Service Model
8901B
NOTE
To
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
(J1)
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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