is operating
correctly check
the frequency response, which
should
remain
essentially flat bet
ween
10Hz
and
10MHz, by
sweeping the
signal
generator
over
that range
while
maintaining
a
constant input voltage
on
the
green
lead
.
4)
Reconnect
A2CI.
1
and
A2Q7 to ensure that the
Peak Detector
does
not
disable
the amplifier (if
it does troubleshoot the Peak Detector). If the
signal
is
still
present at A2 Pin
5
then the
Section V
probable
cause of
trouble in the
oscillator
circuit
is the FREQUENCY RANGE switch or
associated components
.
5-84. TROUBLESHOOTING THE BUFFER AMPLIFIER.
NOTE
AC
output
will be erroneous
if
the
control
voltage to DSVI is the wrong
value.
This
should be
approximately
5 to
7
V
dc.
5-85. The
ac gain of
the Buffer
Amplifier should
be
0.6
or
less than
I.
The dc vol tage
at
A2R44 should be 0 V
±
150 mY
and
the
ac
voltage
should
be between 3 and
4
Y
pop.
5-86. TROUBLESHOOTING THE BALANCED AMPLI-
FIER.
5-87.
The balanced
amplifier
should have an
ac
gain
of
approximately two. The de voltage at both
sides of
A2C32
should be
near zero
(±
300
mY)
with one
side
positive
and
the other negative. The
ac
wavefonns at
each
side
of
A2C32
should be
equal
in amplitude but 180
0
out of
phase.
The
amplitude
should
be
6
to
7
V POp.
a.
If the ac voltages
are
near
equal and
the dc
voltages are
of opposite
polarity but not near
zero
and
the
amplitude control
has little effect, then
the trouble is probably in the feedback loop
A2Q17, Q22
or
Q23
.
This
circuit
holds A2C32
near
zero
.
b. If the balanced amplifier
output
is
clipped or
distorted, replace A2Q17 with
approximately
500
n
resistor between
emitter and
collector,
using pc board holes. If the balanced
amplifier's
ou
tpu t becomes
a good sine
wave, this indica tes
the Q17, Q22, Q23 loop is faulty. If
not,
the
balanced amplifier differential pairs
(Q II
thru
Q21)
are
at fault.
c. If A2C32
ac voltages
are not approximately
equal,
the
fault
is probably in the differential pairs Qil
thru Q21.
d. If the instrument will not pass its balance
specifi-
cations check
to
see
that A2R74 and R75 are the
same
value.
5-88. TROUBLE ISOLATION IN THE REMAINDER OF
THE LEVELING
LOOP.
5-89.
The following check will determine
if
the trouble is
in the Average Detector.
a. The input
of
A2C40 should be 4 to
5.5
V dc
and
the ac signal
should
be
0.8
Y POp
to
1.0
V
pop.
5-23
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