[sdiy] U-20 questions

[Paul D Brooks]

--- Bob Weigel <sounddoctorin at imt.net> wrote:

> That seems unusual.  Hmm.  U-20 is a rompler. In
> some designs I can <cut>That's a strange one.   -Bob
 Tony Kalomiris wrote:
> > Only some notes in each octave (I forget which)
> work.I removed every <cut again, for everyone's
pleasure>

Ah, yes... the problem is almost certainly related to
the bottom part of the key contact system (if the
upper contacts have been cleaned properly)... not only
are the circuit traces part of the ribbon-like part on
the bottom, but there are some surface mount
components in there as well.  likely, the key scanning
diodes, iirc.

Hence, one of the key scan lines is out, so the
problem repeats itself.

Of course, this does not show up on the schematics...
the topology is there, but nothing to tell you how to
fix embedded circuit traces or diodes inside a flat
ribbon style plastic assembly. 

Roland is, of course, the only supplier with such an
assembly in stock (besides perhaps a few repair shops
willing to play ball).  They used this type of keybed
in plenty of synths in that era, and hopefully will
have some in stock... they are better than most in
this respect.

-paul