[af024]

Thank you for all of your great comments, links and suggestions. I am encouraged.

Just now I've managed to grab a few moments to drag the 'scope and a multimeter out to see what's what.

Here are the results:-

Motor supply blue/red = 5 ohm
Tacho grey/grey (shown drawing as yellow/yellow on circuit rather strangely) = 408 ohm
Motor supply volts = 38.4 Vd.c.

Scope trace settings (sorry about the bad photo):-
5V/div, 0 Vd.c. on centreline, 0.2ms/div
So I make that circa 29V pk-pk off the tacho, and 1351 Hz (motor running flat out off the 38.4 Vd.c. supply)
The above measurement were taken with the tacho not connected to servo and the motor supply taken from the faulty servo output (38.4 Vd.c.)

There is no difference when the servo tacho circuit is disconnected.

The two diodes that I mentioned seem ok, although I only tested them as standard diodes, not zeners.

I also came across this interesting service bulletin (about the servo and diodes). I'm guessing that they were expecting trouble way back then for some reason!

Regards,

Andy