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

[Mike]

I have one from a '63 Conn organ. The sound is beautiful. I'm paying
attention to this thread because mods seem like a natural thing to do. 

Did you see the video of Moby's analog drum machine collection? 

Mike


On Thu, 22 Oct 2020 17:41:59 -0700
John Ames <commodorejohn at gmail.com> wrote:

> 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...
>