Peavey Classic 30 squealing

[Albert Kreuzer]

Hi all,

C30 came to me with some intermittent issues, otherwise working normal.
After some resoldering and retensioning: No more intermittents, but a high obnoxious squealing (in the kHz range).

My observations so far:

1. Noise does not react to volume knobs, but gets much louder when I switch to the drive channel.
2. Inserting a shorting plug into the return jack makes it stop.
3. Pulling V1 makes it softer (until it fades because the heater string is interrupted).
4. Pulling V2: no change of squealing (until fade out)
5. Pulling V3: Squealing stops immediately.
6. Changing preamp tubes: No joy.
7. And yes, the OT is connected correctly (tried that already).

Can anyone please guide me into the right direction? Pulling this thing apart every time is a real pain.

Thanks,
Albert

[Enzo]

Does turning the reverb down help? A misconnected reverb cable can cause trouble. And if the reverb circuit starts howling, it could get into the -30v rail, which COULD inject it into the signal path via Q2, the loop driver. Does pulling the Molex connectors off the board at the amp end of the reverb cables stop it?

Look at the three main board sections. They are linked together by a bunch of short bare wire jumpers. Get under each one with some tool and tug. If any come away, they were broken already, and need repair.

You did half the test. Shoving a shunt into the return silences the amp, but run a cord from the send to some other amp. Is the oscillation still on there?

Scope the B+, is the osc signal there?

Note the B+ node for the preamp input stages has its filter grounded elsewhere than the other B+ caps. Possible open ground return.

Note this amp uses at least four grounds, the triangle and the rake, each with or without a circle around it. Bottom center of schematic, just to right of bias supply and just below EL84s, a pair of 47 ohm resistors connecting grounds together. FInd them and measure. They will probably measure shorted as there are other ground connections in parallel. But an open one can cause trouble.

[gtrplayr1976]

Have you tried swapping preamp tubes ? V2 with V3 ect

[Enzo]

Here is the whole schematic file, the trace art is visible on the layout. Might make tracing connections easier.

[Albert Kreuzer]

Does turning the reverb down help? A misconnected reverb cable can cause trouble. And if the reverb circuit starts howling, it could get into the -30v rail, which COULD inject it into the signal path via Q2, the loop driver. Does pulling the Molex connectors off the board at the amp end of the reverb cables stop it?

Bingo. Reverb was turned down, works normally when turned up, no change in squealing. Pulling the reverb connector: Squealing gone.
Verified the connector was connected right.

What now?

Look at the three main board sections. They are linked together by a bunch of short bare wire jumpers. Get under each one with some tool and tug. If any come away, they were broken already, and need repair.

Done that before. Fixed one flakey jumper, resoldered all of them.

You did half the test. Shoving a shunt into the return silences the amp, but run a cord from the send to some other amp. Is the oscillation still on there?

Yes.

Scope the B+, is the osc signal there?

Note the B+ node for the preamp input stages has its filter grounded elsewhere than the other B+ caps. Possible open ground return.

Note this amp uses at least four grounds, the triangle and the rake, each with or without a circle around it. Bottom center of schematic, just to right of bias supply and just below EL84s, a pair of 47 ohm resistors connecting grounds together. FInd them and measure. They will probably measure shorted as there are other ground connections in parallel. But an open one can cause trouble.

Not tested yet. Back to the shop. It's 1:00 am here. Sigh.

[Enzo]

So removing the reverb cable from the amp cures the oscillation.

Is the reverb pan the right type - 4EB2C1B? OUTPUT jack grounded to the pan body, INPUT jack insulated from pan body.

FOur reverb cable conductors, 5-place connctor, right? Unused pin hole keyed to match pin spacing on board so can;t invert connector, right?

Verify the cables are in the right holes on the pan. Turn the amp on, reverb up some, with two plugs pulled out of reverb pan. Touch plug tip of each with your finger. One should hum out the speaker. That one goes to the OUTPUT jack on the pan.

The drive end cable - goes to pins J6 and J29 on the Molex connector. NEITHER should have continuity to ground.

[Albert Kreuzer]

Found it.

Checked the 47 Ohm resistors. Both good. Reverb pan: Stock, everything ok with grounding.
Then I started to systematically check the different ground points. First the "triangle without circle" ones.
Discovered a missing ground from the "Post" pot - broken wafer, not visible to the eye.
New pot, everybody happy

Thanks Enzo, the "different grounds" thing brought me onto the right track.

Cheers,
Albert

[Enzo]

Great!

[pecorporation]

I forgot what I have learned.

[Enzo]

And let that be a lesson to you.

[Albert Kreuzer]

And let that be a lesson to you.

Not sure what you want to tell me
I learn something new every day

[Enzo]

SOrry, Albert, that was not aimed at you. I was just making a smart ass reply to pecorporation, after he poked fun at my signature line.

[Albert Kreuzer]

Ah, ok - that was my first thought too, I just wasn't sure

You cannot forget what you haven't learned

[Enzo]

According to my wife, I NEVER learn...