Roland JC-120 unwanted distortion after 20mins

[tubby]

Hi all,

I got the amp from a bandmate who says unwanted distortion sets in after about 20 mins of playing. Regardless which channel he's plugged into. Since the distortion sets in on both channels I'm guessing it's probably something to do with one of the power stages since it seems unlikely that the preamp sections on both channels would go south at the same time.

It's a fairly complex circuit (for me, that is) and I'm not really sure where to start, so I'm thankful for any tips in that direction.
Also, maybe someone already encountered the same problem with a JC-120?

A search in the forum has yielded nothing similar. At least I found out that its a good idea to check the jacks and resolder the board on those amps.

Serial no is 809754, I think I got the right schematic. Roland JC120' 82.pdf

Thanks!

[TomCarlos]

tubby...

To confirm your suspicions where the problem is in the power amp section, you can run a signal directly into the Effects Return Left - that is the mono input. That point in the circuit bypasses the preamp section. See if you can reproduce the problem. If it occurs, we then know it is either a power supply problem or something in the power amp section.

For a signal source, maybe one side of your iphone and play some music? Otherwise, get a cheapo mixer and use that to feed the Roland.

[The Dude]

Agree with Tom. I'll add: If it takes 20 minutes, it's likely heat related. After checking for cold solder joints, I might get some freeze spray and see if you can narrow it down a failing component.

[Enzo]

Or...

I once had a guy bring me an amp with a similar complaint. I asked if he had active pickups. he said he did, "why?" Have you tried a fresh battery? "Battery? What battery?"

Lesson is to verify the symptom actually occurs. Does this happen with other guitars? Does he always use some sort of pedal in his set up?

[BobK]

^^^
|||
This.

Verify the symptom actually occurs as described.

Disconnect the speakers, drive it into load (yeah, it's a JC-120 so you might need 2 loads) and a scope using a test tone as the source and see if it really starts to mis-behave after 20 minutes of signal.

There have also been many times I've gone looking for a "happens after 'x' minutes of play" issue only to find that if I poked it in the right place I could make it happen right away.

[Enzo]

Actually I was suggesting the guy could have a fading battery in huis actives or a floor toy.