Dead LG flatron M1717S monitor

[shamooooot]

Hello Guys

I had this LG flatron M1717S monitor for a quite some time now. Couple of days ago it stopped working, it is completely dead and even the LED is not working.

I have some experience in fixing boards but when it comes to SMPS I am not very good.. But in this particular case it is a challenge for me to fix this.

I found the output caps leaking and I replaced them all, but still no success.
The bridge is producing 319 VDC.
I took the power FET and test it out (using dirty test), and it seems to have no problem
The voltage between the 5v pin and the gnd pin is 1.4-1.12.
The 15v pins and the gnd pins are all shorted!!.
In one of the Schottky diodes I found that all pins are connected together!!.

Here's the board after replacing the caps







Please advise.

[shamooooot]

Re: Dead LG flatron M1717S monitor

I am just wondering if this is a hopeless case!!, could it be fixed or not?

[stj]

Re: Dead LG flatron M1717S monitor

replace the small capacitors - they are related to the startup & feedback.

[shamooooot]

Re: Dead LG flatron M1717S monitor

replace the small capacitors - they are related to the startup & feedback.

Thank you for answering STJ.

I have a question,

I removed the Schottky diodes, and I found pins place on board are all shortened!!. While Following on board connections, only two of them should!.

I also removed a 980ohm resistance on the output, and found pins place on the board also shortened!.

All the components on this side seems good (not shortened).

Is this normal?

[shamooooot]

Re: Dead LG flatron M1717S monitor

replace the small capacitors - they are related to the startup & feedback.

I have replaced the two small electrolytic caps. But still no success.

[shamooooot]

Re: Dead LG flatron M1717S monitor

Any ideas pleaaaaase???!

[budm]

Re: Dead LG flatron M1717S monitor

So the two 8-PIN SMD power MOSFETs in the inverter sections are OK?
The two TO-220 rectifier diodes tested good off the board?

"I removed the Schottky diodes, and I found pins place on board are all shortened!!. While Following on board connections, only two of them should!." Can you please mark the location where it shows shorted?

BTW, the picture are too dark for me to see the connections and traces clearly.
https://www.badcaps.net/forum/attach...3&d=1479994680

[shamooooot]

Re: Dead LG flatron M1717S monitor

So the two 8-PIN SMD power MOSFETs in the inverter sections are OK?
The two TO-220 rectifier diodes tested good off the board?

"I removed the Schottky diodes, and I found pins place on board are all shortened!!. While Following on board connections, only two of them should!." Can you please mark the location where it shows shorted?

BTW, the picture are too dark for me to see the connections and traces clearly.
https://www.badcaps.net/forum/attach...3&d=1479994680

Dear budm


Thanks for answering, and sorry for the late reply. I just started to loose hope with the forum.

I have taken a better pics for the board, I also marked the shortened place. Please note that the Schottky diode itself is tested fine off board!!.







Someone proposed that it is obviously the transformer, but I am not convinced as I heard these are very hard to break down. Is there a way to test it using the DMM?.

I can reach and test the board with load and power connected.

[budm]

Re: Dead LG flatron M1717S monitor

So when you check the resistance between the GND and the center pin of that marked diode it shows very low resistance and the diode tested OK off the board, right? Then the loads may be shorted it that is why i asked you to tested those two MOSFETS.
BTW, the soldering works look bad.

[budm]

Re: Dead LG flatron M1717S monitor

BTW, that diode (D201) in question is for the power supply for the Inverter circuit.

[shamooooot]

Re: Dead LG flatron M1717S monitor

BTW, that diode (D201) in question is for the power supply for the Inverter circuit.

Dear Budm


Regarding to your first question, all the marked pins are shortened together and at the same time shortened to the ground (on the output connection).
This short is not resulted from the output as I test the board with the output not connected.

The soldering is bad yes, but I am sure it is not the problem, and these two Schottky diodes are tested fine off board.

And the D201 I don't believe it is related to the inverter circuit!.

[budm]

Re: Dead LG flatron M1717S monitor

"This short is not resulted from the output as I test the board with the output not connected." Well the output of that power supply section is CONNECTED to the inverter circuit, so you can disconnect the connector but if there is shorted circuit in the inverter section then it will short will still be present.

"And the D201 I don't believe it is related to the inverter circuit!."
How did you figure that?
I suggest you to at least trace out the connections to see it for yourself where the output of that D201 is connected to.
You can also lift up one end of the JUMPER for the 15V that feeds the inverter circuit to see of the short will go away.
That soldering works need to be cleaned up to make sure there is no solder bridge.
I try to help but if you insist then that D201 is not related to the inverter circuit and not going to test the MOSFETs then I cannot help you, good luck.

