I have had two 840av machines, and had similar problems in both cases, involving erratic booting/ non-booting. The machines would power on sometimes, but generally would get no further than a grey screen. This is going back as many as 6-8 years.

The trouble, I discovered, was that capacitors had leaked on both near the main power connector on the logic board (look closely for residue). The rest of the board looked fine. A good cleaning cured the problem and both then booted normally.

One unit was subsequently sold working; the other is still running fine here in the house, is more or less pristine, and is very unlikely ever to leave my collection. I have not yet needed to recap, though obviously that day will come eventually.