On 02/28/2017 06:46 AM, N_Cook wrote:
On 25/02/2017 14:28, N_Cook wrote:
Stored indoors for some years and then powering up, missing notes and
some top panel sw functions missing.
full sm here
http://www.synfo.nl/servicemanuals/R...VICE_NOTES.pdf
same 2 notes ,not each octave , but as keyboard is scanned as 8x8 matrix
keys 8n and 8n-2 are missing , n= 1 to 8.
Hopefully that means the 74LS138 3 to 8 demultiplexer did not survive
1/3 century,internal metalisation creep or something, rather than main
micro problem.

no factory reset sequence found for this model. Reseated the socketed
memory JIC, but no change
As some of the function switches are out as well as keyboard, part of
same polling structure, i'll go with a problem on the digital side
rather than VCF/analogue. Continuity back to 40245 ok , but as 2 of the
octal lines out , presumably back farther than that, have to do some
digital probing after finding bench space.
One tip for dealing with these, small cable ties between top and base,
to act as a hinge for probing access to the internals and keep
everything in reasonable registration so no strained cables.

My Alpha Juno 2 from around the same timeframe had a problem with the
membrane switches on the front panel not activating correctly; I traced
it down to a bad pullup resistor in one of the SIP packages attached to
the bus headed from the switch board to the demultiplexer IIRC.

Might be worth looking at too...