Some time ago, I added a resonance pot to the filter, and here i find a schematic to add a CV in
The Gate in also moving the height of the Pitch, this tiny circuit allows with the potentiometer to adjust it so that it is adapted to 1V / Oct.
i find it here
My built progress
not tested yet (probably tomorrow), but hope it works!
This is rad as hell. i love seeing mods for these cheaper synth dealies.
i 've tested it and it work !
not at the first time, I had to reverse the 2 inputs Gate and Pitch for it to work?
That is very cool ! So is it worth buying a Monotron even though I built Sam’s and @Dud ´s PT2399 Delay?
The Korg Monotron Delay is a very little but complete voice of synth with VCO, VCF, LFO + an intern PT2399 Delay circuit, not just a Delay FX 
(btw and it’s still a gadget)
Hi! I wish to do this mod, it’s going to be pretty much my first ever aside from some gameboy stuff. I was wondering, how does the schematic change when you say “reserve the 2 inputs”? Just switch the gate and pitch labels around?
Hi and welcome to the forum 
I think I had to reverse the “Pitch jack in” with “Gate jack in” or label if it is not already written, so that it works well, but there is a moment so nothing sure, best to test
a good thing and i’ve made is to add a resonance pot to his actual filter
