I this video I show how I replaced the existing permanently mounted power cable on my Korg Poly-61 with an IEC C14 power socket. The reason for this was that the old cable was pretty crusty and also it had a British connector (I’m in Sweden and we use Schuko style connectors). If I were to do this again I would have drilled a lot of small holes in the area where metal was to removed. As I used files it took more than an hour to remove all the material.
Please be aware that this can potentially be VERY dangerous if you are not very careful (both for you and/or the synthesizer). If you are uncertain about any step here please ask someone with more expoerience for assistance so you do not get hurt!