I suspect the issue is power related based on what Paul suggested. It makes sense that calibration wouldn't work if there was noise on the power supply.

In fact, one time we calibrated, it seemed to be in key. I thought it was just a fluke since we tried again and again after that and it was off by a similar amount to the previous times. We just happened to calibrate it when the power oscillation was in the right phase.

When we had it apart I noticed that the whole system is powered by a giant AC transformer running into two full wave rectifying diodes which then feed linear regulators bolted to a giant heat sink. Gotta love that 1980's tech.

My guess is, since power measured okay with a meter, that the electrolytic caps in the power supply have gone bad. Since they are 30 years old, I'm not surprised. Are there other caps we should check?

Thanks much for the help. It was great fun to see that old synth come back to life.

Brian