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Service
Model 8901B
TROUBLESHOOTING
General
Procedures for checking the Audio Section of the instrument are given below. The blocks or points
to check are marked on the block diagram by a hexagon with a check mark and a number inside, for
example,
(J3)
.
Before performing any check, perform all the checks on Service Sheet BD1.
Tighten SMC connectors to 0.6 N . m (5 in. lb). Hand tightening
of
connectors
is insufficient. Hand-tightened connectors can work loose and came reduced
performance or malfunctions.
Equipment
Audio Synthesizer.
................................................................
HP 3336C
Oscilloscope
......................................................................
H P 1740A
Signal Generator.
.................................................................
HP 8640B
Audio Filters and Gain Check
1.
Set the audio synthesizer to
1
kHz at
-17
a m . (The synthesizer output impedance should be
500.) Connect its output to A2J1 (AM IN).
If the Modulation Analyzer is to be turned
off,
disconnect the audio
synthesizer first to prevent damage to the CMOS switches.
2. Connect a high-impedance, ac coupled oscilloscope (with
1O:l
divider probe) to A2TP4 (AMPL
3
OUT).
3. Set the signal generator to approximately 20
MHz CW
a t 0 a m . Connect its
RF
output to the
Modulation Analyzer's INPUT.
4. Press the blue key, then press INSTR PRESET (the AUTOMATIC OPERATION key) to preset
the instrument. Press AM. Key in 2.1 SPCL to set the audio gain to maximum. The oscilloscope
should show the
1
kHz sinusoidal waveform with an ac amplitude of 2.4 to 2.8 Vpp. If faulty, see
Service Sheet 12 and check the audio path beginning at the 260 kHz Low-Pass Filter at the AM
output.
5. Increase the synthesizer frequency to 260 kHz without altering the amplitude. The waveform
should have an ac amplitude of
1.2
to 1.6 Vpp. If faulty, see Service Sheet 12 and check the 260
kHz Low-Pass Filter a t the AM output as well as checking to see if the 15 kHz or
>20
kHz are
in the circuit. (They should be out.)
8D-40
Service Sheet BD4
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