Korg Polysix board repairs

A defunct Korg Polysix inbound for repairs.. Prior to getting my hands on the synth I briefly checked the net for schematics and typical faults. The owner had taken a look inside and noticed a leaked battery, so I knew a little bit what to expect.. Seemingly, battery leakage is a pretty common root cause for faults in the Polysix.

Among the top search results was Scott Rider’s Polysix repair page. Here, the battery leak repair procedure is described well in detail so do take a look at this if interested. Much thanks to this article, I was even more well-prepared beforehand :)

Eventually, the synth appears at my desk..

Polysix, case open

Open up..

Polysix, battery leakage

Some mess around IC30 & IC 31. Battery removed already.

Nothing even remotely as bad as in Scott’s example, just a few damaged traces under IC31 and a bit of dirt in the near vicinity (namely, IC30).

Whilst removing IC30 & IC31 for the trace repairs, I decided to install sockets for both chips despite there wasn’t any damage under IC30. Clean up and trace repairs took a while longer than expected, but nothing really problematic. Before soldering in the sockets, I checked the repaired traces for continuity and afterwards for short-circuits.

Polysix, foil repairs

Repaired traces running under socket of IC31.

Polysix, new battery

New battery installed.

Seems I got lucky as there isn’t anything else to repair.. Off it goes, yay \o/

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7 responses to “Korg Polysix board repairs”

  1. Roland Marckwort says :

    Nice thanks for this – I am doing the same repairs as we speak.
    Is it better to replace batt with a coin cell type CR 2032?

    Also, as a nit of a newbie, how do I know which traces I should be repairing? ie: if they are black, visibly damaged, etc?
    As for ICs, my IC 31 has some blue battery gunk under its contacts – should that come out for sure?

    thanks!
    Roland

    • arto says :

      I think the battery type doesn’t matter, just use whatever you think fits on the board. Just as long as the voltages match, of course :). CR2032 is easy in the sense that it’s really common and if you install a holder for it, replacing the battery won’t require soldering anymore.

      For trace repairs, first clean away any battery acid spills. Carefully scrape of bigger blobs of dirt and finish up with a q-tip soaked in some alcohol-based cleaning solvent (I use isopropyl alcohol). If you can see spills under any chips, remove them for cleaning. Remember. if you leave any battery acid on the board, you can be sure that the synth will be back on your desk for repairs sooner than later.

      When starting to patch up traces, check every single one that has come into contact with the battery acid. Set your multimeter to measure resistance and check each trace from points that are on the opposite sides of the suspected damaged area of the trace. You’ll see where/how the traces connect from the schematics. Checking resistance, the multimeter should read something between 0.5 to 2 ohms. The value depends a bit on the quality of your multimeter and probe leads. If you get anything higher resistances, you’ve just found a trace that needs to be patched up :)

      • Roland says :

        Hi again- well did the repairs, replaced the !C31 but not the CPU chip, and still have the same issues – 4 or more of the ‘programmer’ patch buttons stay lit at once. Would this be a CPU replacement do you think, and where can you find these?

        thanks!
        Roland

  2. Roland says :

    great, thank you! A big help. I guess any traces that look black/non copper colored would be suspect? Right now, all my ‘programmer’ / unison buttons light up simultaneously, so hopefully the battery and
    IC31 replacement will solve this….

    • arto says :

      Yep, quite so. Check anything that looks out of the ordinary. It’s not a bad idea to measure the entire length of a signal (from one end of the board to the other, between connectors, etc.) for resistance either. Double-sided (or multilayer) boards can develop via breakages over time, f.ex. if there has been issues with the plating process when the PCB was manufactured.

      Oh and if your multimeter probes give a readout +1 ohms when you connect them together without nothing in-between, you should definitely consider getting a new set. Just for comparison, the ones I have measure all between 0.1 – 0.3 ohms. A fairly good habit to test the leads and probes every now and then :)

      Good luck with the repairs!

  3. Roland Marckwort says :

    Hi Arto – well, after a looong process, I stripped the 367 board, bought the clone, and replaced a lot of the parts. Sadly, the filter and EG do not work, but I dont think they did prior to this repair. I have taken it to a tech here in Ottawa, Canada, but do you know of any other web resources that may know the solutions to these issues? I really dont want to give up on this beast ; )

    thanks!
    Roland

    • arto says :

      Polysix uses a SSM2044 chip for the filter and SSM2056 for the EG. They require very little external parts to work, so in your case it’s very likely that the chips themself are shot. But as always, measurements and failure analysis take precedence over randomly replacing parts. The tech should be definitely able to pinpoint the fault and source spare parts if needed.

      One good resource worth checking out is the vintagesynthrepair mailing list @ yahoogroups.

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