Peavey Ultra plus won’t work after potentiometer cleaning.

[Ceredox]

Hello everyone,

I had some scratchy pots on my Peavey Ultra plus and took them apart to clean them out because I wanted to sell it. After that, I couldn’t get the preamp to output anything. I took apart the volume pot for the post of each channel, including the master. It was such an simple thing and I’ve done this before with no problems. This time, I cannot figure out what’s going on. If I plug straight into the power amp, it’s fine. If I plug into either (high or low) Input, I get a lot of popping and loud buzzing whenever I touch the master volume knob. What am I missing here? I’d like to troubleshoot it. I’ve taken it back apart 3 times now and still no change. The pots are working fine, when I desolder them.

Also: Im an electronics tech but Analog Amplification is not my strong point. Does anyone have any good recommendations to learn the art of guitar amplifier electronics?

Thanks!

[Enzo]

Troubleshooting is troubleshooting, audio or not. A system has power supply, and signal in and signal out. Could be audio, could be CMOS logic, or even a string of relays.

it worked, you took it apart, and now it doesn't work. Has happened to us all. Almost certainly it is due to something you did. Look at your connections. ANy chance a ribbon cable is off one pin?

Otherwise put a signal into the amp and follow it stage by stage through the schematic.. Where does it go away?

[Ceredox]

Thanks for the advice. I've checked the ribbon cable connector positions many times, since ive taken the board in and out a few times since ive been troubleshoooting this. BUT i never actually checked the ribbon cable continuity. They are really the only other things that were disturbed when i took the board out. When i get home from work, ill check those and the solder ocnnections of the pins the attach to. Ill let you know what i find!

[glebert]

The fact you get popping when you touch the master volume knob would make that a prime suspect of mine.

[nickb]

What glebert said^^^^

You can check the operation of the master volume pot from wiper one turning slowly looking for anomalies and also from end to end using the ohms range of your meter.

[Mick Bailey]

Maybe the pot wiper is losing continuity and leaving an open connection to the next stage. If you play an audio source through the input are you getting anything at all mixed in with the popping or hum when moving the MV pot - even very briefly?

I like Jack Darr's book (available as a free pdf) as a starting point for amp repair - it's old but the main thing is the philosophy. Jack Darr was pretty good at applying techniques across a broad range of electronic devices and his basis for many books was the approach and philosophy, rather than the specific equipment. An emphasis on technique.

[Ceredox]

Maybe the pot wiper is losing continuity and leaving an open connection to the next stage. If you play an audio source through the input are you getting anything at all mixed in with the popping or hum when moving the MV pot - even very briefly?

I like Jack Darr's book (available as a free pdf) as a starting point for amp repair - it's old but the main thing is the philosophy. Jack Darr was pretty good at applying techniques across a broad range of electronic devices and his basis for many books was the approach and philosophy, rather than the specific equipment. An emphasis on technique.

I did remove the pot and check it functionality. It seems to sweep just fine. I also checked the other pots that I cleaned and they all seem to function fine. I get absolutely nothing Out of it when i put a signal through it. Just loud popping if I'm not careful and buzzing if i touch the chassis (as if im touching an unplugged guitar cable. Would that imply a grounding issue? I did check the ribbon cables and they have continuity, took a look at the solder joints and they seem okay. I'm going to run a function generator through it tonight and start probing around. This has to be something stupid and its driving me crazy.

I will definitely check out that book! Thanks

[tedmich]

Ceredox, what exactly did you use to clean the pots? Not WD40 right ?

[Ceredox]

Ceredox, what exactly did you use to clean the pots? Not WD40 right ?

I used a light amount of Isopropyl alcohol, I remember correctly. But the pots seem to be working correctly.

[Mick Bailey]

With the chassis buzzing when touched it does look like you have a grounding issue. Meter from the guitar plug barrel to the chassis and see if there's continuity. Note that the schematic shows two ground symbols - chassis ground and signal ground. It looks to me that the preamp PCB connects the signal ground to the chassis ground at the input sockets.

[Pedro Vecino]

The amp must have ground connections through some of the screws that hold the board to the chassis.

[Ceredox]

So that was a goof on my end. Since i had taken the board out a bunch of times, i didnt have it screwed back in and grounded. So no more of that buzzing when touched. BUT my original issue is still there. No sound but when i tap on the chassis or ribbon cables anywhere, it pops like crazy. Not like its sound coming out but a loud popping sound. I'm not a big fan of this schematic. I have a 2khz sine wave running through it right now but i cant even find the stupid input on the schematic to probe around and check anything.

Edit: I found another schematic that actually labeled things and i found them. Ill troubleshoot further.

[Enzo]

It is VERY similar to the 5150.

[Ceredox]

Havent had much time to work on this but did a little last night. I'm not sure why i didn't notice this before (Maybe because i had a light bright shining over the preamp tubes) but the heater for V1 and V2 isn't lighting up. The input signal goes through V1 before anything else, so the input signal isn't even making it through that tube and stopping dead there. Not sure how this happened from me cleaning the pots but ill have to find out where that heater voltage comes from.

[Ceredox]

Well, Enzo it looks like you answered this same issue many years ago haha. Ill check out those and see what i find.