NEC Multisync 2470WNX LCD monitor doesn't power up

[mctx]

I need help fixing my NEC Multisync 2470WNX LCD monitor.
It was working flawlessly until couple of days ago it would not turn on.
I've checked the mains cable and found blown 5A fuse.
After replacing the fuse and testing cable, the monitor is still doesn't power up. I took it apart and could not find any caps looking suspicious.
All other parts also looked OK. I used multimeter to check the power board 715G2428-2. With mains connected, I could detect AC voltage up to see
voltage testing.jpg. None of the exit DC channels (24-5V, etc), see DC channels.jpg, appear to have voltage. All USB ports are dead. I have replaced parts on small simple boards in the past, but this board is fairly complex and it's not clear what/where to start testing. Any help would be greatly appreciated. Let me know if you need additional photos/data.

[CapLeaker]

Re: NEC Multisync 2470WNX LCD monitor doesn't power up

check the 5v pins on the PSU to see if there is stby voltage.

[mctx]

Re: NEC Multisync 2470WNX LCD monitor doesn't power up

I know where the 5v pins on the PSU in the computer.
Where are they on the power board?
I've already checked the 5v (and all others) pins shown in the attached picture
DC channels.jpg.
Do I need to short PS_ON pin with something else to get the power board doing something?
Thank you

[CapLeaker]

Re: NEC Multisync 2470WNX LCD monitor doesn't power up

You need to have 5VSB on the PSU first. Without any STBY voltage, the PSU won't turn on. So if you don't have STBY voltage, you have to fix that first.
If you do have STBY voltage, then do this:
You need two 1k or so resistors. One 1k resistor between PSon to 5VSB and the other resistor between On/Off (back lights) to 5VSB. When you plug the PSU in, all the voltage rails should be up and the back lights turned on.

[mctx]

Re: NEC Multisync 2470WNX LCD monitor doesn't power up

Capleaker, I have attached (decent IMHO) pictures to make advising easier for those who wanted to help.
Any chance you could link your suggestions to the pictures of my board?

1. Could you point where do you measure STBY voltage?
As I already said there is no voltage on output pins (DC channels.jpg)
and I cannot see STBY pin(s) on the PSBoard.

2. "So if you don't have STBY voltage, you have to fix that first".
OK, how?
Google search for "badcaps.net+fixing STBY voltage" generates a lot of pages........

3. What usually happens if monitor blows 5A mains fuse before stopping working?

Thanks

[budm]

Re: NEC Multisync 2470WNX LCD monitor doesn't power up

We need good clear pictures of your board, especially at the connectors so we can clearly see the pin's labels.
The second picture in post 1 is from the web but is it exactly the same as your board?
We also need good clear straight shot pictures of the bottom side of the board so we trace out the circuits.
Picture #1 on post one is not well focused either.
Fuse will blow if too much current flow through the circuit so we need to trace out the board to be able to give you the test points.

[mctx]

Re: NEC Multisync 2470WNX LCD monitor doesn't power up

Budm,
I've attached the best pictures I could squeeze out of my smartphone using support to eliminate hand shaking. I don't have better digital camera to improve the quality of pictures. Pin's labels are exactly as they are shown in DC channels.jpg. Can you work with this level of clarity? Tell me what you need to see and I'll do my best to provide the info.

Is it normal that 5A mains plug fuse was blown and on-board 2.5A fuse was not?

Thanks.

[CapLeaker]

Re: NEC Multisync 2470WNX LCD monitor doesn't power up

Sorry man, but the pics are too grainy for me.
With the PSU disconnected, put your meter in Ohm mode, put the red probe to any of the 5V pins and the black probe to a GND pin, in order to see if there is a short of the 5V STBY supply output. Your first post, 2nd picture.

I've zoned some areas of interest of the 5V secondary. There is a cap, a inductor and a diode that's mounted on a heat sink. Check that diode, if it is shorted, it is bad.
However your statement on the fuses, leads me to believe that the problem is in the primary section of the power supply. Seems like something went open?

[mctx]

Re: NEC Multisync 2470WNX LCD monitor doesn't power up

Thank you for your suggestions and apologies for the quality of pictures.

I've tested 3 5V pins in 2K Ohm range - 0.471


diode D904 2KOhm range 0.481, reverse-0.459
0.480, reverse-0.459

cap C916 2KOhm range 0.480, reverse-0.458


rectif L903 2KOhm range 0.000 BOTH DIRECTIONS

Could you expand on "problem is in the primary section of the power supply. Seems like something went open? " (or point me where to research)?

[CapLeaker]

Re: NEC Multisync 2470WNX LCD monitor doesn't power up

Well... second try. I've lost my original post somehow. It looks like the secondary is fine as there is no short at 471 Ohms. The inductor L903 should measure a short. The diode D904 has 2 diodes in one package as it has 3 legs. You have to measure each of the outer pins in reference to the middle pin. It should read what you have measured, but when you reverse the leads of your meter, it should show open. If something doesn't measure correctly in circuit, take it out and re-measure it again outside of the circuit.

Following things backwards, output pins, cap, inductor, diode, transformer, some switching fet and a drive circuit for the fet (some IC). I wonder if that drive IC is actually getting Vcc power, or if it is blown or dead. The FET, should be easy, if it is shorted, it is bad. If there is a short involving the gate of the FET, the gate drive circuit would have to be checked and the IC driving it. Unfortunately I can't see much on these pictures, but maybe you are able to borrow a better camera from someone and snap some new pictures.

[mctx]

Re: NEC Multisync 2470WNX LCD monitor doesn't power up

diode D904 - I knew it had 3 legs. It was tested as you described. Results are already included.
I managed to get much better pictures so may you could take another look?
Let me know if something is still not clearly visible.
Thanks