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Service
Model 8901B
@ Local Oscillator Level Check
1.
Set RF spectrum analyzer to measure a 0 a m , 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 f a d @ .
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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