problem w/ a prophet 600

[Jeroen Bruintjes]

> Hey I got an old prophet 600 that is giving me a hard time..
> Its a digitally controlled analog synth, and some times the digital
> control stops working. I can still play it, I just cant change the sounds.
> Sometimes it will start working again if I play with the knobs or turn it
> off and on, but other times, i just gotta give up for the day. This
> problem used to happen every once I a while but now its like every day. I
> have a theory it could be heat related, but i'm not sure. I realy wouldnt
> even know how to test for that. My worst fear is that its the rom going.
> I'm and EE student and i got and Oscope and a bunch of other toys avalibe,
> but i dont know where to start looking. Thanks Tom

Believe it or not, but it's got to do with a dirty pot. The P600 has 
limited CPU power and scans *one* front panel control between 
each key scan / envelope calculation. If there is no change in the 
parameter, it moves on to the next knob. If it detects a change in a 
parameter, it assumes that that knob is moving and it scans the 
same knob again on the next run, so that you won't have to wait a 
couple of seconds before the new value is put through.

Now, this has one disadvantage: if the potmeter is dirty and its 
resistance varies a little around, the P600 reads a constantly 
changing value and goes into a loop. Mine did exactly the same 
thing when I bought it. The problems disappeared after cleaning it 
up.

By the way, the P600-schematics are on my homepage.

Bye,

Jeroen

----> Brun, met een dakje op de u.

Dutch pricelist, DIY, schematics on http://users.bart.nl/~brun/index.htm