Fixing old Peavey C30, still getting trafo Hum

[Mystovski]

Hi,

Hope everyone is well, that's my first post here. I've done some searching but the best thread was from 2015.

I bought my C30 in a slightly poor condition but managed to fix a few things, however still struggling with mains hum. I figured out that the hum is not related to the master volume and also after removing power amp tubes the hum is still there! Does it mean I need a new transformer? Do you think it's just being old, or do I need to worry about it breaking down? I want my gear reliable as possible as I gig regularly. The intensity of the
hum is rather low, clearly audible when playing at home (like in a blues movie) but it's not there during gig/rehearsal because it's negligible (or just not there) compared to the actual guitar volume.

Fixes that I've done so far:

• Replaced the two diodes on the tube PCB to iGP02-40 4k (speaker pop on power on and off is gone)
• Replaced the dodgy jumpers with some fresh wire
• Quick inspection for cold joints and dodgy connections. Solderless pcb plug for reverb is interesting, the wires are just pushed in and sharp brackets cut through the sleeve so connection happens. Worth redoing if someone squeezed that in 30 years ago.

Jumper wire is a big thing in CL30. In mine, one jumper has burned during a gig, burning a big black dot in the PCB and of course sending the amp on holiday...

I know it's generally good idea to recap them, however I'm sure someone has done it already and there are no spills or ugly ones. Found a good video of a full repair.

Many Thanks
Luke

[Enzo]

Hi, welcome.

As a tech, the term "transformer hum" doesn;t mean anything. Do you mean the transformer itself physically hums/vibrates? Or do you mean a hum sound comes out the speaker. If the speaker, the transformer provides the powr to all circuits, so ultimately in any amp, any hum originally was in the transformer.

A little vibrational hum is normal in zillions of amps.

You get hum in the speaker even without power tubes? That would be darn unusual. The transformers in this should not be coupling, which is one cause of such a symptom.

You are in the UK, so your power is 50Hz. Important to know if the hum you hear is 50Hz or 100Hz.

Isolate the problem. The output transformer primary has a three wire connector to the circuit board - red, blue, and brown wires. Pull that connector off, does your hum stop? Put it back.

There are three fuses, two on the board. On the board is a low voltage heater fuse, and a high voltage fuse. HV on the red wires. Either pull the HV fuse or unplug the two red wires from the board. That removes high voltage from the circuit. Hum gone or stay?

Plug a guitar into the FX return and dial its volume to zero. HUm still?

Pull one of the three small tubes, they all go dark now. Did the hum go away?

We need to know WHERE the problem is, then we can consider WHY it happens and WHAT to do with it. I have more W words...

[Mystovski]

Hi Enzo. Thank you.

There's no significant difference after removing the OT or a tube in preamp or playing through the FX loop. If theres any hum getting into the actual signal, that would be very negligible amount.

I noticed that after removing the high voltage wire, a high buzzing overtone goes away. Two things were happening at once making it hard for me to interpret by ear. So the power transformer is mechanically producing what sounds like 50hz hum which is not extremely annoying and I probably managed to reduce it slightly but tightening the enclosure screws a bit so they hold it down better. Are they just made that way? If it's reliable, then I'm fine.

The interesting bit though - after putting the red wire back in, I am getting some sort of a buzzing noise that's slightly similar to the sound of an old fluorescent bulb (that is being a slight nuisance). I was trying to find the source of it, it seems to be on the PCB somewhere around where the fuses are. Could the rectifier diodes do it?

[Mystovski]

Hi All,

Just to share the solution.

A small ceramic capacitor in anode power supply circuit (C38) was causing the high buzz noise which I found by pressing high voltage supply elements with a chopstick. I tried to redo the soldering points but interestingly it was the element itself causing the nuisance.

To address the trafo noise, I tightened the bolts holding it together and then tightened all bolts of the enclosure.

Together it gave me a very decent decrease in noise levels, happy now. Thanks for your support.

L.