Acorn A4000 monitor problems

Anyone got any experience with Acorns?

I’ve got an antique one I took from the rubbish pile at school a while ago. It worked! With RISC OS 2.

But with once month of neglect I decided to play around with it (I managed to get MAME running on it not long ago). Anyway, I boot it up, and the monitor is just outputting the noise like a TV with no arial.

I’ve gone through the RISC OS manual, but as that is for the OS it doesnt mention anything about the hardware.

Does anybody know how to fix this problem, or a link to fixing it?