Roland JC120 sounds farty if played hard

[Jay Mann]

I've got a late 70's Roland Jazz Chorus.
Recently it started getting what I'd call a farty breakup sound if you play hard.
Volume on about 3 (9'oclock-ish) or above. Seems to be fine when playing quieter. Does it in either channel.
It had a bad hum, so I replaced the caps on the power supply which cured the hum.
It also breaks up less if you turn the bass down. I suspected speakers, so I disconnected both of them and ran out to 2 known good speaker cabs. Same breakup.
Doesn't make a lot of sense having such a big heavy amp, only to have to play it quietly.
Seems to have no other issues other than the terrible, unusable farts.
Does anyone here have any suggestions what to check next?
Thanks!

[J M Fahey]

ALL amps distort when played loud, which is more noticeable in SS amps because they jump from ultra clean to flat top distortion within 1 V increase in output, once they reach power rails.

Tube amps also clip but in a smoother way , you can see clipping on a scope but it does not sound harsh until things get *real* chopped, there is a wide very usable "in-between" area.

JC120 is favored by Jazz players who REALLY donīt play loud, at all, compared to Rock players.

Excess Bass settings will also complicate the job and way too much, yes, can sound farty.

Your description does not sound "broken" but normal in that circumstance.

It "might" have a problem, but only way to find is to scope output, load it with two 8 ohm >60W resistors, inject 100mV 1kHz at the input, all controls flat, start rising volume until you see slight clipping and measure voltage/power out.

Normal spec is 22V RMS or 60W into 8 ohm.

If much less, then you start searching for a problem, but not otherwise.

[Jay Mann]

The break up is a recent occurrence though. I don't feel like volume on 3 and tones straight up should sound that bad. It used to at least turn up to 5 and stay clean.
Thanks for the spec. I'll dig out the scope and see what it shows.

[Jay Mann]

ALL amps distort when played loud, which is more noticeable in SS amps because they jump from ultra clean to flat top distortion within 1 V increase in output, once they reach power rails.

Tube amps also clip but in a smoother way , you can see clipping on a scope but it does not sound harsh until things get *real* chopped, there is a wide very usable "in-between" area.

JC120 is favored by Jazz players who REALLY donīt play loud, at all, compared to Rock players.

Excess Bass settings will also complicate the job and way too much, yes, can sound farty.

Your description does not sound "broken" but normal in that circumstance.

It "might" have a problem, but only way to find is to scope output, load it with two 8 ohm >60W resistors, inject 100mV 1kHz at the input, all controls flat, start rising volume until you see slight clipping and measure voltage/power out.

Normal spec is 22V RMS or 60W into 8 ohm.

If much less, then you start searching for a problem, but not otherwise.

I just got around to scoping the output. I'm getting 15V RMS. Both channels showing about the same. Both speaker outputs about the same.
Clipping starts with the volume between 2 and 3. Seems way too low volume to clip that soon.

[glebert]

8 ohm speakers right? That is only 28 watts clean.

[Jay Mann]

8 ohm speakers right? That is only 28 watts clean.

yes. 8 ohm

[The Dude]

That seems like low output at clip. The symptom (and since output is low on both amp channels) makes me think power supply- maybe failing filter caps. Check your power rails- both that they are up to snuff and clean. If those are good, I'd start scoping from the input and see where clipping starts.

[Jay Mann]

That seems like low output at clip. The symptom (and since output is low on both amp channels) makes me think power supply- maybe failing filter caps. Check your power rails- both that they are up to snuff and clean. If those are good, I'd start scoping from the input and see where clipping starts.

I replaced the caps on the power supply board. There was a bit of a hum going on and the caps were original so I didn't even think twice about replacing 40+ year old caps. It cured the hum, but the clipping was still there. Seeing that it's both CH1 pre/power board and CH2 pre/power board that's acting the same, I am leaning towards something in the power supply, or maybe output transistors?

[The Dude]

It's unlikely, but not impossible that it would be output transistors. It would mean that both power amps failed at the same time. I'd be looking for something common to both amps- early preamp clipping, or a power supply problem. You replaced caps, but did you check voltages?

[Jay Mann]

I
I've tested B1 and B2 and get 50V and -50V
B3 and B4 show 15 and -15

[g1]

Does it have power amp 'in' jacks that you can try? Maybe the clipping is not from the output transistors hitting the rails, but something in the preamp common to both amps.

There are many versions of the schematic depending on the serial number. Can you post the exact schematic for your unit?

[Jay Mann]

Does it have power amp 'in' jacks that you can try? Maybe the clipping is not from the output transistors hitting the rails, but something in the preamp common to both amps.

There are many versions of the schematic depending on the serial number. Can you post the exact schematic for your unit?

there's no power amp in jacks. just a line out.
this schematic is the one my serial number lines up with.

[glebert]

there's no power amp in jacks. just a line out.
this schematic is the one my serial number lines up with.

Just an FYI that Roland serial numbers are weird, they will periodically repeat. Two different amps could have the same serial number and be made 8 years apart. Does yours have bright switches? If so it was later than the schematic above.

[Jay Mann]

Just an FYI that Roland serial numbers are weird, they will periodically repeat. Two different amps could have the same serial number and be made 8 years apart. Does yours have bright switches? If so it was later than the schematic above.

No bright switches. This amp is old. My friend bought it new in 79 or 80 he seems to recall.

[Jay Mann]

Here's what it sounds like.