> A problem has just started where every sixth white key does not
> produce any sound.

It is actually every sixth white key - or is it every sixth distinct key that's
pressed (white or black)?

If it is every sixth key (and assuming that this problem just started to
happen), I'd expect something has become unwell in one of the voice channels
(there being 6 separate voices in the Polaris).

One quick thing to try would be to force a "tune all" and see if it at least
will get you to where you have no 'dead keys' (although the auto-tune might
reject the 6th voice, thus making you have a 5-voice machine). You can see
what happens by watching the red LED when you press [Lower Function]-[Tune All]
(i.e., how many flashes).

> Also, for some time my MIDI in and out haven't
> worked. Any suggestions on where to start to fix these problems?
> I have some experience with electronics. Thanks.

If you can tell us what LEDs light up when you press:

Lower Function + B (Interface)
Lower Function + C (Cassette)
Lower Function + K (Midi Channel)

we can check to see if there's any conflicting configuration which might affect
MIDI.

Best regards,

David Clarke