Before any action on the JC120, try plugging the guitar through some linear power booster preamp (similar EHX LPB-1) to raise the signal for distortion.
LPB-1 Linear Power Booster Preamp
[ATTACH=CONFIG]55281[/ATTACH]
https://hotbottles.wordpress.com/2012/02/14/building-an-electro-harmonix-lpb-1-from-scratch/
[ATTACH=CONFIG]55281[/ATTACH]
https://hotbottles.wordpress.com/2012/02/14/building-an-electro-harmonix-lpb-1-from-scratch/
EDIT 190921
DOD - 210 FET Preamp (Portal requires registration)
http://www.freestompboxes.org/viewtopic.php?t=12099
DOD - 210 FET Preamp schematics
DOD - 210 FET Preamp (Portal requires registration)
http://www.freestompboxes.org/viewtopic.php?t=12099
DOD - 210 FET Preamp schematics
It's worth trying replace C21; C25 (.033uF) with .1uF; Replace C23 (1uF) with 10-22uF
If still not enough distortion, in parallel with R51 (220k) connect 2 diodes and resistor as desired max to 1k (red on schematics)
about JC-120 Distortion
https://www.youtube.com/watch?v=AaeSF-ZNHjk
https://www.tdpri.com/threads/roland-jc-120-distortion-question.430949/
https://www.thegearpage.net/board/index.php?threads/roland-jc-120-distortion-mods.636445/
https://www.diyaudio.com/forums/instruments-and-amps/291757-roland-jc120-distortion.html
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