Help - Roland JC120H speaker output modded - need to know what's been done!

[JoeW]

Hi, I'm not an electronics guy, and I'm looking for someone to please tell me what's been done to a Roland Jazz Chorus 120H (Head only) which I bought.

The amp has two left, and two right speaker outs - for stereo. I think what's been done is this:
one left, and one right have been combined into a new XLR output.
the other left and right have had something else done - there is some very faint writing under each, which I *think* might say "power amp".

Pictures of the wiring are below. My question is - are the two outputs that are now attached to the XLR effectively still the same as they were originally? I just want to use two 1/4" jack outputs, as Roland intended !

JC120H-2
JC120H-1

[Steve Conner]

Looks as if the two sockets connected to the XLR are still the speaker outputs. But the other two (with the little green capacitors on them) are now inputs to the power amps. That would match with the "power amp" writing.

Hopefully the guy wired it so that when nothing is plugged into these inputs, the existing preamp gets patched through to the power amp and everything works normally.

I also notice he has replaced the 6.3 amp fuses with some bits of wire.

[JoeW]

Hmm thanks for that - particularly for spotting the "fuses"...

I assumed the power amp sockets were outputs, not inputs - would that be possible too?

I think I need to take it to an expert to check over, before I start using in anger.

Cheers

[Steve Conner]

I'm almost certain the "power amp" sockets are inputs. The previous owner would have done that so he could use the JC120H's power amps with another preamp.

You can take it to an expert, but I would just replace those dodgy bits of wire with the correct rated fuses and let it rip. There might be absolutely nothing wrong with it. If it sounded bad, or blew the fuses, THEN I'd take it to an expert. Well, I'd fix it myself, but you know what I mean.

Just watch out for DC offset: sometimes when a solid-state amp decides to blow, it'll take your speakers out with it.

Have you tried it yet to see if it works?

[JoeW]

I tried it with a single weedy speaker when I bought it, and it was OK. For various reasons it's been in storage ever since.

I'm now in a position to buy a half decent 2x12 stereo cabinet, and start using it, so started to look more closely. Main concern is that the original spec of 2x 8ohm outputs still holds true for the 2 1/4" output sockets, as that is the spec of the cab I'm looking at (Laney GS212IE).

Yes, I probably could manage to replace a couple of fuses...they're 6.3 amp you reckon?

What do you mean by "watch out for DC offset" ?

Thanks for your help

[Steve Conner]

The loading will work as follows:

If the JC120H manual said it was OK to connect one 8 ohm load to each of the four sockets, then it's now OK to connect 4 ohm loads to the remaining two sockets. (The old speaker sockets would have been in parallel, I expect.)

Connecting a stereo cabinet with two 8 ohm inputs should be fine in any case.

It says 6.3 amp on the board, so yeah, I reckon the fuses are 6.3 amp.

DC offset is the technical name for the process by which an exploding solid-state power amp takes the speakers out with it. Its final act is often to apply a large DC voltage to the speaker and burn it up.

[JoeW]

Great, thanks for your help. I shall certainly watch out for DC offset...though it doesn't sound like there's much I can do apart from duck