Converter for NMB Space Invaders keyboard

[Palatino]

I have an NMB RT8255CW+ (FCC AQ6-OAEZ15) with a split spacebar and black clicky space invaders. Trouble is, not all the keys will register. I'm using a 5 pin to PS2 converter, then a PS2 to USB, both of which work fine in tandem with other AT 5-pin keyboards. The PS2 to USB is the active type with green and purple ends for mouse and keyboard. A whole load of keys won't register at all, including the entire bottom row, Z to M, as well as a load of others, like Enter, P, 3, 4, 6 etc. I've tried swapping to a different PS2 to USB - this actually made it worse: no keys registered. I've even tried one of those short stubby passive adapters, which I think also produced zero output. Does this sort of keyboard require a particular adapter type? A Soarer's converter? It's new in box so I doubt it's a fault in the board itself. Unfortunately I don't have a computer with a PS2 port to test it with, so at the moment it's not living up to its promise! The keys feel great - hoping someone can help make them usable!

[Findecanor]

I would unscrew the case and take a look inside. Even new-in-box keyboards could have electrolytic capacitors that have dried out and would need replacing.

[Palatino]

Doesn't the fact that some keys worked with one converter, but no keys worked with another, suggest it's a converter?protocol issue? I'd like to try exhausting other avenues before I open it up - but I'll bear that in mind if nothing else works - thank you.