520 STFM working but no video output.

[Lurch666]

I have two 520 STFMs One is working but the other refuses to output any video.

The power and floppy lights come on and the floppy is seeking.If there is a disk in it sounds like it's loading and through the composite output I am getting clicks on keypresses so the computer is working but I when tried the TV output it wouldn't even tune into anything.
I also tried composite through the 13 pin connector to an lcd TV and an old crt monitor and I'm getting no output whatsoever-not even a black or white screen-just sound.

I have done a visual check of the soldering,swapped out the PSU and shifter with the known working ones from the other ST and pushed the rest of the socketed chips to make sure they are in properly and am now wondering what steps I should take next?

Thanks dudes.