> From: btmckee9@...
>
> The behavior changes between channels. Channel 5 is more
> consistent than channel 6, but it also tends to go sharp on
> the high notes and flat on the low notes (although that
> didn't seem to be the case before we swapped the oscillators
> with channel 6, see below, but again things are ... confusing
> since we don't know what the proper behavior is. Channel six
> is off by as much as 21 cent on the low end, where as channel
> 5 is typically only 8 or so.
>
> We swapped the switches and it didn't seem to make a
> difference. We swapped the op amps, and it didn't seem to
> make a difference. We swapped the oscillators and there was a
> change, even after calibration, but the behavior is still
> pretty much the same although channel 5 may have gotten
> worse... It's hard to tell...
>
> I'm tempted to try putting in new caps on the op-amp feed
> circuit since I have the right kind, but there are eight and
> pulling the board for no reason seems at best reckless.
>
> While I'm still here, is there anything else we can try?
>
> I'm going to read the previous posts and look at the ADC circuit.

21 cents is definitely bad. Even 8 sounds like there's something slightly wrong.

You don't have to replace all the caps in the sample-and-hold circuits, only the four oscillator ones are relevant.

Another thing you might try is thoroughly cleaning the board in the sample-and-hold area. Remove the chips, douse the area with alcohol, and brush it off with a soft bristle brush, or blow it off with compressed air if you've got it. Those circuits are susceptible to contamination.

Another thing to check: start with a raw sound, like you get from an empty program position, and move the master tuning very slowly and listen carefully. Make sure the pitch changes smoothly, and doesn't have any sudden jumps in it. That can expose bad low-order DAC bits. And you should also put a meter between the DAC test points and adjust the null if you haven't already done that.

I don't think the ADC has anything to do with this. It's just for reading slider positions and performance controls.

--

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