PDP-8/M H740 PSU woes

I'm just in the midst of rebuilding a PDP-8/M which has the H740 PSU in it (also fitted to 5.25" PDP-11/05s). I've cleaned the PSU up and reformed its capacitors, checked the transformer is working fine. On replacing the switch mode circuit board and powering up, I'm getting a blown fuse (F1 on circuit diagram) which supplies the 5V and 15V rails. The -15V rail is working fine. Does anyway have any suggestions as to the best approach to locating the problem? I've been pondering whether it would be possible to isolate either the 5V or 15V circuit so I can see which of them is causing the problem, but I can't see any simple way to achieve this. Also, is my practice of powering the PSU up without any load a foolish one? I've not had any problems in the past using this approach but I have seen people previously mention that running switch modes without a load is a bad idea. A circuit diagram of the H740 can be found here: http://highgate.comm.sfu.ca/~djg/htdocs/cgi-bin/tifftopdf.cgi/h740.pdf?loc=… Thanks, Toby -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean.