Vintage Synth Repair

> I don't think you should have instrument on when changing battery
> and  have it opened.

If it's the kind that you have to desolder (E.g. Lihium) you shouldn't even 
have it plugged in!  The tip on many soldering irons is grounded and if your 
synth is grounded through power or audio jacks you might find yourself in 
emergency waiting for face transplant surgery.  Back up your patches first.