Power Issue - Blinking Amber LED - Dell Dimension E520

So, I'm working on dusting and re-installing windows on a Dell Dimension E520. I did the dusting but now I'm having issues with a blinking amber LED (per the diagnostic manual, this indicates that the system is receiving power but that there is an issue of some kind). It will not power up at all---no fans or beeps.

After looking through some posts for ideas, I'm thinking that it's either a bad power supply or an issue with the heat sink.

I'm thinking power supply because that seems to be a common issue. How can I test the existing one? Is there a way to fire it up without it being connected to the motherboard?

The reason I'm suspicious of the heat sink is that I removed it when I was doing the dusting. I was planning to fire it up tonight and get some new thermal paste in the next few days. I've since read that this could be a recipe for frying the CPU altogether and I'm wondering if the machine isn't firing up as a self preservation check of some kind.

Would it detect if I removed the heat sink? If that's what's going on, will it automatically detect when the new thermal paste is in there? Or is there a reset procedure of some kind?

Any help would be great. Thanks!

As a side note, the design of this box is poor. I can't find any way to effectively dust the heat sink without removing it from the CPU. The openings in the power supply don't match up either, so you can't blow a nice stream of air through the box.

Update: I did a paperclip power supply test and it fired up just fine. When I reconnected the power the PSU fan ran for a split second and then stopped (before I didn't even get the fan).

What can I check on the motherboard? I removed all components and just tried to get the board to fire up but didn't have any luck.

Hi,
I have the same problem with my 5 year old e520. Had shutdown for a couple of weeks when we went on vacation and it won't boot up and the powe button blinks orange. I tried removing ram and resetting cmos but no help.
Which power supply did you replace it with? Dell one or a good 80 certified one from Antec etc?
Thanks
Ram