[shamooooot]

Re: Dead LG flatron M1717S monitor

"This short is not resulted from the output as I test the board with the output not connected." Well the output of that power supply section is CONNECTED to the inverter circuit, so you can disconnect the connector but if there is shorted circuit in the inverter section then it will short will still be present.

"And the D201 I don't believe it is related to the inverter circuit!."
How did you figure that?
I suggest you to at least trace out the connections to see it for yourself where the output of that D201 is connected to.
You can also lift up one end of the JUMPER for the 15V that feeds the inverter circuit to see of the short will go away.
That soldering works need to be cleaned up to make sure there is no solder bridge.
I try to help but if you insist then that D201 is not related to the inverter circuit and not going to test the MOSFETs then I cannot help you, good luck.

Dear Budm

I am truely sorry, please excuse me if I offended you in any way.

I just couldn't trace that diode to the inverter circuit .

By MOSFETS you mean U303, and U304?.

Also as I mentioned before I don't have the 5v rail, I only got around 1v.

[budm]

Re: Dead LG flatron M1717S monitor

"By MOSFETS you mean U303, and U304?" Yes.
You are getting the fluctuation output because the power supply is in the startup/shutdown due to over current draw and that is due to the shorts (low resistance of the D501 output).
So to isolate the inverter power supply from the 15V output is to lift one end of the jumper J1 which feeds the 15V to the inverter circuit, then check the resistance between GND and the Cathode (middle pin) of D201 to see if the shorts/low resistance at the middle pin of D201 goes away.

[shamooooot]

Re: Dead LG flatron M1717S monitor

"By MOSFETS you mean U303, and U304?" Yes.
You are getting the fluctuation output because the power supply is in the startup/shutdown due to over current draw and that is due to the shorts (low resistance of the D501 output).
So to isolate the inverter power supply from the 15V output is to lift one end of the jumper J1 which feeds the 15V to the inverter circuit, then check the resistance between GND and the Cathode (middle pin) of D201 to see if the shorts/low resistance at the middle pin of D201 goes away.

BUDM

Yes I have left the J1 and now the short circuit is gone.. I tested the U303 and all pins are shortened together. I will get a replacement one soon.

Just for being sure, would my monitor work without connecting the jumper - without backlight of course?.

[budm]

Re: Dead LG flatron M1717S monitor

I would replace both MOSFETs at the same time.
The monitor should power up now without that J1 in place.

[shamooooot]

Re: Dead LG flatron M1717S monitor

I would replace both MOSFETs at the same time.
The monitor should power up now without that J1 in place.

Hello budm

I have replaced those MOSFETs. When I plug the monitor now, nothing is shown on it as before. Also the LED on the ON/OFF button keeps flashing with blue color.

I have tested the output of the power supply board, and the voltage is fluctuating. The results as follows:

Without load: 4.7-5.25 for the 5V rail. And 13-15 for the 15V rail.
With load: 1-5 for the 5V rail. And 6-17 for the 15V rail!!!!!!!

What do you think?.

[budm]

Re: Dead LG flatron M1717S monitor

Lets us see the soldering works of those two MOSFETS.
You are sure that those two MOSFETs are installed correctly?
The DCV without load should be steady. Did you check or replace the small electrolytics cap in the primary side?
BTW, when you say with load, do mean with Jumper J1 in place, and no load = J1 not in place?

[shamooooot]

Re: Dead LG flatron M1717S monitor

Lets us see the soldering works of those two MOSFETS.
You are sure that those two MOSFETs are installed correctly?
The DCV without load should be steady. Did you check or replace the small electrolytics cap in the primary side?
BTW, when you say with load, do mean with Jumper J1 in place, and no load = J1 not in place?

Der budm

Yes I was extremely careful in installing.
Yes I replace the two small caps.
With load I mean the connection to the main board. I installed the jumper directly after the MOSFET.

[budm]

Re: Dead LG flatron M1717S monitor

If the J1 is not installed, and the connector to the main board is in place. the monitor should come on without backlights and the Voltages should be steady. The Power light should stay on Blue if the monitor is connected to the running PC, try that.
And then if J1 is in place, and everything are hook back up and you see fluctuating Voltages, it may mean that the Inverter transformer is bad.
The heaviest load is the inverter circuit.