Mac Pro 3,1 turns off straight away after power button press

It's quite difficult to phrase this in a concise way hence the longer title then I would have wanted. Basically, I have a Mac Pro 3,1 early 2008 model, which, when the power button is pressed, a fan turns on, power light comes on, and within 1 second the shutdown click is heard, fans turns off and power indicator light switches off.

After this, the power button does not do anything when pressed unless you wait at least 1 minute.

I tried a few different things at first which did not resolve the problem:

• unplugged everything, waited 30 secs, plugged it back in again (SMC reset)
• took out the Hard Drive
• took out the RAM
• took out the DVD drive
• took out graphics card

Finally I thought I'd look it up and found this post and this post which both described the symptoms perfectly, and all answers pointed to a power supply problem.

Thought I'd check this out, but needed some instructions on how to get the power supply out. Found this scribd.com manual which did the job. Looks like an official manual.

After taking it out, I cleaned it up with a can of air and put it back to no avail. Next I tried swapping with another out of a Mac Pro 3,1. The working one worked in the previously not working machine, and the broken one didn't work in the working machine (is that a good way to put this?).

So, conclusion, the Power Supply Unit is not working correctly. Reading the manual further I found that it looks like the "trickle feed" isn't working correctly.

Apple Part Number: 614-0409

Looks like a new one is about £280, or refurbished is £105 from USA.