[HELP] Boss HM-2 switching problem

[fbsc]

Hi! I'm new here, and I'm sorry if I got in the wrong section.
I have a Boss HM-2 that has a strange problem: when it is bypassed no sound passes! When, instead, it is on, it has a quite low output level. I already tried substituting Q1 and Q4, but no good...
Where could the error be? I've been working on it for two weeks, but it's not easy to use audio probe on this board!

[cmilless]

Q3 is the output transistor. The bypassed audio goes through Q2 and Q5. I'd check those and see if they are working properly.