New PSU no go, but ....
In a moment of weakness I offered to check a friend's computer which "wouldn't start". The description suggested a dead PSU, and component substitution seemed a good place to start. I fitted a 'spare' PSU I had on hand and all system were immediately 'go'. I left it there to keep him airborne, and eventually sourced a nice new Corsair VX450 of slightly higher power capacity than the original and, by all accounts a reliable and highly regarded unit. I had previously installed one in my own primary computer here. Yesterday the time came.
His M/B (now a few years old) uses the older 20 pin power socket with a second 4 pin socket elsewhere on the M/B fed with its own supply lead and plug. The Corsair has a 24 pin plug, so the 'surplus' 4 pin part of the connector was unclipped and ignored. A double check on everything and then the moment of switch-on. Main power leds came on, the CPU fan started up, but things went no further. Wait a short eternity - no progress. Check, recheck. Nothing. The only obvious sign of distress - the HDD led also on. Extreme puzzlement and embarrassment! Had I inadvertently stuffed something? (sense of profound horror at the implications)
The old loan PSU was reinstalled and status returned to 'ops normal' (profound sense of relief!). I returned home and tried the Corsair PSU on a machine I have here with 24 pin connector on board power socket. Worked a charm. Returned to friend's residence with PSU from that machine (860 watt Shaw dual fan model also with 24 pin connector). Installed same, detaching and ignoring the extra 4 pin plug as before. Machine worked immediately. We agreed that it could stay there. Problem solved - but doubts remain.
Begs the question as to why the Corsair supply appears not to 'work' unless all 24 pins are loaded. But why then did the M/B power up top the degree that it did - key board leds as normal, 5 volts on USB external, but nothing further, when a seemingly identical Shaw PSU with similar connections and installation worked OK. The only obvious major difference is that the Shaw was rated at 860 watts - almost 200% of what the machine had in the past (it's a very basic domestic device - nothing suggestive of a larger than normal power requirement - onboard video, one HDD, One DVDrom, 2.9 Ghz Pentium, single memory module etc.).
I need to add of course that due attention was paid to all aspects of the installation - correct plugs in the correct sockets etc. (I've been doing this sort of thing for a while now) One suspects a faulty new PSU, but it works OK on the test bed machine so a return is out of the question.
Has anyone else ever encountered anything like this ??
Thanks in advance.
A very puzzled FOR