[sdiy] Hacking up an old rhythm board...

[John Ames]

A few years ago, I salvaged the percussion generator board from an old
cheapo Kimball organ that gave up the ghost a couple weeks after
someone "thoughtfully" donated it to our church. I've had it kicking
around as one of my "one of these days..." projects ever since, but
now I'm working on a piece that I'd actually like to use it in - I
want those old-school Mini Pops beats, but it seems like every modern
analog drum machine inevitably uses the 808 as its point of reference,
sonically.

Fortunately, I was able to acquire a copy of the service manual for
the organ on the cheap, and it has a nice clean foldout for the board
with schematics and layout clearly documented. From this, I confirmed
that *almost* everything about the rhythm section is done on the one
board, which bodes well. The only things it needs aside from external
triggers are a white-noise source and a power supply; the output
signal is preamped before it leaves the board. It seems like it should
be *fairly* simple to turn it into a standalone drum machine, but I
have a couple questions as low-level electronics is not my strong suit
(my experience is limited to light repair/soldering and wiring up
passive circuits e.g. guitar stuff) and I'd really like to not
accidentally smoke this board that I only have the one of.

First, the power supply. Is it relatively simple to take, say, one of
those generic Meanwell switching PSUs and filter the outputs until
they're suitable for audio purposes, or should I be looking at a
different solution? The board itself requires +5v, -12v, and +15v,
plus whatever is required by the noise source on top of that. What's a
relatively quick-'n-easy way to get a halfway-decent clean signal to
these connectors?

Second, the noise source. Unfortunately, this was part of a custom IC
on one of the other boards that I didn't save, and there's no
information on what kind of maximum levels are expected to come in on
that line; fortunately, there's a trimmer to adjust it. It's possible
for all I know that the internal implementation was just basic
shift-register noise, but while I'm going to the trouble to hack up a
project out of this, I might as well do a proper one. MFOS has what
looks like a pretty simple design at
http://musicfromouterspace.com/analogsynth_new/EXPERIMENTERBOARD/page7.html
but that'd also require +12v from the PSU...interested to hear other
suggestions.

Attached is the schematic for the rhythm board, if anyone's
interested. It looks like the maraca and cymbal sounds are actually
the same circuit with two different trigger inputs; the service
manual's diagram for the pre-programmed beats seems to indicate that
the maraca sound is just produced by rapid re-triggering. Might even
be able to coax a guiro-type sound out of it if you ramped up the
trigger rate from slower to faster...
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