DPS-460DB-4B - Dell 460-Watts Power Supply Dimension XPS 7100/8300/8500/8700

Dell WY7XX

https://www.amazon.com/Dell-Power-Supply-Systems-D460AD-00/dp/B009G28TOW

Shutting off could be bad power supply but could also be failed CPU heatsink.

Recommended replacement is now EVGA 700BR

https://www.bestbuy.com/site/evga-700w-atx12v-eps12v-80-plus-bronze-power-supply-dc-dc-technology-black/6346160.p?skuId=6346160

Is PSU connected directly to a working wall outlet or are you using a power strip or surge protector?

Remove any of those...

I'm personally leaning toward the power supply on this one only because the OP says it only stays on for a few seconds.  Even with a failed heatsink the system should at least stay on long for the OP to get a "Fan failure" or some other similar alert message Usually the processors of today's machines just throttle if they get hot unless it gets EXTREMELY hot then you can expect a shut off I've honestly never heard of a processor getting hot enough to where it would shut a system down in a few seconds. 

I take the 115VAC from an older 6-way outlet with surge protector. 

I ran into another problem.

I was replacing the old thermal paste (PITA!). When I took out the 1st heat dissipator and reinstalled it, the PSU did not react as expected. There is a LED that is supposed to light up after turning on the PSU (green-wire-to-ground-wire trick). Now it lights up right after I connect the 115V to the PSU and turns off when I turn on the PSU. And still no DC power output.

I noticed that there are two TO220 type ICs with pin 2 clipped off but this pin is soldered to the PCB. Strange...

I pushed back together the separated pin 2 on both ICs and secured them with a toothpick on order to make contact. Still no output..

Then I remembered that one of the soldering plates for the dissipator came off the PCB. No big deal, unless it was connected to something. 

And after I made a connection between this leg and the nearest solder lug next to lug that appeared to be originally connected to the one that fell off, the PSU did not power up at all.

Question: is there a fuse hidden somewhere on these PSUs or is there another component that acts as a fuse?

Thanks again in advance.

The 115V (or 120V) come from a good and working outlet. I even checked the voltage at the outlet after PSU didn't turn on.

By heat dissipator I mean the aluminium heat sinks on top of the PCB that are supposed to dissipate the heat created by the ICs that are attached to it.

If there are no fuses on these PSUs, there must be another component that went south. Maybe a resistor?

Problem is that there is silicone stuck in strategic places that is probably supposed to reduce vibrations. And these places are so tight in space that one cannot see what lies below it. 

I could not find a schematic online for this PSU.

Power supplies are not user serviceable parts.  You can be killed by the charge on the capacitors on the input of the switching unit even if the power supply is no longer plugged in.

@Zickezack 

Do yourself a favor and toss that PSU and get a new one ASAP. A PSU will be a lot cheaper than a whole new PC when you fry this one.

We take no responsibility when something bad happens...