Peavey Studio Chorus 210: one channel not working, possible chorus prolblem

[blckmynnse8]

hello everybody... i discovered this forum last night after attempting a google search for a free schematic to this particular amp (which i was unable to obtain). i just started a side job as a technician at a local music store.



on the amp in question both speakers are good, and when i use the headphone jack i get the same result: one channel functions properly with the other one being barely audible.

when i try the chorus function i'm not getting any sort of "phasing sound". all of the potentiometers in this amp appear to be in rather immaculate condition.

not having had any prior experience with this amp, i opened it up and did not find any burn marks or anything out of the ordinary... except i had seen where the previous owner had poked around inside a bit and reinserted one of the ic's with one of the legs bent outside the socket therefore not making contact.... the ic in question appears to be involved with the chorus circuit which does not appear to work. naturally, i re-inserted it properly. apologies for not having noted which ic this was but i can have this information when i get back to the shop.

regarding the issue of the non-functional channel i did the best that i knew how without a schematic, which was to try and compare measurements of the transistors in the bad channel to ones in the good channel... i did this without power using the diode checker in my meter. off the bat i wasn't able to encounter anything significantly different when comparing the two channels. i did not measure any voltages of the transistors either. everything functions fine except for one channel, and the chorus.

any help would be GREATLY appreciated. i can have more specific answers to any questions after getting back to the shop today (friday) where the amp is.

thanks in advance,
mike

[Enzo]

Contact Peavey customer service for the schematic. DO you have the original SC 210 from 1988 or the later 1990 version. Look on the serial number plate. If it says Stero CHorus '90, it is the later. I will assume the older one for discussion, details may vary.

Like most chorusing amps, this one has the two separate power amps. In dry mode both amps carry the same signal. In chorus mode one amp carries the dry signal, while the other carries the chorused signal. That allows the two speakers to work offf one another for a richer sound than simple chorus added in to both as would happen with a chorus pedal out front.

If that power amp dies or the feed to it, then it will make no sound either way, and the chorus will be lost because it only comes out that speaker.

Isolate the problem. Plug a signal - guitar, whatever - into each power amp in jack. Do both work or does one not? If not, we have a dead power amp, but if it does you have a preamp problem.

Likewise, plug a cable into each pream pout jack and send it to another amp for a listen. Are both working or is one dead? A dead one means a preamp problem. Don't assume if you find a dead power amp that the preamp works, make sure.

[blckmynnse8]

First off... Enzo I've done some lurking about in this forum today and I've noticed that you seem to offer quite a bit of your time/knowledge. As a person who's been doing tech work in other industries but who's only just beginning contract/freelance work with music gear I'd like to thank you and others like you for sacrificing your personal time to help others.


ANYWAYS... I just got back from the shop and here is what I've found:

Following your instructions:

The s/n tag says: model- studio chorus 210

BOTH preamp outputs are GOOD.

BOTH power amp inputs are GOOD.

Just for the sake of completeness, I'll say again that BOTH speakers are GOOD.

+/- 15V supplies are GOOD

I'm getting +/-32V on the cases of the large power transistors.

I'm seeing both + and - 15V on the op-amp ic's.

I verified these functions several times to be on the safe side.

This is what I get when using the amp "normally": I plug a guitar/frequency generator/etc... into either of the inputs on the front [high gain/low gain]... and I get a strong signal from the output with the red/black wires, and an EXTREMELY weak signal from the output with the yellow/black wires. This result is the same whether I'm listening with headphones or the speakers. Turning the rate or depth potentiometers of the chorus function make absolutely no changes in the sound whatsoever... there is absolutely no "phasing" effect.

[Enzo]

Both preamp outputs send a good signal out. Good

Both power amp inputs are good, which means to me that sound does come out the "bad" speaker when signal is input to the power amp jack. Right?

SO now take a spare cord and plug one end into the preamp out of one channel, and the other end into the power amp in of the same channel. Apply your test signal to the input jack on the front as normal. Does sound come out that speaker? Now move this jumper cord over to the preamp out and power amp in of hte other channel. Does that speaker now make sound?

If both power amps drive speakers from the power amp in jack, and both preamps output from the preamp out jack, then the likely problem is a bad cutout contact on the offending power amp in jack itself.

[blckmynnse8]

Thanks. Last weekend was a busy one so I haven't made it back to the shop yet. I'll be there early this evening and hopefully this problem will get pinned down.

[blckmynnse8]

Both preamp outputs send a good signal out. Good

Both power amp inputs are good, which means to me that sound does come out the "bad" speaker when signal is input to the power amp jack. Right?

SO now take a spare cord and plug one end into the preamp out of one channel, and the other end into the power amp in of the same channel. Apply your test signal to the input jack on the front as normal. Does sound come out that speaker? Now move this jumper cord over to the preamp out and power amp in of hte other channel. Does that speaker now make sound?

If both power amps drive speakers from the power amp in jack, and both preamps output from the preamp out jack, then the likely problem is a bad cutout contact on the offending power amp in jack itself.

"Both power amp inputs are good, which means to me that sound does come out the "bad" speaker when signal is input to the power amp jack. Right?"
-----That is correct


"SO now take a spare cord and plug one end into the preamp out of one channel, and the other end into the power amp in of the same channel. Apply your test signal to the input jack on the front as normal. Does sound come out that speaker? Now move this jumper cord over to the preamp out and power amp in of hte other channel. Does that speaker now make sound?"
-----This is also correct

I set this amp aside for awhile to keep work flow going but I'm picking it back up. I verified that the cut out contact on the jack functions properly, so this is not the problem. I do believe that I should be getting a mixed signal out of one of the preamp channels? I'm hearing no chorus whatsoever.

I just registered with the Peavey message forum and also emailed a request for the schematic.

[Enzo]

Wait. If you can make sound come out either channel by plugging a bypass cord into the channel's loop jacks, then the channel itself works and the jacks are the problem.

If the contacts on the jack itself are actually working as measured with an ohm meter, then check the solder and pc traces beneath.

[blckmynnse8]

It has been awhile, but here are the notes that I had written the last time I worked with this amp:

--------------------------------------------
When I listened with headphones, this is what I heard:

Signal plugged into power amp in 1:
I heard dry signal in one channel, and dead silence in other.

Signal plugged into power amp in 2:
I heard dry signal in one channel, and hiss in other channel

Preamp out 2 into power amp in 1:
Sound in one channel

Preamp out 2 into power amp in 2:
Sound in both channels

Preamp out 1 into power amp in 2:
Sound in both channels

Preamp out 1 into power amp in 1:
Sound in one channel
-----------------------------------------------


My main focus was on the cut out contact of the power amp in 2 jack when I worked with it, and for the sake of experimentation I switched places with both power amp in jacks with no change in results (to the best of my memory). I will go back and check the traces on the board once again. To the best of my memory everything appeared to be quite intact. Again, never at any time have I heard any "chorused" sound during all of these checks. Also noteworthy may be that when I initially opened this amp up, I noticed that one of the ics had been removed and one time, and upon reinsertion one of the legs was outside of the socket and not making any electrical contact at all. The ic in question was in the vicinity of the chorus pots, and I believe it was the MN3101. Going by memory, I'm pretty sure that pin 1 was the one that was not making contact. Naturally of course it was re-seated properly.

Thanks again, I will double check continuity of the traces asap.