Sony VO-4800 not threading up

[MX70]

I have a Sony VO-4800 that I've owned for almost 30 years, and has operated flawlessly, on the occasions I have set it up ( mainly for powering my DXC-M3 and DXC-1820). I've never even had the top off it in all that time.

But today it would not try to thread up.

The head drum spins up, capstan spins, but no movement at the threading ring.

I can manually move the ring into full threaded position, then the machine will play, wind, fast wind, record as normal.

Ejecting the cassette, it unthreads without a hitch, and cassette pops up.

I'm not that familiar with the mechanics of this machine , and unfortunately don't have a manual (yet) , but I am thinking a switch is dirty, or broken ( don't know how that happens , no one uses this unit except myself), but I thought I would ask anyone familiar with this machine if they have any ideas.

I do want to get this machine up and running again as I use it for my two cameras mentioned above, plus I do actually use it for recording on artistic projects.

Many thanks for any help!

Max