Hi All,
I serviced the Carver HR-772 with a very similar issue and may have a fix. An intermittent problem from the client was that it would power on and then power itself off after 3-4 seconds. After chasing a few rabbits, it turned out to be dirty contact switch of the "stand-by" push button. By squirting some deoxit in the crevice and actuating the switch, the Carver no longer has the issue. Had to remove the front panel to get to it properly.
~Ryan J