This sounds to me like the power supply is the problem. When you take the CRT out of the work load, the "FLUP" sound was much faster, which tells me to check for proper voltage going out of the power supply unit. Is the display completely dark when the 512k is put back together? Or does the screen show a slight blip of light, a small dot of light in the center, or a horizontal line? Maybe a capacitor is bad somewhere on the PSU.

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It used to. Now it no longer does.

I'm thinking that perhaps i'll replace the remaining electrolytics. I'll also get some quotes for the large 200 Volt electrolytics that are likely to be causing significant problems for the system, including the issues reported here.

What makes me think a little is that the machine has started before, after the flupping issue began. After working, including power cycles for about 6 hours, it started flupping again and displaying a small white dot in the center of the display. Now it won't start at all. This gives me the impression the power supply is deteriorating, hence suspecting the large 200v mains supply capacitors.

So... where to from here? Swap out every last capacitor on the analog board, it seems, then pray for a miracle and hope this old warhorse powers up. If it still doesn't go, then the rectifiers are the next step.

Thankfully i'll probably make a run to the supplier again this afternoon, so it could be less than 24 hours before we have a successful boot.

I'd love to see this little machine work again. It's become my favourite of the 68k's over the last month or so.