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January 30, 2023, 10:20:33 PM

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Roland JC-120 crossover distortion

Started by cancon, February 01, 2021, 04:55:09 PM

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Hi all,

I'm a tube guy, I know some SS stuff, mainly simple Wurlitzer EP boards. I'm completely bewildered when it comes to these JC schematics, they're a bit intimidating!
I was hoping someone could point me to the location of the output bias voltages. This is a 3rd revision schematic, serial 533XXX. I'm not sure if I'm allowed to upload the schematic as I've never posted here before.

Mainly, the issues are subtle crossover distortion on both channels, plus the distortion effect doesn't seem to work. Also on the second channel, there is noise behind the test signal. When you kill the signal, the noise persists for a second and then goes quiet. It may be my imagination but the noise has a chorus effect on it. 
The PSU is fine, +/- 43V & +/- 15V rails are making it to each board.

Any tips would be appreciated! thanks for your time :)


I only have this one: https://music-electronics-forum.com/filedata/fetch?id=853791

It looks like yours would be in the next set, but if the power amps are similar to these (which I think will be the case) then the bias is supplied with the Vbe multipliers Q9/Q10 and Q26/Q27 (first schematic, p7) which aren't adjustable.

Teemuk's book on SS amps (free, links at this site) is a great resource for explaining some of the more esoteric designs that are guitar/bass amp specific. It also includes a few variations on the Vbe multiplier. If you already know the tube theory, then his book will be easy to get into. Highly recommended.



Yep that is the correct schematic, its the last one down.
Thank you for the book tip, I'll have a look at it.

I've discovered that both my amplifier boards have 0 idle current.
There is something likely going on with the effects board.