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You can check strobe and select lines with a logic probe.
During Gameplay Mode, the select and strobe lines should
pulse. Test for this pulse at connector SIJ3, pins 1 through
8 and 10. You should find the same pulsing at connector
SIJ1, pins 3 through 10 and 13. If not, either U1 or U2 may
be bad. Before replacing these ICs, check or replace C1, a
one microfarad electrolytic capacitor.
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HEAVILY DISTORTED SOUND
Check the negative 12-volt regulator on the Sound Interface
Board. The error amplifier and series-pass device for this
analog regulator reside in chip U3. The circuit also con-
tains two, ten and 100 microfarad electrolytic capacitors.
These tend to fail before the regulator chip. As a rule of
thumb, larger electrolytics fail before smaller ones.
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Check the voltage across the 100 microfarad capacitor.
You can also take this measurement between SIJ2, pin 3
and ground (pin 5). During your test, the Sound Interface
Board should be connected to the Sound Board.
You should find -12 volts DC and not more than 12 millivolts
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of AC ripple.
Check for the ripple with the
AC
setting on
your voltmeter.
If you find excessive ripple, change the 100
microfarad capacitor and recheck the circuit.
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