Strange Power Supply Problem

About a month ago our main PC started showing some worrying symptoms.  It would wait about two minutes following power-on before its lights came on and it began start-up.  Once started, the PC would then run normally, and could even be even shut-down and restarted again, without any problems, so long as the power was not switched off at the wall.  It was as if the PC needed to soak up a certain amount of power before it could begin start up.

I recognized this as probable partial failure of the PC's power-supply unit (PSU)  but, as it was only a minor nuisance rather than a catastrophe, I put off doing anything about it for several weeks.  However, recently the delay between power-on and start-up had grown to over ten minutes, so I thought it was time to do something about it. 

I opened up the PC and read the label on the  PSU.  It said: "ATX 12V.  With PFC.  Ball Bearing Fan.  CHIEFTEC Model HPC-360-202.  AC Input: 115V 8A, 230V 5A.  Frequency 50-60Hz.  Max Load 360W."   I also used a ruler to measure the dimensions of the PSU:  height 8.6cm, width 15cm, depth 14cm.

A quick search on the PCWorld web-site revealed that they could supply a suitable replacement: a "JEANTECH ATX 400W Power Supply Unit", of the same dimensions, for £50.  After a quick walk over to their local store (only 2km away),  I was back with the replacement in my hands in just over an hour.

I carefully read the WikiHow page on How to Diagnose and Replace a Failed PC Power Supply.  I didn't have a power supply tester nor an anti-static wrist-band, as recommended in that article, but I was fairly sure it was the power supply, and, after the walk I was too sweaty to generate much static. 

Removing the old PSU and fitting the new one was a bit fiddly, especially unplugging the various cables without breaking anything.  I had to use narrow pliers to remove one particularly awkward plug.  I also made a note of what I had unplugged from where so I knew where the new plugs had to go.  I also took care to tie up any loose cable and plugs, so they don't snag any of the fans.  After everything was done, and before I put the PC cover back on, I powered-up and switched the PC on on.  It started first time.  I checked all the drives and fans were working and then put the cover on.