Boss BD-2 works when engaged but not when disengaged

[Eamon]

Hi team.

I blew up my bd-2 the other day using a shitty power adapter.. I replaced the cap that exploded (C-3) which managed to get the engaged channel working, but when I switch back to the clean channel I get nothing. All wiring and solders look good. Any ideas where to start? I'm not sure how to follow the bypass signal so any help there would be great.. cheers

[The Dude]

Here's the schematic

http://www.experimentalistsanonymous...OSS%20BD-2.gif

[Mick Bailey]

Was the supply over voltage, or reverse polarity?

The two signal paths (effect/bypassed) are controlled by Jfets that act as switches. From the input buffer Q3 the signal splits off at R15. The section between C15 and Q4 is the effect signal path (pedal engaged). In by pass mode Q6 and Q4 are turned off, and the Jfet to the right of C12 is turned on (I can't read the designation from the schematic - Q8?). Because both the effect and bypass signals appear at C5, you know that this and output circuitry is good. My suspicion is that the fault lies between C13 and C5. Maybe Q5 is damaged, or the Jfet after C12.

An audio probe is a good way of following a signal path through a pedal - look it up. Using one will enable you to check where the signal is being dropped.

EDIT: Just as a test you can temporarily short Q5 base to emitter (in this circuit the transistor is DC isolated by C12/C13 so does not affect any other stage). In bypass mode see if the pedal now passes audio. Be sure of the pinout before you do this.