The most common capacitors to fail, and therefore the most likely to produce benefits from replacement, seem to be those used in the switched mode power supplies in synths of this era, though this doesn't preclude capacitors in other parts of the circuitry causing problems. Usually it is electrolytic capacitors that dry out, or leak electrolyte, that cause issues. Things to look for are the capacitor casing showing signs of bulging, or leaking electrolyte, or usually the dried out remains on the capacitor leads, or visible on the printed circuit board (not to be confused with adhesive that has intentionally been applied to stop component movement during vibrations). Occasionally, non-electrolytic capacitors can fail, usually because the conductor-dielectric stack inside the capacitor fails through stress fractures.
There are related threads here, which might help:
https://www.korgforums.com/forum/phpBB3 ... p?p=820550
https://www.korgforums.com/forum/phpBB3 ... p?p=621633
It is worth considering performing your own "bad capacitor" hunt. There's an article linked below, about using an ESR meter to test in-circuit capacitors. With decent ESR meters, which impart a tiny test voltage (in the order of 15millivolts), there's no need to desolder the capacitor in order to test it:
https://www.yamanelectronics.com/test-c ... soldering/
Suitable instruments for in-circuit capacitor ESR test are the MESR-100 (various "manufacturers" via e.g. Amazon), or the Peak ESR70 meter. The low voltages applied to the components during the ESR test are very unlikely to cause damage to the other components in the keyboard. The tests must be performed on unpowered circuit boards, and the capacitors allowed time to discharge (or discharged using a suitable resistor), before measurement. The Peak ESR70 will perform a pre-discharge step before the measurement, but is slightly more clunky to use.
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