Service Model 8903B
TROUBLESHOOTING---SERVICE SHEET
3
TROUBLESHOOTING
General
Procediires for checking the Notch Filter Assembly are given below. The circuits or points to check are marked
on the schematic diagram by
a
hexagon with
a
check mark and
a
number inside for example,
a.
In addition,
any points outside the labeled circuit area that must be checked are
also
identified. Fixed signals are shown
on the schematic also inside a hexagon, for example,
(t-1.9
TO
+2.1
VDC
>.
Extend the board assembly where
necessary to make measurements. These procedures assume that the source is working properly,
Equipment
Oscilloscope
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HP 1740A
a
Notch
Filter
Check
1.
On the Audio Analyzer,
key
in 41.0 SPCL to initialize the instrument. Press
DISTN.
Short pin
3
of U5 to
A3TPl (GND).
2. Connect
a
high-impedance, dc coupled oscilloscope to A3TP5 (BP OUT). A3TP6 (HP OUT), A3TP7
(OUT),
and A3TP14
(LP
OUT) one at
a
time. The dc offset
level
(superimposed on an ac signal) for each should be
between -200
to
+200 mVdc.
3. On the Audio Analyzer, remove the short at pin
3
of U5. Short A3TP3 (TUNE) and A3TP4 (BAL) both to
A3TPl (GND).
4. On the Audio Analyzer, set the INPUT and OUTPUT switches to ground. Key in AMPTD 2
V.
Key in 1.1
I
SPCL
to
set the input gain to
0
dB. Connect the HIGH OUTPUT to the HIGH INPUT.
5.
AC couple the oscilloscope and connect it to A2TP3 (OUT, see Service Sheet 2). Increment the Audio
Analyzer’s SOURCE amplitude to obtain exactly
6
Vpp
on
the oscilloscope.
Hint: The amplitude at A2TP3 should be approxiniately between
5
and
6
Vpp before adjustment. (The period
should be approximately
1
ms.)
6. On the Audio Analyzer, key in the SOURCE frequency and Special Functions listed in Table 8B-20. For
each setting, measure the waveforms at the test point indicated. The peak-to-peak amplitude should be within
the limits indicated.
Hint: If the waveforms at A3TP14 only are faulty, check the Sum and Output Amplifier. See step
8
below.
Hint:
If
all waveforms are incorrect, chech the Sum Amplifier. which should have
a
gain of
1
(from A3TP5 to
pin 6 of
U
11)
and Sum Amplifier 1.
Hint: If most of the waveforms are incorrect, check the control voltages listed in Table 8B-21. If the control
signals are correct, measure
the
ac signal
at
the drains of FETs
430
through
435
at
the corresponding SOURCE
frequency (choose
1
kHz for 53.1 SPCL). The drain voltage shoiild be
0
Vrms when the
FET
is
on.
If
these
signals are
all
correct, continue
on
with step 7.
7. On the Audio Analyzer. key in 53.1
SPCL.
Key in the Special Functions
and
SOURCE frequencies listed
in
Table 88-22. For each setting, measure the ac signal
on
the sources
of
FETs 414 through 429. A signal should
appear on the source unless the FET is oft
0
Vrms should appear if the FET is on. If
a
signal is faulty, also
check the control voltage listed in Table 88-22.
Hint: If an attempt is made to switch
on
an
FET which has failed
to
an open state, signals
will
usually appear
on the soiirces of
414
through 421 but not Q22 through 429.
8.
On
the Audio Analyzer, hey
in
FREQ
1
kHz.
53.1 SPCL. and 54.147 SPCL. Connect the oscilloscope to
A3TP7
(OUT).
8B-32
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