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If
shorti.ng
the grid
does not remove the ac signal,
move
the short
to the grid
of V-21 (pin
2).
If
the hum now
disappears,
either
Vl is defective,
Cl07 is open, or there
is excessive
hum in B-plus.
If the hum does not disappear,
V2 is defective
or there
is excessive
hum in the power
supply.
(At the plus terminal
of
ClO6,
the hum should be
less
than 100 microvolts
RMS. At the plus
termi.nal
of
C12OA it
should be less
than 5 millivolts
RMS).
If the signal
still
persists,
remove V2 from its
socket
and short
pin 2 or V3 to grcund.
The residual
signal
from
the demodulator
should not exceed about 1 volt,
peak to
peak.
If the signal
seen is still
very large,
replace
defective
diodes RFlCs through
108.
(The diodes are rated
tc pass about 1 milliampere
at a five
volt
forward
voltage
and should have about a 100 megohm back resistance).
It is also possible
that
the siganl
observed
at the plate
of V2B will
be very small in amplitude
and not affected
by the short-circuited
zero control.
In this
case either
Vl or V2 is not amplifying.
To check this,
place the
oscilloscope
at high sensitivity
at the plate
of Vl (pin
6)
and rotate
the short
circuit
ZCI'O control.
If the signal
appears
and
is capable
of being nulled,
V2B ins at fault.
Once the defective
amplifier
stage is located,
the operat-
ing points
may be checked against
the Voltage-Resistance
Diagram at the rear of the manual, and the defect
located.
d.
If no defect
has been found in the ac amplifier
or Ci.5
demodull.:c?,
replace
V& and unshort
the output.
Short
pin 2 of V3 to ground.
Locate Rl28 (DC AMP BAL).
On
the bench model,
this
control
is accessible
from the rear;
on the rack model,
the control
is located
on a ceramic
board to the left
rear of the instrument.
If the dc am-
plifier
is functioning
it: will'be'possible
to swing the
meter pointer
through
zero and against
both stops,
al-
though the action
of the control
will
be very coarse.
If it
is not possible
to swing through
zero,
measure the
voltage
at pin 6 of V3B.
When varying
R128, this
voltage
should
swing through
100 volts.
If it will
not pass
through
100 volts
of the resistors
associated
with
the
stage have changed value.
Check according
to the sche-
matic and the Voltage-Resistance
Diagram.
I
v-2
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