On the power supply driver board, there are two 8 pin comparators. With the audio driver boards out, measure the DC voltage on pins 7 and 8 of either of the two comparators. Then reinstall the audio driver boards so it goes into protect and measure the voltage on those two pins of the comparator again. If the voltage isn't steady, post the range that your meter reads on either pin. Be careful not to slip and short the pins together.
IC?
Driver boards out:
Pin 7:
Pin 8:
Driver boards in:
Pin 7:
Pin 8:
IC?
Driver boards out:
Pin 7:
Pin 8:
Driver boards in:
Pin 7:
Pin 8:
The resistors are OK.
FETs can give misleading readings. Read the FET section of the 'Basic Transistor Testing' link below. It will tell you how to test them for relatively reliable results. I think they're probably OK since they were driving the transformer earlier with no excessive current draw.
Have you tried installing the audio driver boards but not the driver board behind the RCAs?
If so, does it draw excessive current?
FETs can give misleading readings. Read the FET section of the 'Basic Transistor Testing' link below. It will tell you how to test them for relatively reliable results. I think they're probably OK since they were driving the transformer earlier with no excessive current draw.
Have you tried installing the audio driver boards but not the driver board behind the RCAs?
If so, does it draw excessive current?
I'm going to study the board I have here to see if I can find anything else.
This is a long shot but you may want to try it...
On the power supply driver board, there's a transistor (Q41) just below pin 5 of the 494. Its center leg will be directly connected to pin 5.
Pull that transistor to see if the amp will idle properly with the driver boards in the circuit.
This is a long shot but you may want to try it...
On the power supply driver board, there's a transistor (Q41) just below pin 5 of the 494. Its center leg will be directly connected to pin 5.
Pull that transistor to see if the amp will idle properly with the driver boards in the circuit.
There are only a few components on the bottom corner of the power supply board between the sync pin and the transistor. Do any look to be defective or do any have possibly defective solder connections?
Are you 100% sure that the replacement was not defective?
If neither of those are the problem, I think you'd need to post a photo of the waveform on the sync pin (all driver boards in, Q41 out). It would have to wait until you get your scope back.
Did the amp produce audio with Q41 out of the circuit?
Are you 100% sure that the replacement was not defective?
If neither of those are the problem, I think you'd need to post a photo of the waveform on the sync pin (all driver boards in, Q41 out). It would have to wait until you get your scope back.
Did the amp produce audio with Q41 out of the circuit?
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