Nikon D50 Repair - main board/firmware?

Hi all, got a troublesome Nikon D50 here. Powers up, takes photos, changes settings etc, but the Menu button doesn't work. Simple enough for an experienced electronics tech, right?

Hmm...

Tested the button itself, and it works. Traced the pinout from the zif connector to the button, continuity is good. (It's pin 6 on the zif, btw - pin 5 is Play, pin 7 is ISO, 8 is WB, 9 is Enter/Qual). Popped the main board out of the camera to attempt to trace the connection further, but sadly it's a multi layer board and I can't see the traces. Found a few extraneous solder balls stuck to the board, but picking them off didn't help.

So I put the camera back together and played with it some more. Turns out the camera actually responds to the Menu button being pushed (the top screen starts flashing the speed/aperture) but the menu itself doesn't come up.

Any ideas on fixing this, short of sending it to Nikon for a ridiculous sum of money, or replacing the entire main board for a slightly less ridiculous sum of money ($80-ish)? Alternatively, anyone seen this before?

(note: I did actually RTFM, and have tried both types of reset plus manually disconnecting the clock battery)