[sdiy] optical retrofit for minis?

[Gene Stopp]

This is a very interesting topic...

A while back I had no qualms about hacking into second-hand analogs that I
had acquired. I added 1/4" output jacks to my ARP 2600, next to the 1/8"
jacks. I added CV in/out jacks to ARP Odysseys. A friend had a Musonics
Minimoog (s/n in the 1200's, metal panel, full dial markings, all
transistor, which I now own after a trade) and the keyboard contacts were
awful. Cleaning didn't help. I added an Electronotes-style digital key
scanner with DAC, tucked under the keyboard, which works great.

Nowadays I look at these machines and think, "in the far future there won't
be many of these left in factory condition". My mindset now is to get them
going as they were created, to preserve the history. A sympathy I know that
you, Kevin, appreciate. It doesn't bother me if somebody else takes a drill
to some vintage front panel, it's just that I'll think twice now before
doing it. These are all personal possessions, after all.

But what to do if the parts just don't exist anymore, in any practical
sense? Certainly it's the sound that's the bottom line, especially with a
Minimoog. Is there a way to restore full functionality without compromising
the history?

For the Musonics mini, I did remove the resistor string and added pull-ups
to the CV contacts. Then they got ribbon-cabled to the demux chips. The gate
bus isn't used. So I was a little more than completely non-intrusive
(remember this was my old self). But this thing is perfect now! No
mis-fires, ever. No sample & hold droop, ever. Perfect for external control
- hit a key to set the base note, and it stays in tune forever. MIDI-to-CV
all day, no problem. Is this not what you would want on stage?

Another friend had a mini with a messed up cabinet and crappy keyboard. He
gave me the "head" section, a 5-octave Panasonic keyboard, and a new
"stretch" case he had woodworked himself (nice job too), and asked me to
build up a 5-octave Minimoog. So I built up another keyboard scanner, of my
own design this time. I used the same Electronotes method (flip-flop to
block any further key-down pulses after the first is encountered, until the
scan starts over) which gives the position-priority. I used up/down counters
with a switch to select direction, which gives low-note (scan starts at
bottom) or high-note (scan starts at top) selectable priority. I used an
open-collector retriggerable monostable made out of an LM-339 for the gate
generation. I used a CV differentiator to give a trigger (active low open
collector) when the CV changes, and used that to "glitch" the gate which
retriggers the mini EG's. Switch-selectable single or multi trigger. I made
both linear (like the original mini) and exponential (like a Moog 951)
portamento circuits, again switch-selectable. So the result was a 5-octave,
high or low note priority, linear or expo glide, single or multi triggering,
stable, no drift, Minimoog. It was really a sweet machine (wonder what ever
happened to it?). There was a panel gap since the head is only 3 1/2 octaves
wide, so in that space I put a patch panel for external control plus an LFO
with VCA so an external CV could control the modulation depth (and free up
VCO 3 when using vibrato).

So I have blasphemed before, no real regrets, but now I like the challenge
of the retain-the-factory-design concept. I like the non-intrusive nature of
the optical idea, but you will have to disconnect some wires. I would image
that would be at the edge connector for the keyboard/envelope circuit board,
in the chassis where the harness is. Cut and splice with heatshrink.

If velocity and pressure were available, where would that go? Filter cutoff?
Volume? How to control the depth without drilling and adding pots? Hmmm...
if MIDI were added, the DAC could be 12 or 16 bit so all kinds of tricks
would be possible in the microcontroller. Then the mod depths could be done
in the micro. You could even add a soft LFO in there, with multiple
waveforms and sample/hold-like things. You could program it without adding a
physical user interface (i.e. non-intrusive except for MIDI jacks somewhere)
using program change, controller, control change, etc. from the MIDI
controller.

Hey this is evolving as I'm writing....

Back to reality. Maybe it's just getting the poor old Minimoog to play
properly again that is what owners would want. That should be easy. I used
74xx counters, latches, gates, CMOS buffers, and a hand-picked 1% R-2R
ladder. I did once implement this in EPLD (with external DAC) and it worked
fine, but that work is long-lost. So a little programmed part with a DAC and
a demux might be small and cheap. Or even just a handful of CMOS and some
precision resistors. Keep the low-note priority. Oops, what about Glide -
that's part of the analog CV circuit, so it would have to be duplicated. One
difference in behavior will be that the glide will complete even if you lift
the key before it's finished gliding. On the original it stops where it's at
if you lift all keys. I don't see a way around this... nobody may care
anyway.

Ramble ramble ramble... those are my thoughts... if I had anything to offer
it would be the schematics of my adventures, which I still do have
somewhere.

- Gene



-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Kevin Lightner
Sent: Tuesday, March 22, 2005 8:56 AM
To: synthdiy
Subject: [sdiy] optical retrofit for minis?


Lest I never complete a project that I desperately need to have done, 
I'd like to share what I've been spending a fair amount of time on 
lately. Any practical help would be hugely appreciated.

Here's the situation:

<snip>