Vintage Synth Repair

The old two prong connecters that had both prongs the same size were dangerous because either side could be plugged into the 'hot' side of the wall outlet.  And one of the wires would go to the power transformer in the appliance while the other would go to chassis.  I remember being a child and having my first record player always giving me 'funny throbing pulses' whenever I touched the metal plate behind the off/on/volume switch.  I didn't know then that it was 120VAC.

  With modern three wire connecters, the black (hot, short blade on the wall plug) and the white (neutral, long blade on the wall plug) will go to the primary side of the internal appliance transformer (or primary side circuitry of the switching power supply) and the yellow/green  (ground/zero voltage) will go to the chassis/case of the appliance.

  You might want to get a 'ground fault adapter' that goes between the modern three wire plug and the two wire vintage synth, especially if you end up connecting the Neutral and the Ground wires together.  Do a Google search on "connecting the Neutral and the Ground wires together" to see if this is a bat-shit crazy thing to do or if it is acceptable in certain circumstances.  I'm not an electrician.  The ground-fault-adapter is a small box (found at a hardware store) that will switch off the power to the appliance nearly instantly if the current in the ground circuit goes above a certain level.  

  Another suggestion is an isolation transformer.  

  At least replace any old plug that has blades the same size with a two-prong plug (if they are still allowed to be sold) that has one wide blade and one thin blade.  That way the chassis/case will not be accidentally connected to the hot side of the wall power.