Roland Jazz-Chorus adds buzzsaw noise when chorus engaged.

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doubleroger

Well-known member
Joined
Aug 24, 2008
Messages
197
Location
paris, france
Hi all.
I have a Roland JC160. It's been on my bench for two years, feeding DC to its speakers. Yesterday I finally found the culprit, an open resistor. It is now taped to the wall of shame.
I thought it was all over, I could have a little party, I had to find a way to celebrate!! Quick let's get something to drink!
So I plugged my Rhodes into it and played.  :'(
There was this little buzzsaw noise that I had forgotten about (well two years is long isn't it?).
Now that I finally put the cover back on it, I'm gonna have to take it off again, to get rid of that, as it makes any recording impossible, and the amp quite worthless.



OK, now that my little introductive story (cute wasn't it?) is told, let's get to the problem:

You can hear the noise in question and find the schematics here .
The schematics are a little hard to read but they are the only ones I found that related to my amp (sn:651870),  so not really the right ones, but the amp boards were those ones, with the right transistors. There are lots of different versions of these amps, it seems... I'm not sure I have the right schematic for the effect board.

The sound is taken with a DI on the speaker output of Amp Channel number 2. It is supposedly the "effect channel amp" on JC-120s and JC-160s. My Rhodes is plugged into Input channel number 2, wich is the only input channel that can have effects. (input ch 1 is for a totally dry sound).
The JC160 frontpannel has a switch between "normal" (both amp channels amplify the same dry signal), "chorus" or "vibrato" (ch1 is dry amp, ch2 is the effect amp channel).
There is NO problem in "normal" mode. The sound is clean in both channels.
There is that noise with both "chorus" or "vibrato" settings.
The noise is only in the speakers of amp ch2 (wet). Amp ch1(dry)  stays perfectly clean all the time.
The noise seems to get modulated by the effect (the soundfile has a vibrato, for example).
The noise only appears above a certain threshold.

Now what is (or not) funny is that if I plug my rhodes into Input Ch1 (the dry input channel), and turn on the chorus/vibrato, it feeds the usual hiss to the speakers (through the "effect channel" ch2 amp ). That is normal. But if I play into input Ch1 now, I don't get the vibrato (that is normal too), but I get the exact same modulated white noise I had before in amp ch2, like there was a leak somewhere...



Any pointers (or readable schematics) are welcome, there are so many informations that I need some of your analytic minds to help me ;)
 
I had an amp on the bench not too long ago that would buzz the moment the volume passed a certain level. From your long story that seems to be the case but only when using the chorus? am I correct on that?.... Anyway  took the amp apart and started looking for a faulty solder joint.  Take a non conductive object and start tapping parts. you should be able to find the faulty joint as you will get noise when you tap the part it's attached too. Then resolder. Should fix your issues.
 
Well, I've been looking at this board for hours now, and I've tapped everything I could in there. No faulty joints, I'm afraid.
If I unplug the effect board from the circuit, problem is gone (well chorus too obviously). I've cleaned pots and trimmers too.

Do you think it could come from an IC (i'm thinking about the Bucket Brigade MN3002 wich has a "noisy" reputation)?
 
you gotta remember, this thing came out one day after the microchip was invented.

so although there is nothing quite like a true stereo chorus coming from  two speakers,

i remember the noise level being about 2 db down from horrendous, and that was 30 yrs ago.

some of the noise cancels if you put the mic in the rite place, stand back 6 feet and listen

don't even mention Polytone either, or i will kill myself.

i'm jus sayin...

all these guys are dead, either musically, physically, or both.

polytone is the cause.

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That sound clip sounds perfectly normal to me. Those vib/chorus circuits ARE noisy like that. I don't hear anything that could be described as a buzzsaw, just a hissy circuit.

Bring up the volume on your piano, that will mask it. These amps weren't designed for critical listening at low volume, it is a stage amp. It should be perfectly recordable at normal volume.

Totally agree about Polytone, btw. Nasty amps for jazz wimps.

 
I've had the JC77 (2x10") JC50(1x12") & the JC120(2X12") in the past & I can confirm that they all had that slight hiss thing happening ::)

The speakers are very full range for a guitar amp, they're like a cross between a JBL & a Jensen, maybe try a speaker diffuser / beam blocker?

Hopefully the chips on the FX board can be replaced by more modern low noise versions?
 
CJ> I had never heard about the Polytone, they sure know how tu put a good comercial together. I love the two dissonant chords flying in the middle of the poster. I guess that was for a targeted audience.

So you all say this is normal and I'm a pussy then.

I still can hear some weird stuff above the simple hiss. Something that comes and goes with the notes.

I thought the jazz chorus series were supposed to be the cleanest cleans of all cleans, perfectly transparent blablabla... If that stupid noise stays I'm gonna end selling it I guess.

Thanks for all your advices!
 
doubleroger said:
I still can hear some weird stuff above the simple hiss. Something that comes and goes with the notes.

The BBD output is gated, that's why the hiss level increases when playing.


trancedental said:
Hopefully the chips on the FX board can be replaced by more modern low noise versions?

Almost certainly a noisy BBD, or associated buffer. There are also shaping and offset controls for these chips, IIRC.

You are right about many different revisions - some are nearly all discrete, some are more integrated. Depending on which one  you have, implementing an upgrade may be quick and painless or ... not. Start at the BBD and work outward from there.

 

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