1014 multichannel problem

I have run into a problem with my Pioneer 1014 receiver. It has suddenly stopped sending a signal through the left front pre-out when using a 5.1 multichannel source. I have tried various combinations of speakers, cables, pre-outs, surround settings, and sources, and it is clearly only the left front pre-out and only when a multichannel source is used. Are there any thoughts as to whether this may be due to a problem I can fix, or is a trip to my local Pioneer service rep in my immediate future? Thanks a bunch.


Greg