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Verify around 5 volts at U3 pin 8. If not, replace U3.
Check continuity from U3 pin 14 to J7 pin 1. If correct, replace U3.
Apply an input signal at 500Hz sufficient to produce 50 volts peak to peak at the
low frequency amplifier output and observe that the red LED illuminates.
• Does the LED constantly show Green, Amber or erratically extinguish?.
Check for less than -8 volts D.C. at U3 pin 11. If not check that D20 is the correct
type and polarity and that C38 is correct polarity. Check values of R62 (14K
ohm), R65 (10K ohm) and R64 (100K ohm). If all these are okay, replace U3.
Verify -7.5 volts at U3 pin 10. If not check values of R63 and R60 (Both 15 K
ohm). If these are okay, replace U3.
Check that U3 pin 13 is at -15 volts. If not check for check for short to this trace
or reversed polarity (or short circuit failure) at C37. If these are okay, replace U3.
Check for approximately -12.5 volts at U3 pin 14. If the voltage at U3 pin 14 is
close to -15 volts replace U3. If the voltage at pin 14 of U3 is more than -15 volts
but less than -12.5 volts check value of R59 (3K32). If U3 pin 14 is close to +15
volts, replace the LED.
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