Casio PT-87 (in progress)

[Remork]

first thoughts: this keyboard sucks.
didn't get much further than the usual pitchpot at first.

output opamp datasheet says it has fixed gain of 50dB, so nothing much to tweak there in terms of overdrive.
the usual feedback mods (input to output) yield a nasty uncontrollable whine at speakerkiller volume.
haven't really hit any nice results there.

after a while, found a nice little lowpass kind of a bend i could have on a pot.
problem was it would only work with some sounds. here's why:

turns out all lead (non-drums) sounds come from one IC pin, but preset selection determines which one of two possible paths the signal travels.
piano, organ, trumpet, flute and clarinet take a lowpassed route.
violin, celesta and harpsichord take a highpassed route.
the choice is forced upon them through fet/transistor switching, pulling one of both ends to ground.
(this happens through control signals coming from the main IC.)
the two signals pass through a passive mixer, where the drums come in as well.
so 3 of the sounds are on a different line for a bit..

just now i removed that control (=the switching trannies), and should have both HP and LP available for ALL presets.
plan is to bring 'em to the front panel, and be able to control the cutoff frequency.
series, parallel, only one at a time on a switch.. i'd have to try which ever way works best.

keep you posted!