Thank you for your reply.

We attempted to tune the channels independently of each other. Tuning them one at a time does a lot better job, but Channel 6 tunes very poorly. The lows are flat and the highs are sharp. Channel 6 is about 9 semitones off at both ends.

Channel 1 is the second worst with slightly better tuning.

Channels 2 through 5 are decent after calibration and if we shut off channels 1 and 6 they sound pretty decent.

When calibrated with "tune all" the calibration is much worse and when all channels are enabled, playing notes spread across all the channels sounds bad, especially the high notes. If you play the notes sequentially, overlapping them, Ableton shows that calibration varies a lot, especially on the high notes.

There is one more observation: Channel 1 and 6 are slow to change frequency. When we play a high note and then a low note with only one of those channels enabled, we can hear it sweeping down to the correct note, where as that is not as prevalent on the other channels.

Does this sound like the oscillator caps are the problem?

We have poly styrene replacement caps that we purchased just in case. We were thinking of replacing the caps on channel 6, since it's the worst to see if it gets better...

Does that seem like a good approach? Is there something else we should check? We have a meter and a o-scope.