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Roland Jazz Chorus 120?

Started by d4v1d5hu13r, April 19, 2007, 09:13:55 PM

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d4v1d5hu13r

Anyone know how to mod the gain/distortion circuit to achieve hi gain output instead of sloppy over-mud?

teemuk

My advice: Forget the idea of modding and instead try it with an outboard distortion effect or perhaps with an external graphic EQ for better voicing of the distortion.

If you want to mod it try varying the values of R32 and R43 for starters. These series resistors "round" the transfer curve of the clipping diodes. Switching R22 for smaller or R23 for higher will provide more "drive". You can also try adding a bit more gain to the first stage. C32 and C33 naturally control what frequencies the diodes pass. There is also C31 that shunts the whole circuit; since it cuts off high frequencies it might cause "muddiness" as well.

Just to make sure, I'm talking about the circuit in this schematic:
http://www.blueguitar.org/new/schem/_ss/roland_jazz-chorus1.gif
There is also Jazz Chorus that has discrete stages:
http://www.freeinfosociety.com/electronics/schematics/audio/Roland-JC120-JC160.pdf
Which one do you have?

The distortion stage in that amp is a pretty generic circuit but no-one can tell how to mod it to suit you except you. If you really want to mod it I suggest you simulate the concerned stage with SPICE until you nail the thing you are after. Remember that there's also the the tonestack following the stage causing a midrange scoop. It might also prove that the circuit does not bend to those tones, after all that amplifier is famous for it's clean tones.

d4v1d5hu13r

I get you for the most part. But when you say change the values, how would I do that? Meaning what kind of parts am I going to need to change the distortion?

teemuk

By "values" I mean either resistance or capacitance depending on which component is in question, resistor or capacitor.

d4v1d5hu13r

Do you mean changing the size and power of the capacitor/resistor?

joecool85

Just the size.  IE - instead of a 0.047uF cap, try a 0.1uF etc.  Or instead of 2.2k try a 5k resistor.
Life is what you make it.
Still rockin' the Dean Markley K-20X
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zuzu

David - if you're thinking of an external distortion then I highly recommend the MI Audio Tube Zone.  It's the most tubey sounding pedal I've ever played, and it doesn't have a tube in it.  It can make any amp sound good provided you are prepared to work with the eq on it.  The EQ section is extremely responsive to changes and a massive variety of tones can be had from this pedal. 

teemuk

This is hillarious: I just found a third JC-120 / JC-160 schematic (dated 1979). Again completely different fom the other. How many of these are there?

d4v1d5hu13r

#8
I don't know, mines pretty old though. The light doesn't even come on when I turn the amp on. My signal is a popping sound.  :P

My serial number is 529195 if that helps any. Help me find the right schematic please. And as far as anything goes, I understand for the most part on how to find the right thing and change it. I just would like to grab the caps and resistors the crate[gfx?] series uses. Not for the solo part, but at least so I can get a nice crunch.

teemuk

Quote from: d4v1d5hu13r on April 25, 2007, 02:46:22 PM
I don't know, mines pretty old though. The light doesn't even come on when I turn the amp on. My signal is a popping sound.  :P

Changing the indicator LED and checking the condition of its series resistor should be a pretty easy fix. What kind of popping sound? There might be more problems in your amp: If it makes sudden popping and crackling sound now and then it means the filter capacitors are beginning to die. (That wouldn't be a surprise since the amplifier is likely at least 20 years old). That in turn will increase hum and those pops are not very healthy for speakers either. One day the capacitors will eventually die and when doing so might fail short circuit, which will of course blow your amplifier in common folk terms. Maybe you should do some basic maintenance for it before worrying about the distortion circuit.

About the right schematic: I did some searching on this. There are likely at least eight or nine versions of JC-120. Roland states that the model name is actually expressed as JC-120-xxx, where the xxx is a letter code that is not explained any further. The letter code is not shown in the schematics either, but then again they are just scans from the service manuals. The schematic from 1979 is the only one showing the serial number range of the concerned version (for JC-120 that is 481650-502499) so at least this is not for your amplifier.

So far I have only two legit schematics of JC-120 / JC-160 that actually say they are for those amps (and those two differ like night and day). The 1984 schematic (http://www.freeinfosociety.com/electronics/schematics/audio/rolandjazzchorus.pdf) might also be one for JC-120 but I'm not 100% sure of that anymore. If it is, it might be pretty close to your amp judging the serial numbers. Then again it might be completely different. I'm afraid you have to find out yourself but if you have skills for modding the circuit this should not be a big deal. Contacting Roland might help as well.

d4v1d5hu13r

The pop is [i assume] power running through the amp and it's like "HEY!" Even on max volume in the high gain input, it's a very low pop and the speakers jump a tad. I thought it was just the power getting to the power amp in a ball of energy but I'll make sure to check that out. I'm probably going to save money and just have all the caps and resistors and whatnot replaced with new upgraded stuff that will even improve the clean tone[it'll be super super clean!].

teemuk

If that popping sound is just the normal startup "thump" you get when you turn the power on (and sometimes off) then it's no big deal. If you hear those popping sounds now and then, then it's the aging capacitors.