Peavey XXL 60hz hum

[misterc57]

Peavey XXL 60hz hum

My crazy ideas and story.

I have an XXL combo chassis (unit 1) in good working order that I was going to build a head cabinet for. Last week I found a non working XXL head (unit 2) with burnt components on the amp board.

The combo chassis (unit 1) does not fit into the head cabinet so I decided to try swapping the burnt out amp board in unit 2 with the working board from the combo chassis (unit 1).

I ohm'd the PT and OT from both units and they had identical readings.

After getting the good amp board installed into unit 2, and all volumes at zero, I slowly brought up the voltage on the variac and immediately (at low voltage) was hearing a loud 60 hz hum which would get louder as the voltage increases. I quickly backed off on the variac.

There were a lot of missing screws connecting the back panel and OT to the rest of the chassis, I did replace most but not all (mentioning this in case there is a ground problem).

Before I abandon this idea and go back to my original plan (build a head cab for the combo chassis) I thought I would check on here for ideas on what the problem could be.

I have attached pics of the original burnt out board (in unit 2) in case that is a clue to other potential problems.

Thank you!

[Enzo]

MAny amps are unstable at low voltages. Always power up with NO load first, no speaker. The loud hum could have been DC on the amp or any number of things. By bringing it up with no speaker load, you can monitor the amp. What matters is whether the thing is drawing excess current from the mains. SO monitor mains current while you variac. If the current ramps up, we have an issue, but if it doesn't draw a lot, we can power all the way up and see what gives. There may be nothing wrong with it other than it isn't right at 40 volts.

You appear to have a burnt up R38, which tells me U2 is likely shot, and probably your power transistors.

[misterc57]

Hi Enzo.

You were spot on with that. Amp is working at full voltage with no more 60 hz hum.

Now there are other issues. Any thoughts on these?

The two gain channels have all sorts of noise in them (at any voicing). Sounds like a wind/wooshing sound. Also there is a burnt smell coming from somewhere, I cannot tell where. Clean channel seems to make an occasional noise.

I found that the NJU211D on the clean channel preamp board is hot to the touch (not sure if it should be). There was also a black mark drawn on the board next to that chip (perhaps someone identified it previously as an issue?).

Not sure where I want to go next with this. Options are;

1. swap all the control boards from the working chassis into this one
2. abandon this unit and go back to building a cab for the working unit
3. try to fix the issues on this units preamp boards

[misterc57]

Hi Enzo.

You were spot on with that. Amp is working at full voltage with no more 60 hz hum.

Now there are other issues. Any thoughts on these?

The two gain channels have all sorts of noise in them (at any voicing). Sounds like a wind/wooshing sound. Also there is a burnt smell coming from somewhere, I cannot tell where. Clean channel seems to make an occasional noise.

I found that the NJU211D on the clean channel preamp board is hot to the touch (not sure if it should be). There was also a black mark drawn on the board next to that chip (perhaps someone identified it previously as an issue?).

Not sure where I want to go next with this. Options are;

1. swap all the control boards from the working chassis into this one
2. abandon this unit and go back to building a cab for the working unit
3. try to fix the issues on this units preamp boards

The hot to the touch IC is U3 (DG211 CJ) on the pre-amp circuit.

[DrGonz78]

Have you tested the preamp voltages that feed the chip yet? If the chip is getting hot to the touch then it might be bad and that might cause issues with the power supply that is feeding the chip.

[misterc57]

I had proper supply voltages at all the DG211 chips. This was the only one running hot. I replaced it and the amp is operating perfect now.

Thank you to all! I often need validation for things I am trying and appreciate everyone's input!

At some point I may try to fix the burnt out amp board that was swapped out.

[Enzo]

I can't speak to your experience level, but as a professional amp tech, I would enter into repair of that burnt board without fear of failure. A little alcohol and a Qtip will clean most of the soot away, then plain old troubleshooting will get the amp running. It looks dramatic, but it will be easier than it looks.

[misterc57]

I consider myself a repair hobbyist. I have a big interest in amplifier electronics with no training. I am always learning!

Thanks for the encouragement, I will start on that board next.