[ARC Tech-109]

On the right hand side under the three blue connectors is the threader motor & gear assy, you can undo the three screws and lift it straight out. Under the threading ring are a number of "cams" or grooves that are used to move the various arms along the way, the ring can be removed and this will allow you to get after the stiff grease that's holding everything up along the way. Do not force the gear to move as these are actually quite fragile and are easily broken, a strong threading motor will break them and they're difficult to find.
I've had a number of the 4800's over the years and they're good decks. just take your time and numerous photos so you have an assembly record.

I like your Sony in your avatar, I had an AV8650 years ago that came out of a school auction for $20 along with the later video rover with the plastic body camera of which I don't recall the model number of.