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Messages - Psc

Amplifier Discussion / Re: Add effect loop to jc120
January 21, 2023, 09:31:42 PM
Thanks for the input. Yes, I want to be able to have a mono input that would go to both power amps if possible.

Phatt, you're correct, the in/out between board 1 and 2 is exactly like that in the amp, very easy spot to access, very reversible. I think I'll try it there when I get a chance. Free time is hard to come by these days, but thinking at work I can manage!
Amplifier Discussion / Re: Add effect loop to jc120
January 20, 2023, 11:31:27 PM
Thanks for the confirmation. I'll update when I get around to doing it.
Amplifier Discussion / Re: Add effect loop to jc120
January 20, 2023, 01:09:44 PM
I'm guessing I would put a jack in between where the two boards connect just after the volume, and just pick one of the two? Either use switching jacks or just run an in and out and jumper with a patch cable?

I'm assuming between the output of board 1and the input of board 2. Is there a better way to do this?
Amplifier Discussion / Add effect loop to jc120
January 20, 2023, 11:25:13 AM
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.
Amplifier Discussion / Re: Speaker switching
July 03, 2020, 11:11:55 PM
I wasn't going to do aby because I'm making them into heads.  I should have clarified that. What I was thinking was more like having my clean effects y off to the jc120, then all dirt pedals to the peavey. I just want one 212 cabinet for the whole affair though.
Amplifier Discussion / Re: Speaker switching
July 03, 2020, 09:47:26 PM
It's probably unnecessary, it's true, but I have most of the stuff on hand. So I have it set up to switch the positive and the negative, but I probably have to keep the rings separate from each amp, do I not? I would imagine you need to keep each power supply isolated.
Amplifier Discussion / Speaker switching
July 03, 2020, 07:09:54 PM
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.
Amplifier Discussion / Re: JC 120 speaker question
March 11, 2020, 11:43:34 PM
I'll definitely double check.  It looks correct so far.  The amp was modified, so I'm going to have to go through it pretty extensively.  I'm not sure what the guy was up to.  He added two inputs to the back, and three relays.  He also put a small circuit board in there, and a transformer.  The end result is that none of the effects work, so I'm going to try and put it back to stock.  I don't think he changed much on the boards, mostly the connections between the boards.  The schematic I posted matches the serial number of my amp.
Amplifier Discussion / Re: JC 120 speaker question
March 11, 2020, 01:15:03 PM

Ok, that makes sense.  I have another stereo amp that needs a 4 ohm load.  So I could build one 2-12 speaker cabinet with 4 ohm speakers and turn both amps into heads?  That might simplify things for me.
Amplifier Discussion / JC 120 speaker question
March 11, 2020, 10:15:50 AM

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.;topic=4786.0;attach=8708

Schematics and Layouts / Re: JC 120 schematic
March 04, 2020, 09:59:52 AM
This looks perfect.  Thanks!
Schematics and Layouts / JC 120 schematic
March 03, 2020, 10:51:58 PM

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.

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.

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.
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.