Anyone considering purchasing a Korg Polysix should be aware of the following: The rechargable battery installed by Korg to power patch memory while the unit is unplugged would inevitably go bad after a period of 10-15 years and spray battery acid on parts of the nearby PCB
Battery Acid Modulation is not a feature considered desirable at this time by most synth owners. The damage will disable a number of features, and requires extensive circuit retracing and component replacement to repair. So be sure to test functionality as thoroughly as possible, as well as opening the unit up to do a visual scan for damage, before purchasing a Polysix.
The battery is located near the middle of the unit, slightly on the left. It is a sky blue plug about 3/4" in diameter and about 3/4" long. It will be quickly apparent if it has leaked -- there will be brown sticky crud on the PCB around the negative terminal of the battery.
If there is no damage (rare these days but possible if the synth wasn't used much), it is a simple fix that will cost less than $50 to repair -- this involves basically replacing the battery with a modern Lithium Ion battery.
These synths sound great. In my opinion, they are much more organic than the Roland Juno 60, which is feature-wise very similar. Additionally, they are much less common, so the sounds are less familiar to listeners, which is frequently a good thing. I highly recommend this synth if you can find one in good shape.