EAY36781302 PSU Partial power

[Mongor62]

This is a TV power supply and it powers up the speakers and everything seemingly except the LCD panel.

What i have done so far is replace the 2 large CAPs, the fuse that was blown near it, 2 large MOSFETs, 1 smal MOSFET and 1 Diode. I pulled a couple of the bigger transistors and they seem to be working & i pulled the main bridge rectifier and it seems to be working correctly. Please keep in mind when i say they seem to be working correctly this means i used my Fluke 87 and used the stripes on the transistors to see what it should read and i read a few things on how to check the bridge rectifier (seen a video too) and it works according to both of those.

After i replaced those 2caps, 3MOSFETS, 1Diode & 1 fuse i turned on the TV and after load up a popping noise came from the PS within a few seconds so i switched it off asap and unplugged it. When i gave it sometime to cool down and disconnected everything the stuff i changed is now like the damaged ones i replaced, So now i am where i started basically

I am looking for help to where to look next.

[Behemot]

Re: EAY36781302 PSU Partial power

I worked on similarly looking board which tells me LG makes more or less the same boards for TVs.

It seems that PFC is probably OK otherwise the very first, input, fuse would blow.

So now that middle fuse is blown again?

[Mongor62]

Re: EAY36781302 PSU Partial power

Yes it blew again and i think it was the thing that made the popping sound but not sure.

[Behemot]

Re: EAY36781302 PSU Partial power

Have you tried only this board without anything else attached, or with other board as well? The problem may also be in the inverter(s) directly.

[Khron]

Re: EAY36781302 PSU Partial power

For what it's worth, i remember having to fix an LG 42" LCD-TV last winter. The fault ended up being two shorted MOSFETs in the inverter. Some sort of funky full-bridge converter, supplied straight from the main primary rail, if memory serves.

I'd start by pulling out all the power semiconductors "downstream" from the fuse that popped, test them, and replace as necessary.

[Behemot]

Re: EAY36781302 PSU Partial power

May very well be some of the smaller diodes or something. Good pciture of the other side of the board would help as well.

[Mongor62]

Re: EAY36781302 PSU Partial power

This board the way i understand it is a Power Supply/Inverter board (there are no inverter boards for this TV just the PS/Main/TCom). As far as testing diodes could i get some sort of guidance please, reason i ask is because i am a novice and i am confused between Schokky FRD and semiconductors. They are currently on board and i can pull if needed (i am new to this but like using my new Hakko FX888D) i own a Fluke 87 (not V) DMM

[Behemot]

Re: EAY36781302 PSU Partial power

Right you are, I haven't noticed those two connectors. Those new images are terrible man, I guess you are using compact and some kind automatic settings. And there is not light enough. It uses too high ISO for the sensor quality making the images look like mosaic paving.

Anyway, start from transformers: measure resistance on their secondary side, usually it is arround 1200 ohm and the difference between both transformers should not be higher than 1-2 %.

As for semiconductors, there should be settings for diode testing on the multimeter.

[Mongor62]

Re: EAY36781302 PSU Partial power

These are transformers right?, also i need to remove them from the board or can i test on board?

I am falling a sleep i am sorry to say because it is past 4am here but i will take any direction you are willing to provide me with after i wake up and please understand your help is very much so appreciated.

[Behemot]

Re: EAY36781302 PSU Partial power

No. 4 and 5, you can test in circuit.

Even semiconductors may be test in circuit but sometimes there are small resistors in parallel which may make you think something is short yet the resistors only have small resistance.

[Mongor62]

Re: EAY36781302 PSU Partial power

Ok the readings on the transformers read the same no matter which lead i put where and this to my math is more than 1-2% difference
#4 25.6 OHM
#5 24.1 OHM

I included a pic of the board and circled the diodes with blue and red. and measured with the meter set to diode and to hold/beep.

All the Reds were -0.001V no matter which direction I think these are diodes not sure.

All the blues were between .438 to .455 and no reading when i reversed bias so other than being just below .5 they seem to be working according to the way i understand how to read them.

[Behemot]

Re: EAY36781302 PSU Partial power

Those FBxxx are ferite beads, a type of inductor, it is basically wire for DMM.

About the transformers, 25 ohm seems somewhat low on the other hand they don't look like ordinary inductor transformers. They seem to use somewhat thick wire and less turns. So I don't think they are not the reason for blowing-up the transistors.

[Mongor62]

Re: EAY36781302 PSU Partial power

Why if the ferite beads (inductors) are just like wire why would the diode setting read then like that?

[tom66]

Re: EAY36781302 PSU Partial power

0.001V = approx 1 ohm equivalent on a typical diode mode.

[Mongor62]

Re: EAY36781302 PSU Partial power

well since they aren't diodes and i had it in diode mode i am sure since they aren't diodes there has to be a better way of testing them other than DMM in diode mode right?

[budm]

Re: EAY36781302 PSU Partial power

It is very low value inductor, in nano Henry range, you just test to see if it does not show open (high resistance), 1/4 of wire can represent high reactance to high frequency signal, DC resistance will be very low. That is one of the reason you do not check the DC resistance of the transformer, it will show as low resistance, but for the high frequency it will represent high impedance to the switching device connected it, the DC resistance can only tell you if the transformer winding is open or not, you use ringer to test the transformer to find out if it has shorted turn or not.

[Mongor62]

Re: EAY36781302 PSU Partial power

Ok so if i have this clear the inductors on the middle of the board should be measure for resistance and should have high resistance (i don't mean to be repeating you but just trying to cross my T's and Dot my I's), can they be tested on board? if so they are testing bascilly wide open.

[budm]

Re: EAY36781302 PSU Partial power

Are you talking about the FBxxx component? If that is what you are referring to, then it will have low resistance for the type you are testing which consist of a piece of wire and the ferrite bead, the DMM will show less than 1 Ohm reading.

http://www.tpub.com/celec/56.htm

http://en.wikipedia.org/wiki/Ferrite_bead

[Mongor62]

Re: EAY36781302 PSU Partial power

Yes i was and that is a nice wiki, i never thought to wiki it i only was using the basic search engine and according to that i think they are working.

I don't have a ringer meter so what would be the next step?

[Behemot]

Re: EAY36781302 PSU Partial power

Check also small glass diodes and SMD components (diodes, transistors).

Are you able to make yourself safety load? For example lightbulb in series with the board.