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I don't know how many of you work or have worked on these, but I've seen a ton of these over the years, all with the same issues requiring replacement of the VCO and VCA/VCF hybrid IC's, which are no longer available.
I always noticed that these IC's tended to age and drop dead at the same rate, indicating a time-related issue.
It has come to my attention that the issue is the potting compound itself. Rather than being chemically-inert epoxy, it is a petroleum-based compound that apparently develops partial conductivity as it ages (reminiscent of the old SPX90 PSU problem). The GOOD thing is that you can soak these chips for a few days in acetone, and the potting compound comes right off. Using dental picks, you can clean up the leftover compound stuck between the legs of the SMD IC's, being careful not to damage the resistors that are silk-screened on the board. In most cases, this cleanup is all that is needed. Reinstall the chips, sans potting compound, recalibrate the unit, and all SHOULD be fine.
I cannot take credit for this idea. I found it on the web, and there is someone on eBay charging $160 for chip overhaul. Anyway, I have a set of chips soaking in acetone right now. I removed most of the compound and am soaking them again to assist removal of the small stuff. I'll keep y'all posted on the outcome. I am hopeful and have my fingers (AND toes!) crossed.
I always noticed that these IC's tended to age and drop dead at the same rate, indicating a time-related issue.
It has come to my attention that the issue is the potting compound itself. Rather than being chemically-inert epoxy, it is a petroleum-based compound that apparently develops partial conductivity as it ages (reminiscent of the old SPX90 PSU problem). The GOOD thing is that you can soak these chips for a few days in acetone, and the potting compound comes right off. Using dental picks, you can clean up the leftover compound stuck between the legs of the SMD IC's, being careful not to damage the resistors that are silk-screened on the board. In most cases, this cleanup is all that is needed. Reinstall the chips, sans potting compound, recalibrate the unit, and all SHOULD be fine.
I cannot take credit for this idea. I found it on the web, and there is someone on eBay charging $160 for chip overhaul. Anyway, I have a set of chips soaking in acetone right now. I removed most of the compound and am soaking them again to assist removal of the small stuff. I'll keep y'all posted on the outcome. I am hopeful and have my fingers (AND toes!) crossed.
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