Roland Jazz

[Valvehead]

I have a JC-120 here . This is no longer a 2x12 combo but has been converted to a head.

Im seeing that 3 of the 4 output transistors are shorted. and 2 fuses popped.

Heres my question concerning the wiring --

the speaker outs used are the amps original 2 jacks on the back , labeled Extension speakers 8 ohm each.

Inside the amp , someone left the wires that went to the original 12" speakers , and they are connected together like so... left + to left - , and right + to right - .

Does this seem correct or is the amp seeing a "wrong " load ?

i found 2 scems for the amp

[The Dude]

The only thing I can think of is that the extension speaker jacks are shorting jacks and the extension speakers are put in series with the existing/internal speakers when the extension speakers were used in the original design. I don't know why else you'd do that. This would work fine if there's a speaker plugged into the extension speaker jacks. If nothing is plugged in to the extension jacks, you'd have a dead short across the output, which would obviously be bad. It should be easy enough to find out checking continuity. If that's what's going on, you should nix the shorting jacks and reconfigure the speaker connection section.

[Valvehead]

The only thing I can think of is that the extension speaker jacks are shorting jacks and the extension speakers are put in series with the existing/internal speakers when the extension speakers were used in the original design. I don't know why else you'd do that. This would work fine if there's a speaker plugged into the extension speaker jacks. If nothing is plugged in to the extension jacks, you'd have a dead short across the output, which would obviously be bad. It should be easy enough to find out checking continuity. If that's what's going on, you should nix the shorting jacks and reconfigure the speaker connection section.

i was thinking of removing the Ext wires and connecting the original 4 wires to those jacks , or putting in normal jacks.......is that what you mean ?

Yes--the Ext jacks in it are shorting style.

[The Dude]

Solid state amps don't like shorting speaker jacks. I suspect someone turned the amp on with nothing connected to at least one of the shorting jacks and shorted the outputs.
Yes, that's what I'd do- remove the extra wires and wire the output section to "normal"/non-switching jacks. You could even re-use the jacks if you cut traces or remove the switched pins of the jacks.

[Valvehead]

ok thanks Duder--------looks like i have more problems....i have the 2 ac wires that go from the PT to the fuses (f2 /f3) un-wired from the board and they are going right to ground

[The Dude]

Sounds like quite the hack job you have there!

[Valvehead]

mine has the 2 Orange ac wires as shown in the 2nd scem ....they are up in the air , un hooked , and show short to ground with my meter .....but test 61vac across them....and the main fuse didnt blow.............what am i missing here... ???

[g1]

Isn't there a third wire going to ground (the center-tap)?
That would give you a low resistance to ground but that is normal.

[Enzo]

You are missing that the center tap of the orange winding is grounded. Does your meter show short to ground? Or just low resistance or even just continuity? I suspect you actually were reading the resistance of the winding wire, wwhich won't be very high.


In my experience, when I see wires removed like that, it is because the circuit they are feeding has a problem.

[The Dude]

......In my experience, when I see wires removed like that, it is because the circuit they are feeding has a problem.

My guess is that problem was the shorted output transistors.

[Valvehead]

yes--it was .2 ohms. Amp is up and running..cleaned pots / jacks...it sounds REAL good.

BUT...i have 1 problem remaining ...the vibrato works perfect..speed and depth. The chorus is on..but the speed and depth controls dont do anything. Its there but sounds like both controls are set on 1 . No change by the 2 pots .

Where should i start looking ??

[g1]

This is normal. The chorus uses preset levels, the vibrato is adjustable. I think it's mentioned in the owners manual.

[The Dude]

^^^^^^ What he said.

[Valvehead]

really?? Thats odd....to me at least --ok , well i have a JC -120 done ..and holy crap...they werent kidding about the clean sound of these---NICE !!

[g1]

Yes it is odd. And everyone of us that ever tried one (without being told that) thought it was broken too!