Sonopuls 490u power source

Hello,

What’s so special about the original power source for Enraf Sonopuls 490u?

I tried a laptop type 18v 2.6A and the diode is still yellow. 

Tried 15V 1A - same yellow diode.

I refuse to pay over a £115 for a power source from enraf.

SHould it be exactly 18V 1.8A?

Thanks for any input.