Hey gang, I'm looking for documentation for a
Realistic mpa-20 PA amplifier. It's not the discrete version, it's the one with two kia7217ap chips. I'm looking to drastically modify it into a guitar amp. I'm hoping to leave the power amp section mostly the same, possibly tweak a few things and add a new preamp section.

The board is labelled P-200902.

I've only found a snippet that someone traced of the preamp section. Hoping I can find something a bit more comprehensive.

Thanks for your help!

Hello all, I'm looking for some help adding an effect loop to a jc120. I mostly want this so that I can use it as a power amp in for other preamps, not so much for adding stereo effects like delays. I have attached the schematic for the amp in question. I'm just not clear exactly where to split off.

Hello all.  Hopefully this is the right place to ask this.  I have two solid state stereo chorus amps, a peavey stereo chorus and a jc120, and I had a thought to use them both with the same speaker cabinet and make a foot switch so I could do it on the fly.  I came up with a schematic that involves six relays. It works very much like a reversing starter.  You push the button for the peavey, it seals in a relay that closes the contacts for the speaker output for that amp and disables the jc120 relay.  To change you hit a stop button, then hit the jc120 button.

I want to do all that to prevent the outputs of the amps from connecting.  I am wondering if this is safe, and more importantly is there anything I haven't thought of.  I expect I'll need three separate plates to mount the jacks, one for each amp and one for the output so that the negatives won't be connected?

If there's a reason I should abandon this please let me know. It's one of those things that I couldn't stop thinking about.

Hi all.  Quick question, hopefully someone can show me what I'm missing.  I understand that the speakers used are 8 ohm.  The schematic shows that when you connect an external speaker you should also use 8 ohm, but it shows them connected in parallel, which would make it 4 ohms.  I would have thought 4 ohms was too low.

https://www.ssguitar.com/index.php?action=dlattach;topic=4786.0;attach=8708

Thanks!

Hello all.  I recently picked up an older JC 120.  I wasn't really in the market, but the guy was pretty much giving it away.  It is not without its quirks though.  The effects don't work, and it has been fairly heavily modified.  There are two extra inputs added to the back along with a toggle switch and two XLR jacks.  There are also three relays that don't look stock, but I'm not sure about those. He also added a small board with a chip on it.  He may have been trying to add delay, but it's hard to be sure.

The amp came with a binder full of notes and graphs, hand drawn schematics but unfortunately it doesn't have the original schematic.  The amp I have is an early model, I believe in approximately 1978-1979 or so.  It is one without the bright switch or the distortion footswitch.  I haven't had any luck finding that particular model.  I'd like to start putting this thing back to something approaching stock.

If anyone can help I'd appreciate it.  Also a drawing that shows component layout would help me.  It looks like he hasn't messed with the boards too much.

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!