I recently bought a used M13 and so far it's worked great.  However, the FX in the entire leftmost bank (and some of the second-to-left) suddenly decided not to make any noise.  It's like they're permanently bypassed: the guitar sound still goes through but none of the FX colour the tone.  Even diming the Mix control on the effects doesn't help. 

Has anybody else had this problem? Any recommendations? Thanks in advance.