[sdiy] Fun with the Oberheim M6

[Tim Parkhurst]

Hi All,

A few observations from a weekend of getting my pair of Oberheim Matrix 6s
working - 

M6 #1 had a problem with two voices having a higher VCF cutoff setting than
the others. The problem persisted even after running the calibration. This
resulted in two voices sounding 'buzzier' than the rest, and was especially
noticeable in patches with a low cutoff setting. The M6 keyboard uses the
wide body version of the Curtis 3396 synth voice chip, and since I had a few
spares (two from Chipforbrains on ebay, another from John Mahoney), I
thought a simple chip swap would cure the problem.

Well, it turns out that
1) The 'voice in use' indicators on the M6's display indicate the voices
from left to right (#1 on the left), but the voice numbers on the PCB go
from right to left (#1 on the right)! 
2) Swapping the chip for the wrong voice didn't cure the problem ;)
3) Swapping the chip for the correct voice didn't cure the problem either.
4) There are a lot of pins to bend on the 3396.
5) When you try to remove an IC and slip, IC pins are capable of perforating
a thumb.
6) I really need to buy a proper chip removal/insertion tool.
7) Swearing, bandaging my thumb, and reseating all the 3396's didn't cure
the problem!

I was running the calibration routines after every chip swap and I did
notice that the problem was getting slightly better (still noticeable, but
not as bad). Well, on a lark I decided to run the calibration three or four
times and THAT CURED THE PROBLEM! John M and I both thought that maybe the
VCFs were out of 'tune' far enough that they couldn't be brought back by a
single recalibration, and that the internal processor was able to bring them
back a little closer each time (each run building on the results of the
previous).

Has anybody had this sort of thing happen? Perhaps it’s a lesson for all of
us using micro-driven analog equipment.

By the way, M6 #2 wouldn't make any sound, but that was just because it had
really weird, scrambled patch data. Loading in some new patches brought that
one immediately back to life! I wish every repair was that easy!

Anyway, just wanted to share those findings. Ciao!



Tim (needs more than one recalibration) Servo

"Imagination is more important than knowledge." - Albert Einstein





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