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Author Topic: Peavey Stereo Chorus NEW noise problem  (Read 1100 times)

Psc

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #15 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.

Psc

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #16 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.


Psc

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #17 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? 

willpirkle

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #18 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.
« Last Edit: February 09, 2020, 06:26:08 PM by willpirkle »

Psc

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #19 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.

willpirkle

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #20 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.

Psc

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #21 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?

willpirkle

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #22 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).

Psc

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #23 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. 

Psc

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #24 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. 

phatt

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #25 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

Enzo

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #26 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.

DrGonz78

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #27 on: February 15, 2020, 02:34:57 PM »
Here is the Stereo Chorus 212 1989 version...
“A person who never made a mistake never tried anything new.” -Albert Einstein

Psc

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #28 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.

Psc

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Re: Peavey Stereo Chorus NEW noise problem
« Reply #29 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.