Vintage Synth Repair

> Oops, my post should have read "PCB contamination" not BCB contamination. The issue of re-capping does come up a lot, and both answers are true - many caps get replaced unnecessarily, and many machines are also laboring under a load of bad electrolytics. 

Some reasons to replace electros in very old machines are:

Obvious leakage. 
Obvious lack of bottom end response.
If the machine has had periods of years where it has not been powered up. Electro's need voltage across them to keep them properly formed.
Situations where the model is known to be reaching the end of it's cap life, and as far as I have seen, Korg PS and MS series are definately there.
Caps that are being overheated, such as those mounted next to heatsinks and power resistors.

Reasons NOT to replace electro's:

If the caps are high quality, such as Spragues, and are in machines that have been kept in service. Outboard gear in large studios is a good example.  IN this case, removing a few caps and testing capacitance and ESR would be the obvious thing to do, and if they are still in spec, and the machine sounds good, then I would say leave it alone and re-test every 3 years.

The other option, is a "mini-re-cap" where you replace the caps in the audio path and VCF's only with high quality audio grade electrolytics such as Panasonics.