Roland JC120 Output Noises

[little cyress]

I need some help with a Roland JC120.
I have some noise issues with the output to the speakers.

I have troubleshooted the speakers and they are fine.
I have troubleshooted the line outs and they are fine. Line outs sound great and all of the chorus tremolo ciruits and all pots on the amp work like they should.

The reverb circuit did not work. I noticed a wire disconnected to the middle of one of the rca connections to the reverb pan and soldered it, but this did not fix the reverb circuit issue.

I tightened all of the cabinet and speaker screws to make sure there is no vibration to cause any noise.

But I am still getting some buzzing and other noises that I can only guess what the cause is.

Could it be caps? The amp turns right on with a loud popping noise when I turn it on. I thought the speaker(s) were blown, but they tested just fine. I double checked and made sure the speakers are wired correctly. I took several pics of the inside of the chassis that I can post but I did not notice anything disconnected or that looked wrong, but I'm not an amp tech. The amp looks to be one of the older USA models. 1980's ???

Any help or advice on my noise issue would be greatly appreciated.
Can I troubleshoot anything else?
Should I post a few pics?

thanks!

[little cyress]

Oh, I forgot to mention that when I tested the amp out when I bought it, it seemed to work just fine. I only picked it up and placed it in my car, drove it home, and now it is acting like it is with the buzzing issue. The bass notes sounded clear with no buzzing when I tested it out to buy it. So my best guess now is maybe a cold solder issue.

[Roy]

Try unhooking the speakers and hook it up to an external cab to see if the prob is the amp or the speakers. might just need new speakers if it was left somplace very hot for long enough to dry out and crack the speaker cones. You migh be able to get them reconed.

[little cyress]

Like I said, the speakers are fine. I tested them with another amp and they work flawlessly. I also tested the preamp via the left and right line outs into my daw and the preamp works perfect.
I can get a slightly better output signal with all of the pots zero'd out. When I turn up the bass or mid pot, I really start to get a signal that is very noisy. BTW, I am using electric guitars with the amp. I know when my friend used a Jazz Chorus in the late eighties with his Juno 106, he had one of the pots always zeroed out, but I can't remember if it was the treble or mid pot, or something else. Anyway, the bass pot really seems to effect the problem more than any other pot. This problem sounds like an easy fix for a seasoned tech. The power amp part of the circuit has an issue, and it's in STEREO

[torquil]

I have a question:

You said that the line out works/preamp fine. But did you use the amp speaker(s) while you were testing the line out? I.e., did the amp vibrate while you tested the line outs? If not, the problem could be related to vibrations in the circuit, and you should try to disconnect the speakers and instead use an external cabinet to verify this.

A schematic is available if you search using google. Looking at it, it seems to me that the line out is connected at the same place as the speaker output, apart from a voltage divider. So the signal should be essentially the same as the speaker signal.

[donnyjaguar]

The schematic I have shows the line out before the power amplifiers (there are two in this amp). So my question is, do both speakers sound fuzzy?

[little cyress]

It is not the speakers. I tested each speaker separately with an old GMT amp and they sounded just fine.
RE: using the amp speakers while I was testing the line out.
If you run both line outs, it defeats the speaker output to the speakers so no speaker activity when I was testing the line outs which sounded pristine in my Mixer. The cabinet was very loose when I first received the amp, and I know there was some vibrating going on there, so the first thing I did was tighten all of the nuts and even added a few nuts where needed to take the cabinet out of the equation. So No, the amp did not vibrate when I tested the line outs. I have not tried an external cabinet yet, and that is a good idea as the higher frequencies are having a less adverse effect on the noise issue. Yes, the line out cable essentially comes from the speaker itself. The orange and red wires are connected right to the board and go to the left and right speaker hot terminals, and the brown and black wires come off the other terminal and go straight to the left and right line out. Then there is the green ground wire that grounds both speakers at each of the speaker mounting holes.

[teemuk]

Originally Posted by donnyjaguar

The schematic I have shows the line out before the power amplifiers (there are two in this amp). So my question is, do both speakers sound fuzzy?

NOTE!

JC-120 is not a single design, in fact, it's several of them, each issue of that amp (starting from ca. mid 1970's) being more or less different than the others issued later. I have seen at least three completely different circuits, all of them which were JC-120 amps. And there's probably around eight different circuit versions of that thing. Basically, JC-120 is a design that shares some common features and a certain cosmetic look but circuit-wise there's a lot of variation.

So, get the correct schematic that matches your amp's serial number, otherwise chances are the JC-120 schematics you are reading are mostly useless.

Also, the most common JC-120 schematic you likely find first from the Internet (the 1984 schem) is actually for the JC-120H, a head model of the amp, which is quite different than the combos.

[little cyress]

I tested the amp with an external cabinet (4x12), and I'm getting the same buzzing.
The buzzing is touch sensitive depending how hard I hit those low pitch E A D strings. If I lightly touch them, no buzz, but anything harder and it's buzz city. I'm no tech, but it must be vibrations in the power amp circuit, right?

[Ronsonic]

The usual cause of loud power-on thumps is filter caps. Those may just need resoldered or may need replaced.

Specific to Roland and it's sorta hard to see in the photos with my color vision .... Roland used the same yellow assembly glue as Yamaha and a lot of other Far East manufacturers. Over time it discolors and turns brown, this also turns it from a beneficial assembly aid into a corrosive, conductive and occasionally semiconductive mess that will cause all sorts of weird problems like crackling, buzzing and hum. By the time it has turned brown it has become rock hard.

Like I say, it's hard for me to see in the photos, but it looks like you've got some on the caps. Sometimes this glue pollutes the solder joints and creates a bubble around the component lead. Very common problem in a lot of Yamaha products and an occasional issue for Roland. This is especially a problem on ribbon cables soldered to the board, but you'll find it elsewhere.

You will want to scrape this off your circuit board. If it means desoldering something to get under it, so be it. Any of the glue that has changed colors will have to go. At least where it touches component leads, traces or terminals. Any of the glue that hasn't changed colors and turned to evil eventually will. The good news is that when it's still yellow it's got the texture of extremely aggressive bubblegum and relatively easy to remove.

[little cyress]

I just got the amp back from my tech and it sound amazing!
He replaced a cap and some other black parts that look like space invaders and now it sings! It was a seventies Roland with 1970's parts.
Thanks for all of the helpful comments! The Gear Page rules

[torquil]

Originally Posted by little cyress

I just got the amp back from my tech and it sound amazing!
He replaced a cap and some other black parts that look like space invaders and now it sings! It was a seventies Roland with 1970's parts.
Thanks for all of the helpful comments! The Gear Page rules

Sounds like you took your amp to the right person!

[Canon]

I just read this and was going to tell you check solder joints in that distortion circuit and filter caps, mine sounds good again too!
By the way, I put 2 scumback speakers in, and it was a big improvement.