Polysix - Basic signal tracing.
2015-07-20 by scott.hamblin@...
Hi there,
I hope you don’t mind me cold calling on you like this, I can imagine you get quite a lot of questions from technical folks attempting to repair battery leaks on their Polysix instruments.
My Korg Polysix has avoided the common acid leakage problem and the CPU board is clean and operating with a CR2032 lithium battery. The front panel buttons and their LEDs are responsive. The PWM and ARP LEDs pulse and the frequency responds to their respective knobs but frustratingly the instrument is silent. Turing up the volume, raises the background hiss but none of the keys trigger a note. A few basic meter checks show the power supply voltages are all present (+5V, +/- 15V) on the CPU and the KLM366B boards. Oscilloscope shows there are pulses coming from the 8049-217
1. Starting with the KLM366B, what is the best way to confirm that the keys being decoded normally? I presume that when a key is pressed, a voltage is generated in the R/2R ladder and appears on pin 1 of IC9 - the schematic suggests this MPX-CV signal is 5V per octave. Can anyone help me understand how this voltage should look?
2. IC29 and IC25 are in an unexpected configuration, maybe a
kind of analogue sample and hold. Is the purpose of this section to hold voltages to then control the envelope generators in each of the 6 voices?
3. Is
there a convenient way to trigger each of the VCOs or indeed inject a signal to
trace the circuit and see if the envelope and filter sections are working? I see there is a lot of analog multiplexers used to gate
signals here and there. The service manual doesn’t give a lot of information
and if the key assigner is not working then I don’t see how I can generate a
test note.
4. Lastly perhaps there is a logical starting point, after the basic power supply checks are completed. I think it's probably around the 8049-217, but I'm a bit vague on how this MPU functions. Any background info would be awesome....
So I hope this message is a nice change from the usual battery acid horror stories.
Any suggestions or technical advice will be greatly appreciated!