Discussion about the Korg PolySix synthesizer

So I managed to complete my first replacement PCB.

Things don't look too bad but it has a number (?) of problems that are a
bit difficult to debug for me.

Basically it runs somewhat unstable. Occasionally it will more or less
reset itself or will exhibit some other behaviour which will explain in a
second.

Nearly all chips are new, except processor, LM13600's and RAM. All
functionality is there; calibration was more or less successful. I haven't
yet a battery installed.

THe most common symptom at the moment is that some LED's on the KLM371
start to blink or that a number of LED's light up permanently (and others
get turned off). This has happened with different KLM371's so I suspect the
error to reside on the KLM367. Oddly enough often the sound isn't affected
at all initially and the switches still work fine. I can call up other
programs and other banks.

Sometimes shortly (or several seconds after this) the LFO modulation is
turned off (I have programmed this particular test sound to modulate the
VCF). There might some other parameters change slightly as well; I have not
yet narrowed this down completely. Sliding the LFO switch back and forth
restores the modulation; so the actual LFO depth isn't affected, just the
target.

So, it appears to me (grain of salt...) that the parameters controlled by
IC28/29/34/35 may be affected, but not the ones controlled by IC18/19
(post-DAC).
This and the working switches leads me so far to the conclusion that the
data lines D0 to D7 may not be the culprit here and that the problem could
be related to lines P24 to P27.

Could this be another "bad RAM" issue? My first salvaged RAM was definitely
dead; it wouldn't store anything. The current one seems to work as far as
storing goes.