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MAINTENANCE
WJ-8718-19/FE HF RECEIVER
4.3.4.19
Audio Amplifier, A4A10, (7459) Testing and Troubleshooting
Audio
Amplifier Testing and Troubleshooting includes a checkout
procedure and fault isolation information.
A Signal Generator and an Oscilloscope
(Table 4-1) are required to perform the tests outlined below.
Perform the following procedure in the sequence given.
If any specified
result is not obtained, refer to Table 4-18 for fault isolationn
1.
Deenergize the receiver.
2.
Remove PC boards A4A6, A4A7, and A4A9.
Place PC
board A4A10 on an extender.
3.
Set the receiver Line Audio Level control to Maximum
Clockwise and the Phone Level control to mid-range.
4.
Connect the Oscilloscope Vertical Input to A4XA10 pin
55 using a short coaxial cable with clip leads on one
end.
Connect shield to IF Motherboard ground plane.
5.
Connect the Signal Generator AM output to A4XA10 pin
51 using a short coaxial cable with clip leads on one
end.
Connect cable shield to IF Motherboard ground
plane.
Set the Signal Generator Modulation Frequency
to 400 Hz, set Audio Output Level to 0.2 V rms and set
AM switch to INT.
6.
Energize the receiver and select the AM Mode.
The
Oscilloscope should display a level of 0.3 V p-p at 400
Hz.
The waveform should be a clean sine wave.
7.
Use the Oscilloscope lead to probe A4XA10 pin 13 and
A4XA10 pin 11. The Oscilloscope should display a level
of 15 V p-p at 400 Hz on each pin.
8.
Connect the Oscilloscope clip lead to A4XA10 pin 41.
The Oscilloscope should indicate a level of -10.8 Vdc.
9.
Move the Oscilloscope clip lead to A4XA10 pin 19. The
Oscilloscope should show a level of 7 V p-p at 400 Hz.
8.
If the signals in steps 6-9 are incorrect, check the test
points given in Table 4-18 with an RF Voltmeter and
high impedance probe. When a faulty signal is
encountered, replace the key components indicated and
repeat the Checkout Procedure.
Figure 6-21, Audio
Amplifier Schematic Diagram should be referred to if
additional signal tracing/fault isolation is necessary.
9.
When the Audio Amplifier has been repaired and is
operating satisfactorily, deenergize the receiver and
disconnect test equipment, if no further tests are to be
performed.
4-38
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