[sdiy] Polymoog - was Organ circuits & Mini-Korg K-2

[Gene]

It sounds like you have severely corroded contact spring-wires and bussbars!
That is not a pretty thought.

The polymoog can be a real time-chewer. I bought 2 last December in a broken
state... I promised to post my story some time ago but time is in short
supply these days (some spent on fixing polymoogs!). I had to rebuild both
Pratt/Read assemblies but the contacts are in good shape.

Anyway one of them is working now. Things I did:

Rebuilt dead power supply (using LM323/LM317/LM337)
Replaced:
3 bad polycom cards (one which was smoking a power resistor in the -5
supply)
Many front panel LED's (like two dozen)
8 divider chips
2 50240 top octave generators
Many LM1458's
A few discrete transistors
A couple of 555's
A bad resistor in one of the exponential converters (man was *that* one hard
to find!)

The low-profile LEDs are a pain in the ass. They were burning out on me
right and left, forcing me to disassemble the thing far too many times. If
you use normal-height LEDs, they poke through and possibly rupture the panel
plastic overlay. I had to make my own low-profile ones by taking normal LEDs
and grinding away their base with a Dremel. Thank God for Dremels.

Once you come to an understanding of what they meant this machine to be, it
actually became easier to troubleshoot because I could predict what the next
problem would be! I think I have developed an intuition for Moog engineering
styles of the mid-70's. It's just like you would see in Electronotes.

The thing is massively complicated, but conquerable. It's probably the most
complicated purely-analog programmable machine ever marketed - anybody have
another candidate?

Of all the parts the only ones I stole from the other dead polymoog were the
divider chips. These are going to be hard to find at a good price. I found
some for $15 each, but if I spend that much I might as well spend a little
more and buy a used D-50 which is certainly a more capable instrument. But
where's the fun in that?

I was afraid that after all of this I would just end up with a glorified
Omni. Fortunately this beast can be severely fat-sounding. With separate
vibrato on the two stacked top octave divided ranks, plus yet another LFO
for PWM on one rank, the thinness of top-octave generation is completely
obliterated. Regarding the sounds - the presets are not very good imitations
of what they are labelled, but maybe useful somehow in a mix. The strings
preset gives a fantastic analog-type detuned-oscillator sound, which is
totally polyphonic, and the VCF can help a lot here. You've got the
capability of long sustain times, so you can get that decaying chime effect
without note stealing. However it's not easy to justify so much mass and
circuitry for one sound. Fortunately the VAR mode (front panel mode) tips
the scales in favor of keeping the thing around because you can get some
pretty spacey noises.

The trick I've found is to combine the multiple outputs - there's DIRECT,
which is the raw unfiltered tone generator signal, then MODE, which is the
same thing except sent through formant filters (different ones depending on
which preset is selected), then RES which is DIRECT through a triple
state-variable parametric filter bank, then VCF, which is the DIRECT through
the 4-pole lowpass with ADSR and LFO mod. If you use the MIX output jack,
and change the relative mixes of the multiple output modes, then there's all
kinds of phase cancellation going on. Especially if you use the resonator
section with high Q settings - you can tweak things until there's all kinds
of sweeping spectrum weirdness. This mode of operation I didn't expect and
kind of stumbled on, and it's not easy to change everything and then find
the exact sound again.

Since all of the preset parameters are determined by hand-soldered resistor
networks, it would be concievable to alter the presets to something other
than bad imitations of other instruments. This would affect the vintage
resale value, but these things are far from mint anyway so I might consider
this someday...

- Gene



-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Tony Allgood
Sent: Saturday, July 21, 2001 6:51 AM
To: Synth DIY
Subject: Re: [sdiy] Organ circuits & Mini-Korg K-2


>Or, for a real true life horror story, "The broken synth that ate all
the spare time."

I think I'm in the sequel right now.

I have just got hold of Polymoog as payment for a small job I have just
done... and after the first five hours of trying to get the thing work,
I am seriously thinking of bursting into tears.

I have never seen a keyboard action so badly affected by age. I would
say 10% of all the notes actually work, and the only way to get them
going is to hit them very very hard. I can now see why Gordon Reid
(Sound on Sound)) despises his so much.

But it has got two working uA726 inside it... :-)

Regards,

Tony Allgood, Penrith, Cumbria, England

Oakley Modular Synth and TB3030:
www.oakleysound.com/projects.htm
My music: www.mp3.com/taklamakan