Neal,

Yep I was a little worried that it was putting out 19.5v against the service manuals 18v so the first thing I did was run it directly from a DC bench supply (taking the external PSU completely out of the picture) and varying the voltage from 16 – 20v made absolutely no difference to the speed of the motor.

When I get a bit more spare time I’m going to have a good look at that reference generator circuit, there is a zener in there that could be the issue. Problem is that it is near impossible to measure anything while running the unit.

I placed the resistor in the brown (common leg of the speed change switch) wire of the external pitch control, seems to be doing the trick at the moment.

The main speed controls work perfectly.

Cam