Vintage Synth Repair

<< if i'm having trouble getting the keys on my Realistic by MOOG synth to 
trigger, does this sound like a contact-cleaning job? >>

possibly, possibly not. 
the mg-1 (which actually predates the rogue, it's distant cousin) has a 
"polyphonic section", which is a top-divider circuit. however, it uses a good old 
normal synth keyboard with just the usual wire contacts for voltage division 
for the mono synth.
the second set that would normally be used to derive a trigger for the mono 
synth are pressed into service as the polyphonic keyboard. it's a bit like an 
elka or a crumar, if you've seen their string synths and organs. the trigger 
for the mono synth is created by detecting the minute voltage changes whenever a 
new key is pressed. I'm doing this from memory, so apologies if this is a bit 
sketchy. I do have the schemo somewhere but I know it's been on the net too. 
let me know if you're still stuck. there is an account somewhere on the net 
too of it's design, which was supposed to be a lifeline/partnership/marketing 
exercise when moog were struggling in the early 80s. 

my other moogs are two prodigys, one from each end of the run (they sound 
identical though) and a LAMM memorymoog, which is the absolute dog's nuts and 
which I routinely use to terrify the neighbours.

the foam that was intended to stop dust getting into the fader-pots is famous 
for disintegrating on several moogs of this period, and it really is 
disgusting once it goes. I don't think it harms the faders too much. my mg-1 (which 
has a similar triggering problem) and my rogue both went this way, but have 
cleaned up alright. if you gig them or smoke in the studio (I do both) you may 
want to fit a replacement. thick or layered polythene with slits and holes cut 
appropriately may suit, though as I say, my little moogs seem to be fine 
without. 
it's just this triggering problem on the mg-1.... I will get around to it one 
day!

duncan/r.m.i.

In a message dated 1/24/04 1:23:15 PM, ferrograph@... writes:

<<  the trigger for the mono synth is created by detecting the minute 
voltage changes whenever a new key is pressed. I'm doing this from 
memory, so apologies if this is a bit sketchy. >>

Hello Duncan,

It sounds like you're describing the way certain "multiple trigger" circuits 
work on synth keyboards that only have a single contact bus (one bus to 
provide both pitch CV and gate/trigger).  An example of this would be the Moog 
Multimoog.  It has exactly the type of circuit you describe -- the circuit looks at 
the pitch control voltage and creates a VERY short trigger pulse whenever the 
pitch changes, even if it's just a half step change.

However, this doesn't actually provide the all-important Gate signal -- it 
only provides a Trigger when you change to a new note (without releasing the 
last note).  The Multimoog, Micromoog, Prodigy, Rogue, Liberation, and yes, the 
MG-1 all derive their Gate signal from the single-bus keyboard by using a c
lever system which superimposes a very high frequency oscillator signal (triangle 
wave, I think) onto the actual pitch CV bus.  There is an accompanying circuit 
which looks at the signal coming from depressed keys and detects the presence 
of that high frequency tone, and puts out a Gate as long as the tone is 
detected, which would be any time you are pressing a key.  The tone is of a high 
enough frequency that it does not effect the operation of the pitch CV circuitry 
(there also might be some kind of lowpass filter in the pitch CV 
sample-and-hold circuit to keep the tone from interfering with the CV -- I can't recall).  
It's a strange circuit, but it works surprisingly well, and it allowed them 
to keep the synth's cost down by allowing the use of a simpler, cheaper 
keyboard assembly. 

BTW, the aforementioned Multimoog Multiple Trigger circuit doesn't actually 
connect to the keyboard bus -- it looks at the output of the pitch CV 
sample-and-hold, watching for pitch changes.  It actually creates the "trigger" by 
interrupting the Gate signal for a very short time (less than 1 ms).  This, in 
effect, gives the same sonic result as an actual separate Trigger signal would 
(as you would find in a 3-bus keyboard such as the ARP 2600).  I have built the 
Multimoog Multiple Trigger circuit into several Minimoogs, with very 
satisfactory results.

Michael Bacich