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Author Topic: Peavey Stereo Chorus NEW noise problem  (Read 856 times)

Psc

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Peavey Stereo Chorus NEW noise problem
« on: January 25, 2020, 10:02:26 PM »
Hello all,

I just bought a stereo chorus, a teal stripe one without a master volume.  It sounded good when I tried it out.  I got it home and discovered that it develops a buzz as it warms up.  It is not a constant hum.  If I mute my strings it isn't there.  If I play, the clean sound is slightly distorted and a bit buzzy.  It appears to be on both channels.

Also, as this amp has two power amp sections, one per speaker, I plugged a jack into the power amp in jack, one at a time to defeat that section.  The sound remained the same in all cases. 

I have done work on tube amps in the past, so I did some poking around in the amp.  I checked voltages at a couple of places and they seemed ok, specifically at the two larger capacitors. Just voltage, no scope. No smoking gun.  I also swapped some of the op amps around to see if I could make the problem move around but no change.

If anyone has any ideas I'd love to hear them.

Thanks!
« Last Edit: February 07, 2020, 10:52:29 PM by Psc »

Psc

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Re: Peavey Stereo Chorus noise problem
« Reply #1 on: January 26, 2020, 02:11:10 PM »

Well I appear to have answered my own question.  I hooked a different any up directly to the speakers and they buzz.  Nothing wrong with the circuit at all.  Have to figure out what's up with the speakers though.  Nothing obviously cosmetically wrong.  Also it doesn't happen just after its warmed up anymore, so that's that.

Enzo

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Re: Peavey Stereo Chorus noise problem
« Reply #2 on: January 26, 2020, 04:30:14 PM »
You might have two bad speakers.  Or maybe just one, but together in the cab they will vibrate sympathetically, so one shakes the other.  You might have a guitar pick between cone and frame in the back.  Or some other bit of debris in there.

Psc

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Re: Peavey Stereo Chorus noise problem
« Reply #3 on: January 26, 2020, 04:51:54 PM »

Thanks, yeah I've been reading that the scorpion speakers have some foam in the voice cool that dries out and crumbles which I'm pretty sure is what's going on.  Fortunately you can take the magnets off of them so I'll be able to have a good look.  It's worked for a lot of other people anyhow.

Enzo

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Re: Peavey Stereo Chorus noise problem
« Reply #4 on: January 26, 2020, 06:54:21 PM »
That does happen, so it bears checking.  But in my 30 some years as an authorized Peavey repair center, I have to say I only saw it a very few times.

Certainly won't hurt to disassemble those magnets and clean the gaps, regardless.

Psc

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Re: Peavey Stereo Chorus noise problem
« Reply #5 on: January 26, 2020, 07:42:14 PM »
It would also make sense because they were working fine at a lowish volume but once I got it home and turned it up a bit, nowhere near maxed or anything, I knocked something loose.  It probably hadn't been played in years.  It was also in storage for part of the time.  I'm hoping to pull them apart tonight.

joecool85

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Re: Peavey Stereo Chorus noise problem
« Reply #6 on: January 26, 2020, 08:25:39 PM »
It would also make sense because they were working fine at a lowish volume but once I got it home and turned it up a bit, nowhere near maxed or anything, I knocked something loose.  It probably hadn't been played in years.  It was also in storage for part of the time.  I'm hoping to pull them apart tonight.

Good luck, and let us know how it turns out!
Life is what you make it.
Still rockin' the Dean Markley K-20X
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Psc

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Re: Peavey Stereo Chorus noise problem
« Reply #7 on: January 26, 2020, 10:51:56 PM »

Well the foam was disintegrated, but the inside of the speaker was totally clean.  The cones are both great, no debris in there.  I cleaned them for good measure, but they were already good.  What I noticed though is that the coil seems to be rubbing in the gap.  I loosened off the bolts and kept adjusting until there was no friction.  One speaker is clean and clear, still working on the second.  There isn't much adjustment, so just trial and error until I can't feel friction.  Hopefully that's the end of it!

