Has this non-power up until plugged in for a few hours problem been identified?

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That's usually a sign of a dead or absent PRAM battery.  Leaving it plugged in for a while lets either the battery or the PRAM startup circuitry get some charge into it.  A diagnostic step to see if this is the problem is the old double startup trick - plug it in, attempt to start it up, wait a while, then very quickly turn it off and on again.