Vintage Synth Repair

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