This Yamaha DX7 did not power up. I thought that it was due to one or more fuses being blown but they had not. After a while I discovered that the power cable was broken internally close to the chassis. This was probably due to the cable being sharply bent there many times over the years.
If you are going to do this; please remember to be careful an disconnect the mains cable from the wall. There is 110/220V here and you could easily be shocked if you are not careful!