HP Draftmaster plotter; setting y axis encoding?

I have a problem that the pen head was moved on a Draftmaster plotter
and not it is up the clackers.

The main area is starts up, rotates pen holder, pen head goes across
(screech from belt at left hand end), comes back, reads paper width (I
think), then pows/collides with the pen holder and I get 1 or 2 messages

1) Load plot paper or
2) Y axis failure

I believe the encoder on the Y axis (L-R pen head movement) is out of sync.

Can anyone tell me how to sync it again.

Alternatively, does anyone know where I might get a service manual?

TIA