Service Model 8901B
@
Local Oscillator Level Check
1.
Set RF spectrum analyzer to measure a
0
am,
0
to 1400 MHz signal. Connect its input to the
end of the cable connected to A17J3
(LO
IN).
2. Key in 57.0 SPCL to cause the
LO
to
sweep sequentially across bands DBLR through 3. The
LO signal should sweep slowly from above
1300
to below 40
MHz.
The sweep will occur over five
bands.
As
the low end of a band is reached, the sweep
will
stop, jump up slightly in frequency,
then continue
to
sweep. Throughout the sweep, the LO should maintain an amplitude of at least
0
dBm.
If
it
does not, see Service Sheet
17
and check the dividers and gates associated with the
bad band.
NOTE
The sweep can be halted
by
pressing the
SPCL
key.
If
the power level is
marginal at a particular frequency, halt the sweep at the frequency and
make a more precise measurement with a power meter.
3.
Set the spectrum analyzer to view a
0
to 40
MHz
signal.
4.
Key in 56.0 SPCL
to
cause the LO
to
sweep sequentially across bands 4 through
8.
The
LO
signal
should sweep slowly from above 40
to
below 1.25 MHz in the manner described in step
2
above.
If
the amplitude
is
not at least
0
dBm, see Service Sheet
17
and check the dividers and gates
associated with the bad band.
NOTE
The low-frequency bands
can
also be viewed on an oscilloscope.
The
oscilloscope should have a
50R
termination. The signal should be a
squarewave with an amplitude
of
0.5
Vpp or greater.
Track Mode Check
NOTE
This check assumes that
@J
Local Oscillator Tuning Check
and
(J4)
Local Oscillator Level Check
give positive results, but that track-mode
tuning
is
suspected to be fad@.
1.
Set signal generator to approximately
20
MHz CW at
0
dBm.
Connect
its
RF output
to
the
2.
Connect high-impedance, dc coupled oscilloscope
to
the rear-panel FM OUTPUT.
3. Press the blue key, then press INSTR PRESET (the AUTOMATIC OPERATION key) to preset
the instrument. After the Modulation Analyzer
is
tuned, press MHz, then
S
(Shift) FREQ
ERROR. The oscilloscope should show
-1
to
+1
Vdc.
If
it does not, see Service Sheet
11
and
begin by checking the Charge-Count Discriminator.
4.
Adjust the oscilloscope to vertically center the trace. Adjust the signal generator’s frequency until
the displayed frequency error is
500
kHz. The oscilloscope display should move down to between
-3.2 and -2.8V.
If
faulty, see Service Sheet
11
and check the Charge-Count Discriminator.
5.
Adjust the signal generator’s frequency until the displayed frequency error is
-500
kHz.
The
oscilloscope display should move up to between
2.8
to 3.2V.
If
faulty, see Service Sheet
11
and
check the Charge-Count Discriminator.
6. Adjust the signal generator’s frequency until the displayed frequency error is
0.0
kHz. Key in
3.1
SPCL to set the IF to 455 kHz. The oscilloscope display should move down to between
-8
to
-6V. If faulty, see Service Sheet
11
and check the Charge-Count Discriminator. If not faulty, see
Service Sheet 20 and check the Track Loop Amplifier.
Modulation Analyzer’s INPUT.
8D-22 Service Sheet BD2
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