Princeton Chorus hum

[dk712]

I have a Princeton Chorus that was bought in 95, it eventually dies and I set aside. But, I thought I'd get it out again, couldn't remember exactly what was wrong.

Power it on, nothing but hum. twist all the knobs and push all the buttons, no change. since this is a SS, what is the prob ? I feel pretty sure this has happened to someone else, and how much is the Dr bill ?


thnx

DK

[Twist]

Is it an extremely loud hum or low level hum? Does the hum level vary as you turn the controls? Sometimes one of the main filter caps burn loose from the PC board at the solder joint or it could be a 1/4" jack or pot has come unsoldered.

[dk712]

I would consider it a low level hum. any adjustments on the knobs does nothing... the hum stays the same as when you power the amp on.

[Twist]

The closest schematic I have is a Princeton Chorus DSP. I think it's a 1999 model. It sounds like your problem is in the power supply or output IC's/transistors (whichever it has). I'm sure someone will have the right schematic on here. It's hard to diagnose unless you can take some voltage readings inside the amp. Don't give up.

[Enzo]

Isolate the problem. If the hum is fairly loudm and there is DC on the speaker leads, then the output stage is blown and can be repaired fairly simply.

If the hum is always present, but there is no DC on the speaker, look inside. Is there a pair of relatively large filter caps near the center of the board? REsolder them. If they are the problem, the amp would still pass signal.

Not passing signal?

Look for power supply trouble. or a bad op amp. Are both + and - low volt rails present and clean? Probably 15v maybe 12v. Got both at the chips? If so, go down the board and check for DC offset at the output pins of each dual op amp. Not worried about a few millivolts, but anything over say 200mv is suspicious, and several volts is definitely bad.

If an op amp output is shorted to rail, then the amp will be amplifying power supply ripple instead of signal.

[Valvehead]

Is it an extremely loud hum or low level hum? Does the hum level vary as you turn the controls? Sometimes one of the main filter caps burn loose from the PC board at the solder joint or it could be a 1/4" jack or pot has come unsoldered.

I just repaired a Princeton 65 Dsp with a loud hum---this was EXACTLY what was wrong---1 solder joint was cracked open on a filter cap.