Peavey Classic 30 problem

[mozz]

I don't like circle within a circle.

I have seen cracked traces hiding next to solder joints, these days i use a magnifying glass loupe, 8x and i search the entire board.

[Smada]

Thanks for your help eschertron. Yes with the amp on and out of standby, with one probe on ground and the other on the plates (pins 1 and 6 for the 12ax7) I got those DC readings. I haven't actually tested the amp since I messed around with the tube socket and I will make it my next move. I will do that and answer the rest of your questions but it will have to wait till tomorrow. Thanks!

[Enzo]

Plates all have like 1 volt? Is the fuse open? You have two fuses on the end of the circuit board, one is higher current and probably ceramic instead of glass. That is the heater fuse. The 1A glass fuse is the high voltage fuse. Always remove fuses from the clips to test.

[eschertron]

I will do that and answer the rest of your questions but it will have to wait till tomorrow. Thanks!

I guess my followup question would be when you reported in post #1 that there was a drop in volume, how severe was it; and under what conditions? Intermittent? Suddenly and permanently? I probably have the wrong idea in my head about what your experiencing.

[Shawnobi]

How are you measuring the tube pins? That amp is a pig for live working as you have to remove everything and unwrap the PCB. Are you measuring in this state with the tubes and everything else connected?

On another point gently tug all the link wires between the PCB's and check they aren't broken. Do that again when you refit.

[Smada]

The volume drop is quite severe, both pre and post knobs could be at 6 and it would output bedroom practice levels, whereas before you could gig at those settings. The volume drop is constant, the amp does not start at its ordinary volume anymore, and the problem is getting worse as time passes.

I plugged in my guitar today and the amp still works but there is now a loud buzzing which was not there before.

The plate voltages seem to be between 0.2 and 0.3 VDC. I have a rubbish usb oscilloscope so I decided to attach it to the plates of the preamp and here is the picture.



This buzzing was not a problem before so i am not sure what has happened.

[Smada]

Yes Shawnobi it is all unwrapped but with everything plugged in, a pig to work on indeed!!!

[Shawnobi]

Confirm Fuse F2 is good an nothing has shorted with the chassis out.
B+ should be around 330VDC
V1 pin 1 should be 160VDC, V2 pin 1 should be 150VDC.....

Also, on the middle PCB at one end there are 2 ceramic power resistors. R71 & R72. Check the connection either side of these is good.

[Smada]

I think perhaps I have removed the ground from the PCB? The only ground connection I can see on the chassis is directly from the mains cable but this is pre input transformer.

[Enzo]

The various metal bushing jacks pull the grounds together. if you try to fire it up with the circuit board out of the chassis, you will need a bunch of clip wires to ground those jacks to chassis.

[Smada]

So I attached a wire from the negative side of C41 to the chassis to act as ground and the buzz has disappeared. So the plate readings I now get from the pre amps are:

V1: pin1: 179 pin2: 172
V2: pin1: 170 pin2: 187
V3: pin1: 193 pin2: 232

These look in the normal range to me?

[copperheadroads]

Terry from D-lab has an interesting video about the Classic 30 i think the problem was loud hum .I know it maybe not related to your problem I thought I'd share .
https://www.youtube.com/watch?v=IpDcRPAjLP8

[Smada]

Hi guys I was just comparing the schematic of the peavey classic 30 to what I have in front of me and I have come across a couple odd components that I wanted to ask about. In two places (there may be more I haven't checked), capacitors C1 and C10 are the examples, instead of seeing what I expect a capacitor to look like I find a small green resistor like thing. Here's a picture of C1, I apologise for the poor quality and can get a better one later if need be.



Are these a type of capacitor I have not come across yet or is something not quite right?

[52 Bill]

Are these a type of capacitor I have not come across yet or is something not quite right?

Yes, they are ceramic capacitors. Peavey has been using them for 20-30 years now. They confused me the first time that I saw them too.

You read the color bands just like resistors for the value, tolerance, etc.

[Enzo]

Color bands like resistors, except the result is picofarads instead of ohms.

And keep in mind the amp worked for 20 years before now, so it would be unlikely those parts were "not quite right".