Roland JC120 Distortion

Carlosraj

Member
2010-01-17 11:12 am
Hi

This amp has a problem with the distortion circuit. When I plugged in a "line out" signal from my CD player and turn on the distortion knob, this is what I noticed.

1) the volume drops to a low level
2) the distortion sounds abnormally distorted.
3) when I turn the distortion knob up even to full, there is no difference in distortion volume, the level remains the same .

I tried another thing, instead of a "line out" signal, I took the signal from the "headphone out" . Now I can control the level of the signal going into the amp by using the volume knob on my CD player. As I reduced the level from full downwards , somewhere around 2/3 volume, I realised that the distortion works as it should. When set at full volume on my CD player, then the amp shows all the problems indicated above. Please could someone assist me with this problem.

Thanks

Carlos
 

Carlosraj

Member
2010-01-17 11:12 am
When the distortion pedal is pressed, a lower voltage is sent to Q4, switching it off and reducing the volume. That allows for over driving of the pre amp section producing heavy distortion.
It sounds to me like the signal level you are using is not the right amount.
Has someone twiddled with VR5?

Sorry, I forgot to mention that I have changed all the JFETs (2SK117) on the input board. The distortion pot VR5 is the same as on the schematic (10KC). SM attached