Thanks for this - I have an F1 which has recently died, sounds like similar symptoms. I started taking it apart, too (figuring there was nothing to lose), but ran out of time to do much investigation. I will investigate yon screw-like object ASAP.
I've actually just acquired TWO F1s (one internal, like the old one, one USB) on eBay, because I couldn't live without the Audio Master mode (and I actually use the T@2 gimmick as well), so I have plenty of backups if this doesn't work!
I'm so gutted that Yamaha pulled out of this market... and I can't believe no one else has picked up on the need for writers for audio. Still, that's life eh?