Just to verify - Uncle Dave is correct!
Always seems a bit sad when this sort of glitch happens but it doubtless down to complexity of internal architecture and memory and therefore cost.
Interstingly enough, my Ensoniq SD1 doesn't glitch, so long as the effect category associated with the voice is unchanged. If the associated effect changes from (say) reverb to distortion, you get a glitch. I don't actually think this would be a factor on the PSR, but I'll give it a go and set up a few registration entries with no effects on any voices and see what happens when I change registrations.
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John Allcock