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Solid State Amplifiers => Amplifier Discussion => Topic started by: Psc on January 25, 2020, 10:02:26 PM

Title: Peavey Stereo Chorus NEW noise problem
Post by: Psc 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!
Title: Re: Peavey Stereo Chorus noise problem
Post by: Psc 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.
Title: Re: Peavey Stereo Chorus noise problem
Post by: Enzo 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.
Title: Re: Peavey Stereo Chorus noise problem
Post by: Psc 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.
Title: Re: Peavey Stereo Chorus noise problem
Post by: Enzo 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.
Title: Re: Peavey Stereo Chorus noise problem
Post by: Psc 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.
Title: Re: Peavey Stereo Chorus noise problem
Post by: joecool85 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!
Title: Re: Peavey Stereo Chorus noise problem
Post by: Psc 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!
Title: Re: Peavey Stereo Chorus noise problem
Post by: Psc 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.
Title: Re: Peavey Stereo Chorus noise problem
Post by: joecool85 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!
Title: Re: Peavey Stereo Chorus noise problem
Post by: Psc 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. 
Title: Re: Peavey Stereo Chorus noise problem
Post by: Psc 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!
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Enzo 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?
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: phatt 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.
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Psc 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.

Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Psc on February 08, 2020, 08:16:17 PM

I just listened to a clip of crossover distortion and it does sound a bit like it.  The clip I heard was a severe case, where this is considerably milder sounding.
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Psc on February 08, 2020, 10:01:53 PM

I've done a bit of reading about crossover distortion.  Is the best way to test whether this is my problem to measure the voltage across the resistors coming off the emitter and then calculating current?  I'm assuming my problem is more related to the power supply as you mentioned.  It doesn't seem likely that the same component in each power amp would go at the same time.

Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Psc on February 09, 2020, 01:14:34 AM
I had time to check some voltages.  Here's what I got.

Across C82 and C84, the two 4700 uf capacitors: 42.3V and 42.4V

Across C114: 15.36V
Across C120: 15.56V

Across CR40: 4.93V
Across CR49: 4.90V

So the + and - 5 volt is a bit low.  Would that do anything? 
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: willpirkle on February 09, 2020, 06:20:53 PM
Get an oscilloscope to look at the output waveform - crossover distortion will be easy to see. If the output transistors are biased just into conducting at idle, then there should be little/no crossover distortion. While you can measure the voltages across the emitter resistors, the scope will tell all.
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Psc on February 09, 2020, 08:22:51 PM
Ok, I think I can get a scope from work tomorrow.  Just to clarify, am I attaching the scope to the speaker leads?  It's been some time since I've used a scope.
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: willpirkle on February 09, 2020, 09:00:22 PM
Honestly, if you haven’t used the scope in a while then I would be weary of poking around the output section of a power amp without some guidance. The scope probe has a ground ring and can easily short stuff and damage your amp if you don’t have the protective sleeve (condom) for it. Or if the alligator clip snaps off and drags across the PCB. I’ve done it - not fun.  :(

To test the power amp alone, you’ll need to run a sinusoid through it via the fx return. Do you have a dummy load to connect to the output of the amp? Then you can run a signal into that rather than the speaker and look with the scope hooked across the dummy load. That will keep you out of the amp chassis and is safer. You can also probe the output jack (speaker leads - be careful) with no load connected, but my old boss (guru) always made us test for that with a dummy load. With the speaker itself you are going to get a noisy signal and the crossover distortion “glitches” do not necessarily occur in the center of the waveform, so might be hard to see, or obscured. Be really careful and get a dummy load if you can.
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Psc on February 10, 2020, 02:58:55 PM
For a dummy load are we just looking for a higher watt resistor?  I have access to lots of that kind of stuff.  I assume I need 4 ohm of resistance able to dissipate 150 watts or so?
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: willpirkle on February 10, 2020, 06:37:23 PM
Yep. That’s it. But you can use 8 ohms or even 16. You won’t need to crank up the amp to see crossover distortion (and you get the noise at relatively low volume I believe).
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Psc on February 10, 2020, 07:09:12 PM
Yes, that's correct.  I've got a bunch of resistors that will do the trick.  I also got a small portable scope that should do the trick.  I'm trying to get my hands on an old techtronix scope, but it might not be for a day or two. 
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Psc on February 11, 2020, 11:54:02 PM
Well, I am having a bit of trouble with my mini scope, but I did find something that seems odd to me.  I disconnected one of the speakers and attached the the dummy load, then fed a sine wave into the power amp.  I was surprised to find that I could hear the sound come from the still connected speaker.  It was quieter and not as clear as when you put it into the other input.  I also tried reversing it and got the same result.

Should the signal bleed to the other section?  I would have assumed it wouldn't.  But maybe it's normal?  It does cut out if I put a jack in the other power amp in. 
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: phatt on February 12, 2020, 06:35:02 AM
Yes the 2 Ch's are interconnected via the switching sockets, as shown on schematic.
Signal at Pre out sockets also crosses over to the other poweramp via internal switching,, look at links around R70 & R80 for clues.
So yes nothing to worry about.
I'm wondering as Enzo already mentioned, a Common supply to both Amps.Hum?
The chances of the identical issue on both power amps is rare but still possible.
Maybe keep monitoring those 42 volt rails while the amp is working to see if one of those rails fluctuates. I'm not the expert but I can only assume that if one side sags too much it could cause the bias to change giving the crossover fizz,, on both circuits.
BTW, in case you are not aware the vertical string of diodes (on the schematic) running down from the Collector of Q119 sets the bias. The dotted line around CR148 normally denotes that it is strapped to the heat sink.
Maybe check that both are intact and in full contact with the heat sink.
Phil
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Enzo on February 15, 2020, 07:25:30 AM
Peavey is great.  Call them again and ask for the Stereo Chorus 212 1989 version schematic.  Your 1992 version schematic is different though similar.
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: DrGonz78 on February 15, 2020, 02:34:57 PM
Here is the Stereo Chorus 212 1989 version...
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Psc on February 18, 2020, 12:03:54 AM

Thanks for all the great info.  I really appreciate it. 

My mini scope is acting up and I've had problems getting the other one.  Hopefully tomorrow.  There's one guy who can approve it and he's been away.
Title: Re: Peavey Stereo Chorus NEW noise problem
Post by: Psc on February 29, 2020, 07:49:01 PM

Well hate to say it, but I have blown my speakers.  The coil rub kept getting worse, then I blew one.  So that's that.

BUT!  I posted locally about finding speakers in someone's closet and a guy is trying to sell me a functional JC 120 for $120 Canadian.  Haven't had the conversation with my wife about dragging another amp home yet though.  One of the speakers isn't working, but all of the amp does apparently.

Still looking for speakers though.  I'm considering making both amps into heads and just building a cabinet I can use for both.