It happened with my RC-838.
And now wiht my 9475. Adjusting speed ... imposible.

To reach motor you need to take the main board (unsolder many wires) and take out the mecha. Nothing like a Panasonic, where motor is here when you took out the front. Speed adjust is a game in this case.

To test the mecha (I put new belts and motor) and adjust the azimuth I done the following:

I powered the motor with a supply
I used a crapy BBX's (a nasty Philips) head amp
I soldered 2 wires in it's head amp and then put in the head cable.

Now I can test and adjust in a confortable way.
I playbacked for five hours, listening for noises or some flutter event. It was ok.
So I put the mecha in the JVC again. I know that when I finish connecting it no surpirses will be waiting for me.

Yeah.... Sometimes life can be a BIiTCH! et us know how it goes!