At a first glance, modifying the PT-80 seems fairly straightforward given that you more or less get both Audio and Gate (VCA CV) outputs. On a closer inspection there are many quirks needed to be accounted for; the VCA CV output is fairly unusable with the sounds that has a constant amplitude since there's no way to retrigger ADSRs etc. using it. My idea of rectifying and low-pass filtering the Audio output instead didn't work either since the audio does not stop instantly, but does in fact stay on for about 0.5s after key-release. Possibly a combination of ANDing the VCA CV and Audio output could work.

My approach on generating a usable Gate is very crude and works okay but is far from perfect; By selecting one of the sounds that has a immediate VCA CV decay (piano etc.) and a comparator with a carefully trimmed threshold, you can indeed make a Gate output that does retrigger fairly reliably. My (very) simple circuit is pictured to the left. This output is directly routed to the EGs. This somewhat works with the constant-amplitude sounds also, but tends to make the EGs's flip-flops get the hickups.

Another problem is the fact that the PT-80's VCAs seems to be one-transistor VCAs (i haven't actually looked into it carefully enough) which gives you quite a tiny window of headroom (1v - 4.5v ?) without distortion. As stated elsewhere i used a Ken Stone Synthacon VCF which i had a *lot* of problems with due to the VCA (i thought the filter was broken for several days). It all started working after putting the circuit to the right inbetween the filter output and the VCA. Somehow the 10nF cap is the hard of the bit, unfortunley i don't really understand myself why (anyone care to fill me in?).

Luckliy though, there's quite a lot of space avaliable if removing the ROM-pack board, which in fact is nothing but a very simple connector - so no harm done, just solder it back to get it into it's original shape.