Roland JC 120 UT - distorted sound from Right output fx loop

[spunko]

Hello friends,

I have now a JC 120 UT that came with almost no output, it sounded like a 1 watt amp in all the inputs (High and low, both channels). Also, when I switched the vibrato/chorus knob to OFF, it made a squeaking super loud sound, that didn't happen if the knob was in vibrato or chorus. That super loud sound made me think that maybe the power section was ok.

I took the amp chassis out from the cabinet, and started to poke around with a plastic stick, and found a loose cable in the fx-loop PCB that was making intermittent contact (see picture).
The other end connects to the PCB where the chorus/vibrato knob switch is located. I plugged in the loose cable and the amp now sounds super loud, but I encountered another problem: The amp sounds distorted, both channels/4 inputs.


So I sent a 500Hz signal to each of the "Return" jacks from the FX-loop, I also used an mp3 player and a guitar, and the left one sounds ok, but the right one sounds distorted and really low output.
Basically I tested this:


* Signal injected into the four front inputs, both channels (I had to disconnect the JC120 speaker leads):
- from Left Line Out to another power amp = sounds good
- from Right Line Out to another power amp = sounds good
- from Send CH2 Loop to another power amp = sounds good

Speaker leads connected.
* Signal injected into JC120 CH2 Right Return = distorted
* Signal injected into JC120 CH2 Left Return = sounds good

With the two above, I also tested disconnecting each OP-transistor socket (picture) and the "CH2 Right Return" distorts no matter which socket is disconnected, but the Left Return sounds good.


Edit: I'm sorry, the tittle is wrong, it must be: Roland JC 120 UT - distorted sound from Right Return fx loop

[The Dude]

It's best to include a schematic or schematic link in your post, if you want quick and accurate responses. Especially in the case of Roland, who may have several different versions/schematics for the same model number.

[Enzo]

yes, that is great advice. I recall my service manual which had several sections for groups of serial numbers, as they made changes over the production life of the model, and that was a couple decades ago. There have been even more versions since

[spunko]

I found these schematics in another thread, provided by Jazz P Bass.
Mine is a new model I guess, the amp doesn't have more than 2 years.
JC120UT_Pg 1-5.pdf
JC120UT_Pg 6-9.pdf

I found a sticker on the back of the map that says model JC120B so I guess thats the model. The owners manual I found said JC120TU and it has the same number of inputs/knobs/loop jacks.

[Enzo]

Many times the difference is inside. Look closely to see if your boards look like the ones in the schematic.

So the right power amp is bad in some way.

Apply a sine wave to the channel, and scope or signal tracer through the power amp from the jack on in.

[The Dude]

1) I'd first try a known good speaker on the right channel or swap speakers and see if the problem follows the amp or speaker.
2) Is there DC on the right output? Unhook the speaker and measure. If there is, leave the speaker unhooked until the amp is repaired.
3) The right return goes first to IC1. What is the voltage on pin 1 of that IC? It should be about half of +VB since the op amp has a single supply. You can compare it to pin 7. It should be about the same.
4) What are the voltages on Q18 and Q19 output transistors?

There are other things in between, but those checks might at least narrow down the problem.

[spunko]

I’m sorry, I tend to reply super late because of my work, I have to travel a lot.

It turned out to be a ribbon cable that was broken inside.
I was taping again with a stick and boom! Full blast of volumen, and it when crazy making intermittent contact.
I cut the broken part and patch it.
Now the amp sounds good with both power amps

[Jazz P Bass]

Nice find!