Korg 900PS

[Brainstormer]

Hi, I recently managed to get my old Korg 900PS back from a friend. As it is, it's pretty useless, the sounds are weak, so I decided to open it up.

Inside there are 17 variable resistors, and a row of smaller preset resistors. They are all labelled up, most of the labelling is pretty obvious, such as cutoff freq, Q, a few less obvious, such as Balance, fC, etc, but there are more obscure labels on the smaller preset resistors, such as TP1.2, TP3, TP4, etc (Test Points?) these have been position fixed with paint, check the pics. There are a few more variable resistors (not pictured) under the control panel to the lower left of the machine, including a white noise level controller.

I was also thinking of making it semi modular, - similar to the MS range, as all the wiring is clearly listed on the board. As it is, the LFO (vibrato) & envelope seem to only control the VCO, it would be nice to have the option of routing the filter through them. I wouldn't mind putting CV/Gate sockets on too, but not sure where I would need to tap into the circuit.

I do have a bit of experience with electronics, but might possibly need a bit of guidance in some areas, and maybe a schematic if anyone knows where I can get one (preferably free). Basically any help will be appreciated.

[Gordonjcp]

Well the bit marked KEIO TS201012 is a diode ring filter IC, similar to the ones in later MS20s.

[Brainstormer]

Well the bit marked KEIO TS201012 is a diode ring filter IC, similar to the ones in later MS20s.

I can hear that when I wind up the Q control, when I did a search for the filter chips, it appears to share the same as the 700. I wonder if increasing the value of those pots will give me more range, or if I'd need to short some nearby resistors to open it up a bit more.

Also if there's a way to shut off the oscillators (variably), so the filter self oscillates, that would be pretty cool, a simple mixer circuit would be beneficial here methinks.