This is Pt.2 of my serie about modding a Monotron Duo to add Midi and other features.
Every Monotron offers slightly different pad points on the pcb to allow us to to fiddle with. The duo is no exception and is actually fairly comprehensive:
It seems one of the focus here is to give access to the various waveforms the oscillator cores go through internally. We can also find these point on the duo’s schematics:
First of all, we see the default output of the oscillator is a square wave (SQR1) – to the contrary of the regular monotron which outputs a saw. There is a SAW1 point where we probably can tap an equivalent saw to the original and a PLS1 with some kind of pulse train. Interestingly, the pulse train is labeled “VCO1” which feeds the TI chip. This means probably the pulse train’s existence is not for any “musical” purpose but for frequency measurement. To be sure of what we’re dealing with, let’s quickly power the monotron, grab a scope and have a look:
While PLS1 and SQR1 go nearly rail to rail (0->5v), SAW1 is way quieter (although still roughly centered around VBIAS). We can easily bring SAW1 to the level of SQR1 by amplifying it by a factor 5 and, possibly, make a SAW/SQR mixer like mutable instrument’s Anushri does.
PLS1 looks as expected and it will be interested to see how one could connect it to a teensy pin to measure it’s frequency, providing a feedback for proper self-tuning of the voltage sent.
We’ll look at that next time.