Hi,

start by checking the connector itself, if not heavy corrosion.
then maybe check solder joint on the video chip itself and video ram.
thoses machines are a bit complicated when problem come from motherboard, as mostly all part of the chipset use custom gate arrays.

Check voltages with a multimeter?

Thanks.
I'll admit I didn't check the voltages, not sure why! I'll get that done ASAP.
All the solder joints look good, but I'll have a closer inspection. Will also test the Model 2121 I have, see if that complains about not having a monitor. If not, then I guess that rules out a bad connection on the VGA port.
Andrew

Sorry it's taken so long, life has got in the way a little!
However, I can now confirm that the voltages are all present and correct and that visually, all the solder joints look fine.
Does anyone have any further ideas, or am I just looking at having to find a replacement motherboard?
Cheers
Andrew

I'd normally do something a bit Neanderthal now like pressing and holding down ICs one by one when you turn it on to see if behaviour changes which could indicate a fine solder joint failure that you can't see (like TubeTime and CuriosMarc's PS/2 - https://www.youtube.com/watch?v=wsLpe4lLhbo) . Or blow hot air onto different ICs to heat them up and turn on to see if that does anything (it did for my old AST)