Model 8903B
TROUBLESHOOTING---SERVICE SHEET
7
Service
TROUBLESHOOTING
General
Procedures for checking the Output Amplifier/Voltmeter 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
make measurements. These procedures assume that the source is working properly.
Equipment
Digital Voltmeter
..........................
HP 3455A
Oscilloscope
..............................
HP 1740A
a
Ripple Filters Check
1. On the Audio Analyzer,
key
in 41.0 SPCL to initialize the instrument. Set the INPUT and OUTPUT switches
to ground. Key in AMPTD 3
V.
Key in 1.11 SPCL to set the input gain to
0
dB. Key in 3.1 SPCL
to
set the
output gain
to
0
dB. Connect the HIGH OUTPUT to the HIGH INPUT.
2. Connect one channel
of
a high-impedance, dc coupled oscilloscope to A2TP2
(RMS).
Connect the other
channel
to
pin 1
of
UlOA. Both channels should read between
+2.9
and +3.1 Vdc and both should read the
same.
3.
On the Audio Analyzer, connect and disconnect the HIGH INPUT several times and observe the two traces
on
the oscilloscope. The signal at
pin
1
of UlOA should lag in time slightly behind the signal at A2TP2 when
the input is connected and disconnected.
4. On the Audio Analyzer, reconnect the HIGH INPUT. Connect one channel
of
the oscilloscope to A4TPI
(DC OUT). Connect the other channel to pin 7
of
U 18. Both channels should read between +2.9 and
+3.
I
Vdc
and both should read the same.
Hint: If the signal at A4TP1 is faulty, see Service Sheet
6
and check the Output RMS/Average Detector.
5.
On the Audio Analyzer, connect and disconnect the HIGH INPUT several times and observe the two traces
on the oscilloscope. The signal at pin
7
of U18 should lag slightly behind the signal at A4TP1 when the input
is connected and disconnected.
6.
On
the
Audio
Analyzer, reconnect the HIGH
INPUT.
Move the oscilloscope connection
from
pin
7
of
U
18
to pin 11. Both channels should read the same.
7. On the Audio Analyzer, connect and disconnect the HIGH INPUT several times and observe the two traces
on the oscilloscope. The signal at
pin
11
of U18 should lag behind the signal at A4TPI when the input is
connected and disconnected. (The lag will be longer than for the previous steps.)
a
Voltmeter Input Selector Check
1.
On the Audio Analyzer,
key
in 41.0 SPCL
to
initialize the instrument. Set the INPUT and OUTPUT switches
to ground. Key in AMPTD 3
V.
Key in 1.11 SPCL to set the input gain to
0
dB. Key in 3.1 SPCL to set the
output gain to
0
dB. Connect the HIGH OUTPUT to the HIGH INPUT.
2.
Key in the Direct Control Special Functions listed in Table 8B-28. For each setting, compare the dc voltage
at A4TP9 (MUX) to the pin on U18 indicated. The two voltages should be the same to within the repeatability
of
the voltmeter. If faulty, also check the
logic
level
of the pins indicated.
Hint: If the selector has a failure in which one input is always connected to the output, pulses will usually
appear on the faulty input when another line is slected. Use Special Function 49.N to select the lines.
a
Voltage-to-Time Converter Check
1. Measure the anode of VR4 with
a
dc voltmeter. The voltage should be between
-6.51
and -5.89 Vdc.
8B-5
1
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