Philips PM3394 startup failure

[Biscayne]

Hi all,
this seems to be the place to visit for experience with the PM3394 so I hope someone can help.
The problem is that on power up all seems well for about 2 seconds, then there are 2 beeps from within the 'scope, the fan slows to a slow speed and the graticule illumination drops to 'very dim'. None of the controls 'click' when rotated as they did when the scope was working The 'ARM'D' led is on but there is no sign of the trace on the CRT.
I have looked through section 8 of the manual but have not found anything that describes this fault. I have removed the board with the UHT psu and checked for signs of overheating and swollen capacitors but nothing obvious.
Next step will be to check all the voltages according to the manual but I was wondering if the '2 beeps' might be a clue to where to look.
I also note that there is an RS232 port on the back of the scope and wondered if any diagnostic information may be available there.

Any help will be much appreciated as I would hate to have to replace what has been an invaluable tool.

Rating plate details: 9444 033 94024, No DM754028