Regarding the capstan-speed / servo-lock issue, it may be worth having a look through this (recent post?) - in case you haven't already discovered it:
https://www.radios-tv.co.uk/communit...age-artifacts/
Which I found at the bottom of this posting (where they'd also had belts turn to a gooey mess):
https://www.radios-tv.co.uk/communit...n1500-finally/
- This has quite a few good photos of the insides / the special 'jig stand'
Plus it also mentions the Combiined Electronic Services (ces) = Philips / Pye Service Link publication, which may be available on their to members
(I've got 3 copies from 1974, which includes info & mods etc. on the N1500, so had been trying to find if any others were available on the 'net)
I found I actually picked up 2 copies of the service manual over the years, and both seem to have several addendum inserts (in different places, but same contents?) with details of manuals corrections / changes & production changes.
- Of which some of the first seem to be a revised version of the infamous loading pulley plus they also changed the motor for a slower more powerful one to reduce noise and also had a flat on the side of spindle at the top to sort problems with the gear slipping.