Roland Jazz Chorus 90 Buzz

[Ted]

I have a Roland JC90 here.

It has a buzz around A on the bottom E String of a guitar.

It's there on both channels. (It has separate power amps for each of its two speakers)

It's still there on an external speaker.

No crossover distortion visible on the scope.

Suggestions please?

[eschertron]

When you say Buzz, do you mean a buzz/hum/constant noise issue, or does it only buzz when you fret an A on the low E-string? If the former, then the sound is reasonably close to 100Hz, double line freq on the island. Someone smarter than myself will suggest where in the B+ power supply to start looking for the cause.

[mozwell]

Is this a JC90 (never heard of a JC90), or a JC120 made in 1990....
There are also JC50, JC60, JC77, JC120 and JC160
various schematics attached, lets see if one of them is the right one.
First check, plug your guitar lead into the "main in" sockets & play your guitar. Is the hum still there. The guitar will be very quiet, for now we are just listening for hum. Try this on the other "main in" socket.
Measure the DC rails, using both AC and DC ranges on your multimeter
Measure across each speaker with DC volts range on your multimeter
Check the main DC filter caps, are the cans bulging, or leaking electrolyte
report all findings here

I also have service manuals for JC120 3rd edition & JC120UR-2000, but these are too big, if you need these pm me with your email, asking for the relevant file & i will email to you

[DrGonz78]

The JC90 does exist, but is probably a relatively newer specimen...

JC-90: Jazz Chorus | Roland U.S.

[doctor]

I think that here may be one of two reasons:

1. String is touching at the next fret;
2. Acoustic resonance of a some amp’s component.

If I'll be wrong, I'll send you schematics.

[Ted]

+/- 36V on the supply rails.
0.5V Ripple at idle

4V Ripple at full power at 1kHz with signal very visible on supply rails



Thats saying Main PSU Caps to me.

[doctor]

This ripple cannot be related with "a buzz around A on the bottom E String of a guitar".

[Ted]

This ripple cannot be related with "a buzz around A on the bottom E String of a guitar".

I would have said that a beat effect between a signal of around 100Hz and the 100Hz ripple (as suggested by Eschertron) was quite a credible explanation. Why do you think not?

[doctor]

I understood that you hear “buzz” when you play A-note on E-string.

[g1]

I think what Doctor is saying is that a beat frequency around 100hz would not sound like "a buzz".

[J M Fahey]

FWIW 10% ripple (4Vpp on 36V DC) on a regular musical instrument amp on full power is quite normal.
And the audio signal is well below 1Vpp , nothing to write home about.

And "a buzz" is something very different from "a beat note".

If it's really a buzz at a specific note, I suspect something "mechanical", either the speaker itself, the cabinet, or lesser (but possible) suspects, such as cloth rattle, some loose back panel, etc.

Inject the suspect note but coming from an oscillator and listen from close up.

A doctor's stethoscope is inexpensive and very useful.
And pulling the membrane head and putting the free hose end close to suspect points is excellent to catch whistles and such.

[Jazz P Bass]

Have we ruled out the guitar itself?

[J M Fahey]

Not yet, doctor asked about string/fret buzz at that specific note.

[Ted]

It's definitely not fret buzz - I tried a couple of different guitars and a bass.

I should have made it clear that the buzz is only there when a note is struck.

I suspected mechanical resonance or speaker problems at first but, as i said, plugging it into an external cab didn't make any difference.

Incidentally, the noise is still there with the guitar plugged into the effects return.

[g1]

When you tried the other cab was it close enough to cause vibration of the chassis?
Have you tried pounding on the amp with your fist or tapping on the circuit board with a wood or plastic prod?
It's possible that vibration at that particular frequency (A note on low E or some harmonic thereof) is triggering a shaky connection somewhere.
As JM suggested, inject a tone at the frequency that causes the buzz. See if you can observe the distortion/buzz on the scope.