I am currently using the NEC model pad-125 power cable for my PC Engine Duo. Is this the proper one?
As has been pointed out, you're currently using the power supply that's intended for the Super CD-ROMĀ².
In terms of voltage and amperage,
I've always understood it that you need to match the voltage specs. Both lower and higher could be bad.
Whereas with the amps it must be at least what the device wants but there is no harm in it being higher. If Johnny wants to eat two apples, he's only going to eat two whether you put 2, 3, 5, or 20 apples on the table.