Is this a DX7 or the DX7IID/DX7IIFD?
Years ago I had my 1987 DX7IID go dead. Power supply was the issue. The local repair shop really didn't want to touch it but said most likely the big 5W resistor in PS opened up, it was a common fault. The supply is internal, a separate module from everything else in t he unit. Even the original DX7 didn't use a wall wart/plug pack.
Opened it up, measured, and sure it was open. I forget the value of it, something like 2.5 Ohms 5W. I could find that exactly but found to resistors at 5W that paralleled was very close to the value I needed. Whacked it in, and it has been working fine for the past 15 years or more.