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Service
Model 8901B
NOTE
T o
repeat steps 5 through 7, it is necessary to press CLEAR first to clear
the
IF
Present Latch.
(J3) AM Demodulator Check
NOTE
This check assumes that the
( J 1 )
IF FM Buffer Check gives positive results.
1. Set the signal generator
to
20 MHz CW at 0 dBm. Connect its RF output to the Modulation
Analyzer's INPUT.
2. Press the blue key, then press INSTR PRESET (the AUTOMATIC OPERATION key)
to preset
the instrument. Key in 20 MHz. Press AM. Key in 49. S (Shift) 1 SPCL. This causes the average
IF level (which the ALC loop
is supposed
to
hold constant) to be displayed. The display should
read between 2.096 and 2.104. If only slightly out of limits, perform the Adjustment 8-ALC
Reference in Section 5; otherwise, see Service Sheet 8.
3. Connect a high-impedance, ac coupled oscilloscope to rear-panel AM OUTPUT.
4. Modulate the signal generator with 50% AM at a 400 Hz rate. The oscilloscope should show the
demodulated AM with an ac amplitude of 750 to 850 mVpp. If faulty, see Service Sheet 8.
5. Key in 6.1 SPCL to set the ALC response time to fast. The amplitude should drop between 40
and
120 mVpp. If faulty, see Service Sheet 8 and check the Bandwidth Control and Level Comparison
Amplifier.
(J4) FM Demodulator Check
NOTE
This check assumes that the
@)
FM IF Buffer Check gives positive results.
1.
Set the signal generator to 20 MHz CW at
0
dBm. Connect its RF output to the Modulation
Analyzer's INPUT.
2. Press the blue key, then press INSTR PRESET (the AUTOMATIC OPERATION key) to preset
the instrument. Key in 20 MHz. Press FM. Allow the instrument to tune then press MHz. Pressing
the MHz key sets tuning to manual.
3. Connect a high-impedance, dc coupled oscilloscope to A4J2 (IF OUT). The waveform should be
TTL pulses with a period of approximately 670 ns. If faulty, see Service Sheet 10 and check the
IF Limiters.
4 . Reconnect the cable to
A4J2. Connect the ac coupled oscilloscope to the rear-panel FM OUTPUT.
5. Modulate the signal generator with 100 kHz peak deviation FM at a
1
kHz rate. The oscilloscope
should show the demodulated FM with an ac amplitude of 1.0 to 1.4 Vpp. If faulty, see Service
Sheet 11 and check the Charge-Count Discriminator.
8D-30
Service Sheet BD3
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