Mono/Poly Tuning

[Shutoku]

My trusted old Mono/poly seems to be slowly developing a bit of a tuning issue.

Initially I noticed one day, that to get it to standard pitch, I had to turn the master tuning knob down considerably, to about 8 o'clock.
I took it to me local repair guy who is generally excellent, but not a synthesizer specialist or anything.
He did something to it, but while the master tuning knob could now be at 12 o'clock, all of the individual oscillators tuning knobs now had to be at around 8 o'clock.

Oh well, no big deal I thought, and have continued enjoying it for another two or three years.

In the last week I noticed I now have to not only have each oscillator at 8 o'clock, but now the master tuning knob has to be at 10 o'clock instead of 12. I'd really like to get everything back to 12 o'clock at A-440, but more to the point I don't want it to continue to drift sharp.

I'm not likely to open it up myself, but if anyone knows what technically needs to be done, I could then show it to my tech, who admits he has not had to work on a nearly 40 year old synthesizer in many years.

Thank you in advance for any information.