It was working just like it should before the moment when I took it to rehearsal place, then sound became too boomy, I thought maybe the batteries died, I came home and yes, batteries where dead. I went to the shop and bought 2 Duracell batteries, best I could find, I changed them, switched on Mu-tron and nothing, I could hear the clean sound but no effect at all.

I decided to take pedal apart to see if I burned it, it looked ok, I plugged my bass while the pedal was opened and it worked again, I almost believed it is fixed and put it together, sound became boomy again.

I took the pedal apart one more time and since that no effect at all, however clean sound could be heard.

What could it be?