[Kenwood] TS-940, strange tuning problem

[Martin Sole]

I have recently noticed that my 940 has developed an odd tuning problem.
It's only really evident when tuning across a carrier, like the
calibrator signal, or if I mark a point on the vfo knob and count turns
up and down in frequency. The manual says 10KHz per turn but mine seems
to vary. Turn the dial very slowly and it does do 10KHz per turn. Turn
it a little faster and it becomes approximately 15 or 18 KHz per turn
and I get different values for increasing or decreasing the frequency.
Tuning across a carrier sounds rough and lumpy as if it's tuning
sometimes in 10Hz ateps and sometimes in something else.

I have been through the book and looked at the encoder outputs with a
scope but they look fine.

Anybody have any ideas as to what might cause this?

Martin, HS0ZED