Psc

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Re: Peavey Stereo Chorus noise problem
« Reply #8 on: January 27, 2020, 12:31:29 AM »

Well this is turning into a bit of a monologue!  I couldn't get the second speaker worked out, but I figured out why.  The coil is perfectly round, but the plastic film it's meant to be attached to is lifting slightly in two places.  It's the film that's rubbing in the gap.  Tomorrow I'll get some thin ca glue and see if I can't reattach it.  If not then I'll try to stiffen the coil with the glue and gently scrape off some of the offending film.  I wouldn't bother if the coil itself didn't look perfect.

joecool85

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Re: Peavey Stereo Chorus noise problem
« Reply #9 on: January 27, 2020, 01:47:35 PM »

Well this is turning into a bit of a monologue!

No worries, we like hearing about progress. Pictures are good too!
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Psc

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Re: Peavey Stereo Chorus noise problem
« Reply #10 on: January 30, 2020, 04:41:25 PM »

Took until now to get back to it.  I took the speaker apart again and carefully cut slits into the blistered plastic.  This was the most nerve-wracking part as I didn't want to cut through the windings on the coil.  Put some packing tape on my thumb so as not to glue my hand to the voice coil, put a drop of ca glue in the hole and pressed it down with my thumb until it was dry.  I took a pocket knife and scraped off the excess glue.  Once it was smooth I put the magnet back on and adjusted.  I had to scrape it off a bit more, but got it to go back together without friction.  I tried it out before reinstalling and it seems to have worked just fine. 

I tried taking a picture or two, but they didn't come out.  It was hard to see the blister.  It was a strange thing.  It didn't look like heat damage.  I almost wonder if it was an adhesive problem. 

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Re: Peavey Stereo Chorus noise problem
« Reply #11 on: February 07, 2020, 10:48:39 PM »

Ok, so my speaker problem is solved, but I still have a buzzing sound.  It is silent if I mute the strings, but buzzes as I play.  It's a faint buzz in the background, following the pitch of the original note, but sounds like an octave off. 

I've isolated the problem to the power amps.  I took the preamp out and put it into the input of a good amp and there was no distortion.  I did the opposite, put my guitar into a preamp and put it I to each power amp input and the distortion was back.  I should note that it was back for each separate power amp. 

I know.i have the speakers dealt with because I checked them both by feeding them from a different amp, but still mounted in the peavey cabinet and there was no buzz.

If anyone has any idea where I should begin I would really appreciate it.  I'll be looking at the schematic until then!

Enzo

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #12 on: February 08, 2020, 02:00:02 AM »
The two amps have power supply in common, so look at ALL power supplies.  Are they CLEAN and relatively even in voltage?

phatt

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #13 on: February 08, 2020, 02:32:03 AM »
My thoughts if useful,
Maybe post the schematic will help.
Sounds like you might have 1 or 2 issues.
The buzz behind the note might be *Crossover distortion*, the bias in the power stage might need adjustment. (or as Enzo said dodgy supply)
Re, Killing buzz by touching strings might be a grounding issue.
Schemo would be a huge help.
Phil.

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #14 on: February 08, 2020, 06:50:33 PM »

I emailed peavey and I have to say that their customer service is excellent.  The emailed me the schematic package I've attached below within a few hours. 

One note, my amp doesn't have the master volume shown on the print, but otherwise I think this is correct.

I should have been more accurate, if I damp the strings and let go of them I don't get a buzz, not until I pluck a string, so I don't suspect a ground issue.  I've checked voltage at a clue of spots when I started all of this, but not enough to rule anything out.  I believe I looked at voltage near the four large electrolytic capacitors, and didn't find a smoking gun.  I'll have to open it up again.

Thanks for responding.  I'd love to get to the bottom of this.