Korg Polysix -- Arpeggiator Timing Errors

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marathonmusicgym
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Korg Polysix -- Arpeggiator Timing Errors

Post by marathonmusicgym » Fri Apr 02, 2021 4:54 pm

Hi, and thanks in advance for your time!

I've got a Korg Polysix on the bench with unusual timing errors in the native Arpeggiator. It will intermittently keep some notes longer than others, rarely falls into time, but cuts out completely as the speed pot is maxed out.

I have replaced/socketed every 4000-series chip in the Key Assigner section to no avail. I have also replaced a handful of the 4051s in the same area -- nothing has changed this strange behavior. I even replaced the 8049 with another Key Assigner IC from another working Polysix.

I have confirmed that the 555 timing circuit is functioning as expected and the Clock signal is making it from the 555 to the 366 board. And the behavior of the timing issues are such that it cannot be traced to any one voice -- the timing errors exist in Unison mode, too. When I trigger the Arpeggiator from the Trigger input, it also gets hung up on random notes just as it would running from the internal clock.

This is the only current problem with this Polysix - it has a new battery, never saw corrosion damage, and everything else (so far as I can tell right now) is working as expected.

Any suggestions would be greatly appreciated, thank you!

EDIT: I somehow missed this post which seems to outline exactly the same problem I've been having, and seemingly never a solution. I've replaced the 8049, I can't imagine I've got two different 8049's with the same problem...

marathonmusicgym
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Joined: Fri Apr 02, 2021 4:31 pm

Re: Korg Polysix -- Arpeggiator Timing Errors

Post by marathonmusicgym » Sun Apr 04, 2021 1:48 pm

For posterity's sake, I'll keep a small log of my progress (or lack thereof) --

I'm realizing that the 14175 chips in the Key Assigner section (the flip-flops ostensibly responsible for triggering voices) share a signal with the 367 board via connector CN05-16. This signal is labeled "ST", does anyone know what this might stand for?

Image

I can trace the 555 timer from the Arp Speed pot all the way to the 8049 Key Assigner (pin 6), but it seems to get lost beyond this. It appears that the Latch 1 signal from 8049 Pin 24 feeds the clock on the two 14175 Gates and would probably be the offending signal.

This ST signal is definitely carrying the mis-timed pulse that ends up as the mis-timed arpeggio, my logic probe follows the exact rhythm of the notes. I have replaced C33 and C32, as well as IC14 (4558) on the 367 board, and I will be replacing Q5 (2SC945) as soon as the replacements arrive.

Image

I'm probably barking up the wrong tree here because this ST signal is fed TO the 367 board from these Gate chips and not vice-versa. I guess the clock signal needs to be fed to the MG section for each voice?

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