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what can i deduce out of this experiments/observations?
Very strange.
Nonsense. It's a perfect (80+) PSU for a 2600 without much harddisks and an extra videocardNormally 300 Watt from a good PSU (not some chinese no-name crap) EASY is capable powering an 2600 cpu.
Then I changed the PSU with an older Chieftec reserver psu that I had for testing.Now the mobo did start, only till I got code 55, but now at least it did something! Very strange.