I have the exact same problem with mine.

I asked for advice some months back.

Not much in there that isn't already in this thread, though. Like yours, mine looked nice and clean inside with no sign of a solder break. I eventually decided it was probably suffering from a deteriorating rca cable. I never did get around to swapping it out with a new one, so my system is still kaput.

Checking out the cartridge slot is an interesting idea, Sweater Fish. I can't recall if I looked at that or not.

If you ever get that sucker working, I'd love to hear about it. If I ever get motivated and work on mine again, I'll post about it too.