Service
Model 89038
TROUBLESHOOTING---SERVICE SHEET
4
TROUBLESHOOTING
General
Procedures 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,
<
+1.9
TO
+2.1
VDC
>.
Extend the board assembly
where necessary
to
mahe measurements. These procedures assume that the soiirce is working properly.
Equipment
Oscilloscope
,
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.
HP 1740A
a
Notch
Filter Tune
and
Balance Check
1.
On
the
Audio Analyzer,
key
in
41.0
SPCL
to initialize the instriiment. Set the INPUT and OUTPUT switches
to
ground. Key in AMPTD 1 V. Key in 1.11 SPCL to set
tlie
input gain to
0
dB. Key in 3.1 SPCL
to
set the
output gain to
0
dB. Key in 53.1 SPCL and 54.147 SPCL to tune the Notch Filter to
1
kHz. Connect the HIGH
OUTPUT
to
the HIGH INPUT.
2.
Connect
a
high-impedance, ac coupled oscilloscope
to
A4TP8 (AMP
1,
see Service Sheet 5). The waveform
should be between
1.1
and
1.7
Vpp. (The period of the signal should be approximately
I
rns.)
Hint: If the signal is faulty, see Service Sheet
3
and check the Notch Filter and Service Sheet 5 and check
Attenuator
1
and Amplifier 1.
3.
Connect the oscilloscope to A3TP2 (NOTCH AMP). The waveform should be between 12 and
19
Vpp.
4. DC
couple
the oscilloscope and connect it to A3TP4
(BAL).
The dc
level
should be between
-5
and -4
Vdc.
Hint: Pin
7
of
U8B
should be
a
0
to
-
15V square wave with
a
period of
1
ms.
The source of 44 should be a
positive, half-wave-rectified sine wave. Pin
6
of
U1
should
be
a
sine wave with an amplitude
of
12
to
19
Vpp.
Pin 14 of U16D should be
-15V;
Q8
should be
off.
5. On the Audio Analyzer,
key
in 44.3
SPCL
to
set the Notch Filter
to
the bandpass mode. The dc level should
be between +4 and
+5
Vdc.
Hint: A3TP2 should be
a
sine wave with an amplitude of 10
to
21 Vpp. If faulty, see Service Sheet
3
and check
the Notch Filter.
Hint: Pin
7
of U8B should be a
0
to
-
15V square wave with a period of
1
ms.
The source of Q4 should be a
negative, half-wave-rectified sine wave.
6.
Connect the oscilloscope to A3TP3 (TUNE). On the Audio Analyzer, key in 44.2 SPCL to set the Notch
Filter
to
the flat mode. Key in FREQ 500 Hz. The dc
level
should be between +4 and
+5
Vdc.
Hint: Pin
1
of U8A should be a
0
to -15V square wave with a period of
2
rns.
The source of Q5 should be a
negative, half-wave-rectified sine wave. Q
1
should be off.
7. On the Audio Analyzer, key in FREQ 2 kHz. The dc level should be between -5 and -4 Vdc.
Hint: Pin
1
of
U8A should be a
0
to
-
15V square wave with
a
period
of
0.5
rns.
The sonrce of Q5 should be a
positive, half-wave-rectified sine wave.
8.
On the Audio Analyzer,
key
in 0.56
S
(Shift)
3
SPCL to discharge the Tune and Balance Integrators. The dc
level should be between
-50
and +50 rnVdc.
Hint: Pin 14 of U16D should be
0
Vdc;
Q1
should be on.
9.
Connect the oscilloscope
to
A3TP4. The dc
level
shoirld be between -50 and +50 rnVdc.
8B-36
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