MAINTENANCE
TROUBLESHOOTING
AND REPAIR
4D-16
or
lO-kHz
digit is inoperative
or the frequency jumps
as
the
I
-MHz
digit is programmed,
the problem is
likely
the triple-modulus prescalar. If the
lower
order (1-kHz, 100-Hz,
10-
Hz) digits cannot
be programmed,
the problem is the sub-synthesizer
or
single sideband
mixer.
4D-38.
Sub-Synthesizer
and MET
(800
MHz), 40-MMz Loop
The frequency at TP24 and TP25 should be
20
MHz. The
frequency at U64 pins 14 and 1
5
should be 40 MHz. If the 40-MHz signal is present,
but not the 20 MHz, the problem is
most likely with
Q4,
Q5,
U35,
or
U34. If the 40-MHz signal is in error, the
problem is in
the
40-MHz
loop.
Check the frequency at the 40-MHz VCO, U64 pin
3.
It
should be 40 MHz. If it is not, lift
the op-amp end of R
169,
and connect it to a variable
power supply set to approximately
6V. The signal at U64 pin 3 should
be
approximately a 40-MHz ECL level
(approximately 3.2V to 4.2V) signal. By varying
the supply voltage, the frequency should
change. A similar signal should be present at
U64 pin 2. Check to see if U64 pin 1 1 is ECL
low (approximately 3.2V).
The output of TTL buffer U65 pin 8 should be
approximately
40 MHz. The output of the
divide-by^,
U66, should be approximately
10
MHz.
Once again, if the frequency is
greater than 10 MHz, pulses should exist at TP52
and
the output of op-amp U60 pin
6
should be low. If the frequency is below
10
MHz,
pulses
should existatTP49, and the op-
amp should be high (approximately 24V).
The
loop should lock when the operator
reconnects R169.
If the TP checks are all
right
and the 800-MHz
oscillator
is not locked when in the HET
band, the problem
is
either with the 800-MHz
VCO,
the divide-by-4(U6l), the divide-by-
5
(U62, U63),
or the logic that controls the
switched +5V,
Program
the UUT to 320 MHz. The frequency
at
TP27 (the output of the sub-synthesizer
gate array
U33) should be 10 MHz if the input signals are correct.
The frequency at TP12
should be 1 MHz, and
TPl 1 should be 20 kHz. There should be
a20-kHzsine
wavealthe
hot end of R33. The signals at the output
of the active quadrature generator,
UlO pin
8
and
UlO
pin 14 should be approximately
300
mv
p-p
sine waves that are
90°
apart in
phase. Use a
dual-trace
Oscilloscope for verification.
The frequency at TP27 should change 500 kHz for
a I-kHz change in the programmed
frequency, and
50
kHz for
a 100-Hz change, etc.
4D-39. FM Circuitry
Program the UUT to
500
MHz,
JNT FM, 99.9-kHz deviation,
and 1-kHz modulation
frequency. There should
be
a 2V
p-p
1-kHz sine wave
at
TP22. Program 50-kHz
deviation, and the level should
drop
to 1
V
p-p.
Reprogram the deviation
to 99.9-kHz, The
level of the output of the KV DAC,
U2S
pin
7 will be approximately 1.5V
p-p
depending
on the FM
correction
value (KV) in the EPROM.
The signals at TP32 and TP33
should
be approximately the same, depending
on how R87
is set. The output of the audio integrator should
be about 1
V
p-p.
To check the FM range,
program the
UUT to 9.99-kHz deviation. The ac
voltage at TP32 should drop to
10%
of
the 99.9-kHz value. Program 999-Hz, and the
voltage should drop to
1
%
of the 99.9-kHz
value.
The INT/ EXT FM
selection is done on the A2A4 Output PCA. The controls are listed in
Table 4D-17.
Tables
4D-12
and
4D-1
3
provide FM range and FM DAC
(10
bits) control
information.
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