(3) Slight changes in component values will
steady input signal. As a final check, a test
affect the frequency.
should be taken at several levels; for example, -4
(4) Deviations from the values shown in the
db, -1 db, +4 db, and +2 db. Plus 4 db is not
signal-tracing chart may point to the
an operating level, but it is possible that defects
cause of off-frequency operation.
will show up quicker when this overload signal is
(5) Low or high RB+ voltage may affect the
used.
frequency.
93. Signal Amplifier (RECORD PHOTO)
a. General. The DB METER circuit is of the
vacuum-tube voltmeter type which measures the
output of the second signal amplifier, V11 (7L7).
a. Signal Tracing. Apply an input signal of
The circuit of the meter includes resistor R70 or
.0015 volt at 1,800 cps to LINE JACK connector
R71, depending on whether the selector switch is
J5. Connect a voltmeter between the grid and
in the sending or receiving position.
ground of V14 with the selector switch at RE-
b. Correct Meter Reading. Resistors R70 and
CORD PHOTO and the GAIN control set at 100.
R71 have tolerance values within 10 percent.
Set Signal Generator SG-15/PCM to produce an
If the meter does not indicate correctly, select
output of 1 volt. Place a 100,000-ohm resistor
resistore within this tolerance range to obtain a
in series with a 200-ohm resistor across this output.
reasonably exact reading. To determine whether
The output across the 200-ohm resistor will be
the reading is correct with the selector switch in
approximately .002 volt. This is enough to insure
the TRANSMIT position, measure the output
an input voltage that will give a test of the
signal across the LINE binding posts after con-
amplifier gain. The voltage on the grid of
necting a 600-ohm resistor to these terminals. A
V14 should be approximately 16 volts. In
reading of 0 dbm across the LINE binding posts
some sets, it may be necessary to raise the input
should cause the DB METER in the transceiver
voltage to obtain the proper reading on the grid
to indicate 2 db. If the reading is correct in
of V14. An oscilloscope (placed in the grid
the TRANSMIT position, it will also be correct
circuit of V14) should show the proper waveforms
in the RECEIVE position unless the value of R71
has changed. This may be checked by making a
forms are not correct, trace the signal in accord-
simple resistance measurement.
ance with the signal-tracing chart to localize the
trouble. The performance of stage V14 can be
checked by measuring the current in the recorder
lamp circuit. The current should be approxi-
a. General. When the selector switch is set to
mately 35 ma with proper input voltage on the
RECORD DIRECT, the signal amplifier circuit
grid of V14. This current will vary somewhat
is changed in the following manner:
with different recording lamps and different tubes.
(1) Tube V12 (7C5) is cut off by opening the
b. Level Changes. Occasionally, recordings will
plate supply circuit. One section (5, 6,
show undesirable changes in density. To localize
and 7) of V13 (7N7) is switched in and
the trouble, apply a steady tone, free from level
takes the place of V12. This is done for
changes, to the input of the facsimile transceiver,
the purpose of increasing the contrast on
If necessary, connect the test meter across the in-
the RECORD DIRECT position. The
put and watch it for several minutes; be sure that
cut-off bias of V13 is supplied in part by
there is no change in level more than .2 db;
bias from the cathode circuit of motor
watch the DB METER in the facsimile trans-
amplifier driver tube V4 (7C5).
ceiver for 6 to 10 minutes. Bounce or jar the set
(2) The output of V14 (1635) is switched to
in an effort to aggravate the troubles caused by
transformer T3. The output of T3
poor contacts. If the indication on the meter
delivers the power to the recording stylus.
remains steady, it is possible that the level changes
The switching connections include two
are the result of a defective recorder lamp. Re-
sets of contacts to reduce the danger of a
place the lamp with a new one. Line it up and
high-voltage flashover. When operating
adjust it in accordance with the instructions in
through transformer T3, the plate voltage
of tube V14 is +450 volts.
