> From: chromapolaris@yahoogroups.com
>
> Unfortunately we now have a new problem to solve. Every note
> is out of tune - one quart to one half step sharp. The sound
> of the oscillators appears to be wavering. We attempted the
> Tune All function, which completed but did not change or fix
> the tuning.
>
> We read in the manual that you can disable the oscillators by
> setting them to Pulse with the width at 0%. When both
> oscillators are set this way, the keyboard makes sound. Is
> that the expected behavior? We tried isolating the issue to
> the oscillators but we don't understand what the proper behavior is.
>
> We tried isolating the 6 voice channels, but all channels
> played exactly the same, too sharp. They all wavered and were
> out of tune.
>
> Using the Pitch Bend lever, we could force it into tune while
> holding the lever in place.

If the oscillators are all over the place, it may be a problem with the DAC, or with a power supply, in which case the pulse widths would be all over the place too, so setting them to 0 won't necessarily shut them up.

> We found an article online of an Australian Polaris owner who
> had the same issue. He had to replace all the tuning caps on
> Channel 1 (C102-C108). Do you think this could be the same
> problem? If not, how could we debug this? We have an
> oscilloscope and multimeter and schematics.

> The original capacitors are polypropylene and my engineer
> friend doesn't know what that is. Can we just use ceramic caps?

Most ceramic caps have high ESR. The polypropylene have low ESR, and low leakage. Those are the two important characteristics. But I doubt that's the problem, because that would result in only minor tuning issues.

Since you have a scope, you should make sure the power supplies are clean (no 120Hz ripple). And you should look at the control voltages to the oscillators on the sample-and-hold op-amps and see if they're moving around in synchronization with the wavering pitch.

If the pitch is stable, but way off, then it could also be a problem measuring the frequency during auto-tune. But "wavering" sounds like something power supply related.

--

Ciao, Paul D. DeRocco
Paul mailto:pderocco@...