It's possible, but I still think it's a capacitor problem. I have replaced the main diode in the 5v circuit BTW.
I sat and monitored the 5v rail on a cold boot, having sat for some time, so it didn't boot right away.
The 5v rail starts about 4.8v and slowly but surely climbs to 5.1-5.2v when it bongs and boots up. Prior to that it sits with garbage on the screen. As soon as I can test again, I will monitor 12v at the same time.
I'll use 2 multimeters and video what happens.
The other reason I think it's a capacitor somewhere it that it takes sometime to get back to the stage where it won't boot, suggesting something is discharging.
As exxos suggested, I think I need to have a look at the values at suggested test points on the schematic, I just need to mark the points on the bottom of the analog board as it's too difficult to measure on the component side with the machine assembled.
Back to the schematic too, see if I can figure out why the 5v circuit, which is pretty simple, would take time to deliver a working voltage.
