Help me diagnose my monitor power supply.

Thread Starter

Nikola Zlatkov

Joined Aug 22, 2017
12
This old power supply for my monitor just died on me, its outputing 0v. I'm not really experienced with electronics but I assume the mbr20100ct is the problem. I will link pictures. I don't know how to test this component, only thing did is check if the pins are shorted, and they are. I can solder like an amateur but first I want to hear some tips here.20180905_163243.jpg 20180905_163254.jpg 20180905_163240.jpg
 

UnnamedUser159

Joined May 3, 2016
501
there is one PDF in the net about repairing LCD. There you will see the groups and their purposes.
You better check by logic the needed elements and then power on.

The diode is double diodes in one corpse. Check it as normal diode.
 

Thread Starter

Nikola Zlatkov

Joined Aug 22, 2017
12
I didn't remove the diode, when I tested it in the circuit it worked like it's supposed to. First I thought it was a mosfet that's why I was curios about the short. It does work like a diode (one way). So my bet is on the transformer. What are your thoughts?
 

AlbertHall

Joined Jun 4, 2014
9,826
The transformer is probably the least likely culprit.
Immediately to the left of the large capacitor in your picture is simething covered in a black heatshrink sleeve. That might be a fuse. If it's blown then that's a clue (but not the cause of the problem). If it's blown you need to find and test the transistor(s) or MOSFET(s) connected to the primary of the transformer.

Also worth testing is the bridge rectifier - oblong black thing left hand side near the top of your picture.
(It's also possible that the round black thing top right of your picture is the fuse).
 

dendad

Joined Feb 20, 2016
3,517
Most times the capacitors dry out, so the capacitance goes down. Among other things, this can increase the transient voltage pulses and pop FETs and other things.
First, change the caps with good low ESR types and then fix the other faults. If you leave the caps there, and they are failing, you will pop the bits again when you turn it on.

But please be careful! These things can be killers.
Wire an incandescent mains voltage lamp in series with the input to help protect the circuitry when testing. You will not be able to run to full power load with the lamp there, but run the supply without a load.
The lamp will flash when you turn it on then dim or go out.
If it lights brightly, the fault is still there.

BE CAREFUL!
 

Dodgydave

Joined Jun 22, 2012
9,157
The small 8pin chip on the back will be the smps chip. Usually this is a UC3843 series or other, and there you can get the datasheet and have a look at how they work. Most of the time it's the fet and the sense resistor.
 
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dendad

Joined Feb 20, 2016
3,517
Just be careful probing around the circuit with a scope or meter as it is at mains potential. A mains isolation transformer is recommend to make it a bit safer. And an earth leakage cutout to drop the power if you get a shock to ground. I know I'm a bit pedantic about this but it is important to keep safe. Burning flesh does smell pretty bad!
 

Thread Starter

Nikola Zlatkov

Joined Aug 22, 2017
12
I have checked the components you suspected are fuses and they are showing 0 ohms so they're okay. Maybe the fact that this power supply died when there was a power surge should help figure out the problem.
 

ebp

Joined Feb 8, 2018
2,332
I think the fuse may be the black round part that is adjacent to the rectangular yellow capacitor and under the brown wire in the upper right hand corner of the first photo, but it really is hard to tell. Some more photos of the board would be helpful, including one that shows all of the back of the board.

EDIT- I missed Albert's comment about that being the fuse
 
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