120 Hz Hum in Solid State Amp

[Fred2202]

My nephew brought me his ailing ASI solid state guitar amp to take a look at. It is a solid state Q-tube 70 guitar amp probably from the 1980's. It has a single 12AX7 in the preamp (along with a bunch of op amps) and a Mosfet power section. It has a low level hum that sounds like 120 hz AC leakage. The volume control does not seem to effect the hum level, so I suspected the large electrolytic caps in the power section were shot due to age. The amp is laid out on about four separate circuit boards so I disconnected the preamp board from the power amp and the hum remains.
I replaced every large and small electrolytic cap on the power amp, and for good measure replaced the bridge rectifier and some small IN4007 type silicone diodes. Tried the amp out and the low level hum is still there. I went back in and reflowed all of the solder joints and checked the grounds and they looked good. Checked all the power resistors around the power transistors and the readings look good. After all this the amp is still humming.
At this point I have run out of ideas as to what to check next. I do not have a schematic for this amp and I have not fooled around with transistor amps very much. The amp seems to have plenty of power (I think it is about 70 watts and is loud as hell) and sounds good (for a solid state amp) except for the low level hum.
There are some small non-polar caps on the power amp board that may be coupling caps... I was going to try replacing them next.
Could a bad output or driver transistor cause this type of hum?? Any suggestions for troubleshooting would be much appreciated.

[Fred2202]

Another thing I forgot to mention in the post above is, this amp makes a strange sound when once power is shut off. If makes a putt putt sound that fades off like a capacitor or something is discharging. It does not do it when the amp is turned on.

[Enzo]

Bad transistors don't make hum as a rule, and the power off oscillation is nothing.

It could just be weak design, you know. I'd be looking for a ground return somewhere not making good earth. Power supply ripple currents sharing copper with signal grounds.

[Mars Amp Repair]

The ususal questions apply,
does the hum change at all with the volume or any of the controls up or down? That might help to isolate the stage the hum is being introduced...if it is a ground loop as Enzo mentioned, then things begin to get a bit difficult to trace.

It may also be possible the the power stage is oscillating at an inaudible frequency & loading the power supply to create the hum. However if this were the case, the power stage would most likely be getting verrrrry hot.

If you have one, hang a scope on the speaker output leads (ground your scope to the chassis, not the negative speaker lead!) & see if you have an ultrasonic frequency there.

Those problem can sometimes be real buggers, too. glen

[Fred2202]

Thanks very much for the replies. The hum stays at pretty much the same volume level regardless of where the volume knob is set. That is why I suspected bad power supply caps. There is only one ground off the power section and power amp circuit boards to the chassis. I probably could separate the ground off of the power supply section to see if that will help.
I did change out a bad output jack to the speaker. Put in a non-insulating type that grounded to the chassis and the amp squealed like a pig. I changed it back to an isolated type like the original jack and that solved that problem.
I do not have a scope...but I did turn the amp on, hooked up a cd player and let it run on 1/3 volume for about three hours. All of the components including the transformer were only moderately warm to the touch.
The amp did work flawlessly up until a few months ago. That is why I thought there is a bad component somewhere in the power amp section. All the caps and rectifiers/diodes in the power supply are new at this point. I will take another look at the ground connections and see if I can separate the ground from the power supply section and power amp section and put connect them to different points on the chassis.