maybe this sounds too easy, but what about just replacing it with another unit? The 500C uses one of the sub-chassis type units that come out fairly easily right? I've bought those things in working shape for under 30 bucks. Heck, I've got one sitting on the bench right now, waiting for me to build a box and power supply for it so I can use it as an external decoder. I applaud your tenacity with troubleshooting this, but personally I run out of patience with things at some point. You're well past the point I'd have fixed it with a hammer. That would have probably happened like 4 1/2 years ago.