The issue started when the eq amp developed a loud hum. Then, I noticed that with no input and volume maxed, there was a similar, but faint hum in the aux input. In testing the power supply board I found AC ripple on the B+ and speaker protect/relay supply outputs( pins 8,[9: not shown],10). The AC measures as high as 1.5 V (full line, current unlimited input) on the DC output of those three supply outputs. I can't find the cause of this AC voltage on the DC supply. The circuit in which this is happening is outlined in red on the schematic.
I have: recapped the electrolytics; recapped the electrolytics again in the offending circuit with the next higher voltage caps (I did not change the capacitance values); changed the bridge rectifier and the two half-wave rectifier diodes; changed the zener diodes; checked and replaced out of spec resistors; checked the ground continuity to board, and verified the output voltages. All the DC voltages are either exact per the schematic or within 2 volts of spec.
Whoever had this unit before me had done some unexplainable rewiring. The AC line in had been rewired to completely bypass the relay board, and the dc supply to the thermal shutdown circuit of the protect board had been moved from that board and wired directly to the power supply board at pin 9. I have rewired the AC line and the thermal protect circuit to match the schematic. Now, both the power relay and the speaker protect relay seem to work fine and the unit plays, but still has hum in phono and aux setting. Just so you know, I had already recapped the entire unit, and it was working fantastic until the hum in the eq amp started. I have been through the whole eq board to include thoroughly cleaning the wafer selector switch and reflowing the solder on all the boards that I've worked on.
I've obviously missed something, but I'm at a loss to figure out what it is. I would appreciate any suggestions that you might have.
I have: recapped the electrolytics; recapped the electrolytics again in the offending circuit with the next higher voltage caps (I did not change the capacitance values); changed the bridge rectifier and the two half-wave rectifier diodes; changed the zener diodes; checked and replaced out of spec resistors; checked the ground continuity to board, and verified the output voltages. All the DC voltages are either exact per the schematic or within 2 volts of spec.
Whoever had this unit before me had done some unexplainable rewiring. The AC line in had been rewired to completely bypass the relay board, and the dc supply to the thermal shutdown circuit of the protect board had been moved from that board and wired directly to the power supply board at pin 9. I have rewired the AC line and the thermal protect circuit to match the schematic. Now, both the power relay and the speaker protect relay seem to work fine and the unit plays, but still has hum in phono and aux setting. Just so you know, I had already recapped the entire unit, and it was working fantastic until the hum in the eq amp started. I have been through the whole eq board to include thoroughly cleaning the wafer selector switch and reflowing the solder on all the boards that I've worked on.
I've obviously missed something, but I'm at a loss to figure out what it is. I would appreciate any suggestions that you might have.
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