How to fix Yamaha DSP-A1

I bought a Yamaha DSP-A1 when it first came out and it provided years of home theater enjoyment. About 4 years ago, I bought larger L-C-R speakers and I began to use the DSP-A1 as a preamp for those 3 channels. The preamp outputs for L-C-R were connected to larger external amps while the surrounds and "front" channels remained on the DSP-A1. This worked fine, and I recently bought a proper 7.1 preamp, so I disconnected the DSP-A1 and was hoping to use it as a stand-alone unit again.

Unfortunately, the years of not having a speaker load on the L-C-R channels seems to have blown something in those circuits. The surrounds and "front" channels still worked fine, but there is only silence in the L-C-R channels. It seems a waste to discard such a classic unit, still in pristine shape. Is this an easy fix? Any insights on how I might begin to fix the DSP-A1 would be most welcomed! Also, if anyone knows of a good repair shop for a Yamaha DSP-A1, please let me know.