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.
Considering the cost of shipping your Yamaha in both directions, it would help if you told us the area you live in. My gold Yamaha RX-V1 is currently being repaired locally at a cost of $432, so be prepared.

I doubt that there is anything wrong with the DSP-A1. When using main pre out, you must remove a pre out/main in jumper bar. This disconnects the pre amp from the internal main amp. These need to be installed for the main amp to work. So you have no idea where these jumpers are, no problem, just use an interconnect cable between pre out and main in on each channel.

See page 21 here,

Another thought is that if you did install the jumpers, are they horizontal and not vertical?
So what did you decide to do with the DSP-A1, fix it, sell it, or did you locate the problems?
I also have a DSP-A1 and used it in the same manner. I concur with Tls49. You probably removed the jumpers and forgot to replace them.