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Post by mlrpa on Jun 29, 2007 0:38:37 GMT -5
Hey guys, I have a Roland Jazz Chorus 77, that the right side has almost no output, and has a fuzzyist to the sound. And Ideas? The output thru the mixer, using the left right outputs, sound ok. Hissy, but none of the fuzz like the speaker. And yes, I did try a different speaker there with the same results. Here is a link to the Roland schematic for those of you who can actually read them. www.schematicheaven.com/newamps/roland_jazz_chorus.pdfHope that helps you to help me! |
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Post by sumgai on Jun 29, 2007 1:58:26 GMT -5
mlrpa, I'm gonna go out on a limb here, and guess that a cable header has become disconnected from the power amp board. If your power transistors had gone south, you can be sure that the fuse would pop even as you turned the amp on. Depending on the age, the problem might be nothing more than a collection of "crud" (technical term for a build-up of crap) on one or more of the header's pins. A bad solder joint, either from age or from 'road shock' could cause this, I'd check the speaker jack solder joints first. About the only other thing I could do would be to thank my lucky stars that there are two channels in there, one being a clone of the other. Since Right channel is perfectly good, I can take comparative measurements between the two, and find the bad component(s). But the power transistors are the usual culprits. If they aren't bad, then it's likely to be a niggling mechanical connection type of problem. Visual inspection is your first line of attack for this kind of problem. Or you can call me to come get it, and I'll put it on the bench. For you, such a deal - shop time + driving time + parts + ACBA 1, and I promise you can keep your first born kid! ;D HTH sumgai 1. ACBA = A Case of Beer for the Aggravation!  |
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Post by mlrpa on Jun 29, 2007 11:53:24 GMT -5
Guiness, or a, gag, American brew?
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Post by sumgai on Jun 30, 2007 3:27:00 GMT -5
mlrpa, Took me a moment, all those commas.  Err, make mine an O'Douls Amber, please. ;D Provided that you've been doing some checking under the hood, and found the culprit......... sumgai p.s. Still expecting a phone call. |
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Post by sneakymarco on Jun 30, 2007 3:57:20 GMT -5
I have the same amp, only there's no output whatsoever out of the one side. Is this a common problem with JC77s?
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Post by sumgai on Jul 1, 2007 12:54:05 GMT -5
SM,
Not per se. In your case, I'd also do a visual inspection first, looking for loose or disconnected cable headers. After that, I'd head over to the Line Out (Effect) jack - those things were notorious for going bad (in many, many amps, not just Rolands). Effectively, they would fail after a few times of being plugged/unplugged. The signal flows through that point all the time, and when the path is broken (by having a cable plugged in), that's fine. It's when the path is restored (unplug the cable), that's where the problem gets started. The tiny connection point is not stout enough to withstand the rigors of mechanical stress, and the result is caa-caa output at the speaker (none, or real low and fuzzy).
Come to think of it, mlrpa, you should check this out too, on your unit. You did try the signal from this jack, and it was good, but unfortunately, that doesn't test the jack itself for conductivity. The only test I can think of would be to open up the amp, and use a wooden stick to physically force the jack's contacts closed. If you get any change in sound at all, then that contact point is bad, and the jack needs R'n'R (removing and replacing).
HTH
sumgai
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rivertramp
Rookie Solder Flinger
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Post by rivertramp on Jan 6, 2008 15:22:45 GMT -5
I just picked up a used JC77, and was having what sounds like the exact same problem: occasional volume drop, right speaker cut out, fizz and crackle. I fixed it, very simply, and now it works great. It may not be your solution but here's what I did.
Check ALL of the plastic nuts holding your inputs in place, especially the ones on the back panel for the footswitch. I didn't realize this, but I had a cracked nut that hadn't completely fallen off, barely holding the chorus input on the back panel in place. Took the one off the reverb input, put it on the chorus input - same problem. So I figure this: the inputs are metal, so is the frame/chassis of the amp. The nuts are plastic and keep the input barrel from touching the frame. If it touches just slightly - snap crackle pop fizz speaker fadeout. Make sure all the inputs are being held in place by some kind of insulating ie plastic nut. If any of them are metal, replace them. Now just the hiss......any ideas on how to get rid of that?
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