Peavy Backstage Chorus 208 - Ground Issue

[map747]

Hello, New member here.
I recently purchased a Peavy Backstage Chorus 208. When I touch the input jacks, I hear a hissing/crackling sound. Also, when touching the top of the cabinet, I can hear a very slight change in tone from the speakers. Is there something I can do to make sure I have a good ground? What else could I check? I have moderate electrical skills...
Thanks,
Michael

[TomCarlos]

Hi Michael....

You mentioned a "good ground..."

Let's start with that. Do you have a 3 prong electrical cord? With the cord unplugged (of course it would be unplugged), do you have continuity between the end of the cord and the green wire that attaches to the amp's chassis?

Next, the PC board most likely has screws that attached the board to the amp chassis. Those screws can provide a ground to the circuit. Are any of those screws missing?

I think this is the schematic - Peavey Backstage Chorus 208

Tom

[map747]

Thank you for the reply and the schematic Tom!

I do have continuity between the green wire and the ground of the plug. Also, all the screws appear to be in place for the board. Do you have a next step I could try?
Grateful member,
Michael

[DrGonz78]

Also, when touching the top of the cabinet, I can hear a very slight change in tone from the speakers.

I assume this means touching the metal chassis and not wooden cabinet? Check that the transformer nuts and bolts are tightened up good to the chassis. Does this noise/crackle occur in both channels? Also, when moving tone controls does this have any effect on the noise?

[map747]

The transformer is tight. The metal chassis is microphonic when on the lead channel. When the volume is up, all you have to do is tap the chassis and you can hear it through the speakers.
I did a search on google and found the following:
Hey y'all. I have this Crate Vintage Club 50 with the 3 x 10" speaker configuration. It has 412AX7s and powered by 4-EL84s. It's been popping, crackling and hissing, and the lead channel wouldn't work all the time. I could isolate a few bad preamp tubes, but since I had them, figured I'd retube the entire amp. It was time.

Granted, tightened and cleaned tube sockets, installed new tubes and still the same problem. Basically, tapping anywhere on the amp is noisy and sounds no different than tapping on a tube that's microphonic. I've done a visual inspection, checked ground connections, pulled various tubes to isolate this problem. But, never run into an entire chassis that's basically microphonic at this point. So, I've stepped back from it at this point to get a fresh perspective. And also, mention it to you guys out there that have run into this, or simply know what I'm missing. What do y'all think? Any input on this subject would be greatly appreciated. Thanks.
Answer:
I've seen this before. I found it to be an open electrolytic in the power supply. An open electrolytic will allow crosstalk between in-phase sections of the amp and make certain stages hyper sensitive to mechanical movement. It would also explain the popping and crackling you are experiencing.

Reply to answer:
Jerry! You are an absolute live saver! I have been working on this Crate V3112 for 2 days straight now. We were tapping the tubes, tapping the chassis and getting a horrible crackling noise almost just by looking at the darn thing. The first thing I did was put all known good working tubes in it. When that didn’t change anything I was stumped. The second day working on it and getting nowhere I decided to take one more look online before throwing in the towel. After reading your post I first replaced the big filter caps, no luck, then I replaced the smaller ones that followed the main ones. The amp works perfect now even with the old warn out tubes in it. I would have NEVER figured this one out without your help –

What do you guys think?
Michael

[TomCarlos]

Look at the Schematic... You have a solid state amp. What you posted is for a Crate tube amp. There is no comparison here.

If you are not familiar with electronic circuits and troubleshooting, that could be an issue. But at least we know one thing... you are saying this problem is with the lead channel.

This amp has a Low and Hi gain input. Those inputs eventually feed the same path. And it looks like there is a switch on the front of the amp that takes you between the Chorus Channel and the Lead Channel. I believe the footswitch also changes the channel.

So... if the problem is only with the lead channel, that narrows things down a bit.

You need to be very careful when poking around in the circuitry while the amp is turned on.... and that is why we use wooden chop sticks or some other non-conductive poking device. What happens if you tap on the SuperSat or Post pots. Do you hear anything? Try poking around a bit to see if you can find an obvious culprit. I would then pull the chassis and look at the solder joints to make sure everything is in tact. And of course, while you are working on the amp, I would use the DEOXIT F5 (cleaner with lube) to clean the pots.

See if you can narrow this down a bit more....

Tom

[map747]

I will try. I realize it was a tube amp, but could "an open electrolytic in the power supply" cause the same problem with a solid state amp?

[TomCarlos]

It could.... but it looks like you are experiencing the problems with the lead channel and not the chorus channel. If you had a bad cap in the power supply, you'd hear the problem with both channels. If the chorus channel is clean and you get plenty of volume, the problem is most likely with a component in the lead channel.

[The Dude]

A few things:
1) You said, "When I touch the input jacks, I hear a hissing/crackling sound". Are you talking about the nut/ground of the jack on the amp chassis, or the tip of a guitar cord plugged into the input?
2) What you describe in post #1 is "hissing/crackling" and then post #5 "microphonic". Is it one of those...... both? That's two different descriptions usually associated with different problems.
3) It's a stereo amp. Does the noise come from both speakers or just one? Temporarily unhook one or the other if it's hard to tell. If it's only one, the problem is likely in the amp circuit for that one amp. If the noise is in both, it's likely either preamp or power supply.
4) It seems like you're describing an intermittent connection issue. The quickest route may simply be to pull the board out and check for cracked/cold solder connections. Check particularly anything that comes through the chassis- jacks, pots, switches, etc.

[DrGonz78]

Please answer question about whether any tone pots have an effect on the hissing/crackling noise. I always admired the way Peavey soldered those old boards, very solid. Still things can get cracked or need touch up soldering.

Could even be a farty old speaker making noises too. How does the amp sound in the clean channel turned up really loud?

[map747]

Haven't been able to check all these, but I hope to get time tomorrow. Thank you for these great tests and suggestions. I feel like I will get to the bottom of this with your help.
Michael