Technics RS-B105

It's not plastic, and it's not yellow, but I'm trying to fix it anyway!

I'm going to throw notes and information as it comes up into this thread. Input from anyone who actually knows what they're doing would be awesome!

Thakker shipped me a new set of belts to the other side of the planet at a very reasonable price! Danke, Thakker!
https://www.thakker.eu/en/belts/tape-deck/technics-rs-b-105-belt-kit/a-2205/

I got a PDF of the service manual here, so I'm well set up to give this a good crack:
https://www.manualslib.com/manual/3432400/Technics-Rs-B105.html

Still looking for a copy of the user manual, for the sake of completeness but it's not important for the repairs.

There's a thread here that sounds like exactly the same issue I'm seeing on mine, but with a B205 model instead. Interesting! Probably the same mechanism!
https://www.tapeheads.net/threads/technics-rs-b205-clicking-how-to-change-belts.64580/

Part two, where I actually replace the belts, is now live!