[Moved] How to repair dead Dell NPS 250KB D PSU ?

How to repair dead Dell NPS 250KB D PSU ?

My Dell NPS 250KB D PSU is dead . Fan is not working at all. My PC didn't start up at all.
Main Fuse is alright. Don't see anything damaged physically, no signs of any blown component.

How to diagnose which component gone bad ?

What are the main components inside it to check . And, how to test these components ?

I also don't have its schematic (electrical drawing) to check

thanks in advance

Re: How to repair dead Dell NPS 250KB D PSU ?

For this operation (testing/replacing mains side components), you are working with mains rated voltages here and they are deadly, so if you are unsure of your abilities please seek the services of a suitably trades qualified person. First read some advices here:
Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies
One idea for the tests to protect PSU using a series light bulb, see details here:
SER FAQ: TVFAQ: The series light bulb trick
In the situation when your power supply does have a short circuit on the mains side, the lamp will light up and stay lite up, so the lamp acts like a power resistor and this should limit current and may save the PSU from self destruction.

If the fuse hasn't blown, it possibly could be the secondary side. Check all the caps first, then the feedback side.

jj_alukkas said:

If the fuse hasn't blown, it possibly could be the secondary side. Check all the caps first, then the feedback side.

Click to expand...

What components are present in the secondary side & in the feedback side ?

The secondary has a lot of hi rated quick rectifier diodes, a bunch of low ESR capacitors and the feedback side has 1 controller chip, possibly along with a TL494 chip and it will be hooked up to an optocoupler wired to the switching elements on the primary. If the opto dies, the supply wont work, same as with the controller chips. Since u have the fuse intact, power up the smps and check for 5 volts on the VSB line (purple wire), then check all caps on side near from where the wire bunches are soldered. That is the secondary side. remove caps from pcb before testing. on circuit will show a false closed ckt due to the transformer's secondary impedance. Also beware that the huge caps on the primary can hold a lethal charge even after u disconnect the supply from mains, check with a multimeter and discharge with a ac bulb if necessary.