[sdiy] Kawai SX240 issue

[Travis Shire]

Hey all....

I have a Kawai/Teisco SX240 that operates fine other than this slight
problem. It seems to be tied only to the portamento function. What it does
is...when set to mono stack mode with porta on, it will occasionally stick
on the previous note gating both the note you glide from and to. It doesn't
seem to follow a set pattern, just does it when it feels like it. If set to
poly4 or poly 8 mode, it will just miss a note sometimes. Turn off the porta
and everythings normal. Moving the pitch bend pot will get rid of the stuck
note. Tapping the LFO disable switch will stop the extra note as well,
but.....if you keep tapping it the note comes back....then goes away again.
Wash, rinse, repeat for as long as you keep tapping that switch.

Now in the block diagram for the hardware there is a comparator and 10bit
DAC that ties into the pitch bend circuit that is operated by the slave
processor (8031). But I believe it to be a digital issue since the LFO
switch influences the erratic behavior.

For the record, all supply voltages are stable and ripple free. There was a
small amt of battery gas damage, but everything even remotely affected by
that has been replaced, discolored traces hardwired, and questionable solder
pads not used in favor of wiring the component(s) to the next good spot down
the line.

Any ideas here? I may look to that DAC first. I do have the full service
manual that I'll try to get scanned and posted to a photo hosting site.