Yamaha ps-55 none of the keys working!

hi, I found an old ps-55 in the basement without a power supply. i connected it to an AC -DC adaptor that receives 230V (i'm in israel) and produces 12V DC at 900mA.

now the lights turn on, the speakers slightly hiss at full volume, but none of the keys remain silent!

could it be a problem with the power supply? The manual says that i need 9-12v DC power, and then states that i must use a PA-4 adaptor. however the lights do turn on with the adaptor I use.

if this is a power supply problem, do you know where i can get a PA-4 adaptor from, abroad?

many thanks,

Tom