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Section V
b. AC waveshape should have
the same
wave
shape as
shown on Schematic
No.2. If
you
have a flatness
problem,
check
the
wave
shape
for symmetry
and
see
if output
capacitor
C9 or C 10
are
leaking.
5-90.
The following
checks
will determine if the trouble is
in
the Control Integrator
or in
the
Meter Differential
Amplifier.
NOTE
Disconnecting the
outputs of
A2CR21
and
CR22
will cause
the
voltage readings
to be incorrect.
a.
If A2CR21
is not zero
±
20 mY,
the trouble
is
in
the Meter Differen tial Amplifier.
b. If A2CR22
is
not
zero
±
20
mY, the trouble is in
the
Control
Integrator.
5-91. TROUBLESHOOTING THE AMPLITUDE
CON·
TROl INTEGRATOR.
5-92. The Amplitude
Control
Integrator
consists of
A3Q6
thru A3Q9. The
output is
+
5
V to
+
7 V
dc. 1l1e
output
feeds back into the Photocell Module of the Buffer
Amplifier.
Check
the resistance
of
the
filament
in the lamp.
It
should
be
approximately 40 n.
The photocell
should
have
approximately
I
kSG
to
2
kn resistance
when
a
+
5
V
to
+
7 V
dc is applied to the filamen t.
5·93. TROUBLESHOOTING THE AlC lOOP.
5-94.
I f the trouble is known to be
in
the ALC loop
(Buffer and
Balanced Amplifier and the Automatic Leveling
Circuits) and cannot
be
isolated
to
a
unit
by
the preceding
paragraphs, then this method
can
be used to break
open
the
loop. The Buffer Amplifier is then driven by
an external
signal generator
(0.25
V
rms
at
I kHz into
2.5 kn) and
it
can
be checked
as any
amplifier.
a. Lift
one side of
A2R33 to
isolate
the Wien
Bridge
Oscillator
from
the
circuit.
b.
Lift the side
of
A2R38 which
is connected
to
A2DSVI.
c. Connect the
signal generator
with a large
(at
least
10.uF) non-polar capacitor ill
series
to A2R38.
1l1e
capacitor blocks any dc present
on the
signal
generator output.
NOTE
If
a
large non-polar
capacitor is not
available,
use two polarized
capacitors in
series,
with
their
+
ends connected
together.
d. Set the
signal generator frequency
to I kHz
and
the
output level
to
about
.25 V
rms (monitor
with
an ac
voltmeter).
5-24
Model
654A
e. Signal
trace the ALC loop. Signal levels, and
voltages should correspond
with those
shown on
the
schematics.
5-95.
TROUBlESHOOTII~G
THE METER
DIFFEREN·
TIAl AMPLIFIER.
5-96. The Meter Differential Amplifier is used
for
the
meter
in all
functions. Use
voltages on
the
schematic
for
locating the trouble. If the base
of
A3Q2 is
not zero,
the
trouble
could be
in
the
offset
current reference circuit.
5-97. TROUBLESHOOTING THE ATTENUATORS.
5-98. If
either
side of the
attenuator
is
shorted
to
ground,
remove the
cover and
make
a
physical
check. The signal
path is probably touching the
metal case or cover. The
contacts
of the
switches
should
be kept clean
and
lubri-
cated
(refer to
CAUTION
on Page
5-]3
and Service Note
M45B). For proper lubricant, use
.Electrolube 2A Or
Electrical
Contact
Lubricant, -hp- Part No.
6040-0300.
NOTE
In
orde
r
for
the attenuator to make
speCifications and
ensure proper
ground-
ing,
all
mechanical
connections must
be
velY
tigh
t.
5-99.
SERVICING ETCHED CIRCUIT BOARDS.
5-100.
The Model
654A
con tains
four
pia ted-through,
double-sided,
etched circuit
boards.
When
working
on
these
boards,
observe
the following rules
to
preven t damage to
the
circuit
board or components
:
a.
Use
a
low-heat
(25
to
50
watts)
soldering iron
with
a small tip.
b. To remove
a component, clip a
heat sink
(long
nose
pliers,
commercial
heat
sink
tweezers,
etc.)
on
the componen t lead as
close
to the
component
as
possible. Place the
soldering
iron directly
on
the
component
lead,
and
pull up
on
the lead. If a
component
is
obviously
damaged
or faulty, clip
the leads
close
to the
componen
t,
and
remove the
leads
from
the board.
EXCESSIVE OR PROLONGED
HEAT
CAN
LIFT
THE CIRCUIT
FOIL FROM
THE BOARD OR CAUSE DAMAGE TO
COMPONENTS.
c.
Clean
the component lead holes
by
heating the
solder
in the hole,
quickly
removing the
soldering
iron,
and
inserting
a
pOinted, non-metallic
object
such
as a
toothpick.
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