LCD Monitor PSU Repair

[giammyzanna]

Hello, this should be a relatively easy fix for you guys, hope I can I have some help.
I have attached schematics of two lcd monitors power supplys. The primary side looks very similar in both of those. Basically they won't power on, no standby, no nothing. The caps seem good at a glance.
The first thing I did was to check the fuse and it's open (on both monitors) so I decided to replace it on the acer (I had only bigger fuses lying around, the one that was on the monitor was rated 4A) and as soon as I plugged it in the NTC on the other side popped (so the main house braker did) so I stopped there.
First of all what is that NTC doing there, I know it's a temperature sensor so I can't see the use of it near a small capacitor.
Second, the dell monitor I think is pretty much the same so I decided to not do anything before anyone more skilled than me can help.
Any ideas on what to check for?
Thanks in advance!

[Armadillo]

The NTC is for inrush current protection.

The first lesson you should learn is, when the fuse is blown, never put another fuse in without checking its causes. Otherwise its create more damages and $.

The first thing you should check is do ohm measurement across the DC capacitor for shorts. The capacitor should slowly charge up to high ohms and not showing low ohms. Anything lower than 10 ohms is considered shorts.

 

[madires]

The NTC limits the inrush current when switching on. The blown fuse and broken NTC are an indication of a short in the primary side. I'd start by checking the bridge rectifier and switching MOSFET. Before replacing a blown fuse check why it has blown. You might have worsen the problem by trying a fuse rated for a larger current.

[giammyzanna]

Thank you, so basically the NTC blew because of too much inrush current?
Of course yes, by replacing the fuse I created more trouble hehe (especially because there is nothing written on the NTC so no idea on what to get to replace). I checked and the main filter cap reads above 1 MOhm, and the right capacitance (150uF). The rectifier is not shorted. No idea on how to check the mosfet

[Armadillo]

The first thing you should check is do ohm measurement across the DC capacitor for shorts.

 

Always remember to ** DISCHARGE **  capacitors BEFORE you do ohms measurement. 

[giammyzanna]

I use the super professional method by shorting it with a screwdriver heh - it has been unplugged for a couple of days btw

[Armadillo]

I checked and the main filter cap reads above 1 MOhm, and the right capacitance (150uF). The rectifier is not shorted. No idea on how to check the mosfet

In circuit measurement or out of circuit measurement.? The ohm measurement I mentioned is for In circuit measurement.

[giammyzanna]

It's all in circuit. Didn't take anything out yet

[Armadillo]

I use the super professional method by shorting it with a screwdriver heh - it has been unplugged for a couple of days btw

KKKKKKKK  Good its super!.... unplugged a couple of days do help.

I checked and the main filter cap reads above 1 MOhm, and the right capacitance (150uF). The rectifier is not shorted. No idea on how to check the mosfet

In circuit measurement or out of circuit measurement.? The ohm measurement I mentioned is for In circuit measurement.

Use the method on the "fresh" DELL to check for shorts.


For the first monitor;
That means something is open maybe due to your second fuse now hiding the shorts. Take the mosfet out of circuit and measure in Diode mode.;
Red on Drain, Black on Source... Either a diode drop or higher reading and cannot be shorted.
Do the same on Gate to Drain and Gate to Source.
After that use a component tester to check the mosfet for functional.

But nowadays, mosfet can be hidden in the integrated switching ICs also.

[giammyzanna]

Luckily these are the older types of monitors. On the dell for example the switching regulator is a 2764AI which I could find on google Tomorrow I'll check that. Meanwhile I tested the main cap on this psu too and it's in the orders of 1Mohm and above (it varies a bit). The rectifier reads 1,3 Mohms on the input and about 5 Mohms on the output. There is also a cap in parallel to the 240v ac input (on both PSUs) and those read about 1Mohm too.

Strange things here. I checked some of the low voltage caps and most of them (on both boards) are in in the 200-300 kOhms range.

[Armadillo]

Yeah, its quite strange that your LCR meter can measure ** in circuit ** exactly 150uF. Must be really super LCR meter.

[giammyzanna]

No it's not, just the 10k count aneng meter, can't remember the model.
I don't know, but that's the reading it gives

[Jwillis]

You have the rest of the PSU circuit to show.All that you have posted is the EMI filter and surge protection.