Roland JC 120 blowing fuses when plugging into input

[pontiacpete]

this amp is blowing the 6A fuses inside the amp on the rectifier board, not the mains fuse. I can plug into the 'mains in' and get sound without a problem. When plugging into any of the inputs my light bulb limiter glows very bright. The output transistors are not showing a short so I'm thinking the power amp section is working.
Also found that one of the speakers is measuring a dead short. Seems that maybe the problem is on the side that the blown speaker is on.
Not sure where I'm going to go from here, does anyone have any ideas how to approach this?

Thanks
pete

[R.G.]

this amp is blowing the 6A fuses inside the amp on the rectifier board, not the mains fuse. I can plug into the 'mains in' and get sound without a problem. When plugging into any of the inputs my light bulb limiter glows very bright. The output transistors are not showing a short so I'm thinking the power amp section is working.
Also found that one of the speakers is measuring a dead short. Seems that maybe the problem is on the side that the blown speaker is on.
Not sure where I'm going to go from here, does anyone have any ideas how to approach this?

First, get that shorted speaker out of there. These amps don't have any clear protection against shorted outputs, and you could melt down the output transistors pretty easily. In fact, you have probably already done this, I just didn't see this in your post.

If you're getting sound from the outputs from inserting signal at the power amp inputs, the power amps are probably OK. It's probably good run some tests on the power amps to see if they're giving full swing into no load with only signal from the Main In. Once you get that checked out...

I'd say that the preamps, one or more, or the effects are oscillating and driving the power amps into some condition that eats more than 6A. I would pull out C12 and C25, the capacitors that couple preamp outputs into the power amps after those two JFET source followers. That prevents the preamps from putting any signal into the power amps, and you can then probe the preamps without blowing the rectifier fuses. Er... I think.

In nearly every "fuses blowing" situation, the rule is divide and conquer. Make temporary disconnects to isolate power sections and see which cuts make the fuses stop blowing.

[pontiacpete]

Thanks for the suggests RG. I forgot to mention that I disconnected the speaker when I discovered that it is measuring .8ohms but still when plugging into either channel inputs, the light comes up bright.
Also when plugging into the 'mains in' there are 2 inputs there, only one of them is giving me sound. So I switched the output wires and there's sound out of the other 'mains in' jack.
I'll check those caps.

[glebert]

When you say output transistors are not showing a short, does that mean measuring them with a DMM or seeing if they bias up correctly in circuit? It sounds to me like you have one of the power amps not working (I think that is why one mains in works and one doesn't). The nice thing about a stereo amp is that if you have a working side you can use it as a reference for the other. I remember when I did my JC120 ('83) I think I read when these amps fail they tend to take out transistors even before the driver transistors, but may be misremembering.

Also, there are a LOT of different schematics for these with small changes, match up the serial number to the schematic and also make sure it agrees with what you see on the circuit.

[pontiacpete]

"It sounds to me like you have one of the power amps not working (I think that is why one mains in works and one doesn't)"

Actually I did try to say that both mains ins are working when exchanging the output wires to the good speaker. Sorry Im not the great at describing things

[pontiacpete]

RG -- I pulled the 2 caps C12 and C25 and the amp is not shorting when plugging into the input. It seems to be working fine without them. Question is, should I just leave it without these 2 caps or look further into what was causing the short?

[g1]

With those caps out the amp sounds normal when using the guitar input?
Only the power amp input jacks (main in) should be working if those caps are removed.

Maybe this is not the correct schematic.

[pontiacpete]

All inputs are working, the 'mains in' both sides and both channels work.

The components are mostly the same. How can I check it against the serial number?
I'll look into that

Here's the serial # 375297

[glebert]

Does it have bright switches? IIRC Roland numbers repeat at some point (the first two numbers is the number of months after the company started or something, so they roll over after 99). They added bright switches in early 80's so that helps date them. "37" serial number would be Aug 75 or Dec 83 don't know if the pattern continues after that. JC120 was introduced in 75 so that is probably less likely.

[pontiacpete]

Yes this amp has bright switches.

[glebert]

Here are two sets of service notes. The fifth edition (11052) matched well for my amp, which I believe was also 83. Fifth edition is for serial numbers starting at "29" for US, so would be eight months before yours if it is 83.

SM_JC-120_11016.zip