LG47LC7DF Fuse Blows

[killian6pk]

On an LG47LC7DF TV the main fuse on the Power Board blows as soon as it is plugged in. What component is likely to be causing this, guess short, bridge or other component?
Owner brought this to me because it had several blown caps. Among them was one of the Large Caps. Also both fuses were bad. I replaced the caps and the fuses, but he did not bring me the power cord so I could not check it. As soon as he got the board in the tv and plugged in the power cord the main fuse blew. His power cord was suspect as he had jury rigged it. He brought the whole tv to me and I replace the main fuse and used a different power cord to plug it in and the fuse blew immediately.

The only two posts that I could find did not deal with this issue.

[vinceroger69]

Re: LG47LC7DF Fuse Blows

bridge rectifier would be one of my first tests then check any transistors/diodes on the hot side of the psu also check your replacement capacitors are fitted the correct polarity as we all make the odd mistake from time to time.

[budm]

Re: LG47LC7DF Fuse Blows

We need to see good clear straight shot pictures of the top and the bottom side of the power supply board, then pictures of the whole backside of the TV showing all the boards.

[vinceroger69]

Re: LG47LC7DF Fuse Blows

This thread maybe worth a read and some possible voltage tests once you have sorted the fuse blowing issue
https://www.badcaps.net/forum/showth...ight=LG47LC7DF
this maybe the psu used in this set please confirm board number
http://www.shopjimmy.com/lg-eay36781...upply-unit.htm

[killian6pk]

Re: LG47LC7DF Fuse Blows

Bud here are the pictures you requested.

[budm]

Re: LG47LC7DF Fuse Blows

Check all the devices in the primary sided that are attached to the heasink, especially the MOSFET's for the PFC circuit.
Pictures are not straight shot and not in focus for me to be able to look for power supply board P/N.
There are prints on the power supply board and also the numbers on the barcode sticker.

[Relayer]

Re: LG47LC7DF Fuse Blows

Hello killian6pk,
Can you please check the resistor shown in the modified picture below:



To me it doesn't look like a white band marking from the factory. It seems to have burned in that section of the resistor and has turned to white ash.
Can you please confirm this?
Thanx.
Regards,
Relayer

[killian6pk]

Re: LG47LC7DF Fuse Blows

Bud here are the Board PN:2300KEG021A-F
Bar Code Number: L82AB367813022533
I will check all the parts on the heat sinks. Get back to you on results.

[killian6pk]

Re: LG47LC7DF Fuse Blows

Relayer, I checked the resistor it is a 39K ohm 5% and reads 38.1K so it appears to be ok. I don't see any signs of burning or ash.

[killian6pk]

Re: LG47LC7DF Fuse Blows

I have checked the bridge rectifier and it tests good. On to other fets, diodes etc. I found a common cathode diode (2 diode symbols facing each other). Tested 1st leg to middle leg got reading. Reversed no reading. 3rd leg to middle leg got reading, reversed no reading. I believe this one is good. Let me know if I am wrong. Will get back with more.

[killian6pk]

Re: LG47LC7DF Fuse Blows

I found a bad mosfet (shorted) checked one regular diode next to it, it was ok. Would a shorted mosfet be enough to blow the fuse?

[vinceroger69]

Re: LG47LC7DF Fuse Blows

yes a shorted mosfet will blow a fuse have you removed the mosfet and re checked it out of circuit.

[killian6pk]

Re: LG47LC7DF Fuse Blows

Yes I have is appears to be shorted. I have ordered two new ones, extra for just in case.

[killian6pk]

Re: LG47LC7DF Fuse Blows

I have also found an Inrush Current Limiter that is broken with one half of it missing (like you took a circle and cut it in half). This is the Digikey part number for the part I am talking about: 317-1226-ND. There are several of these on the power board. Does this one need to be replaced also?

[Relayer]

Re: LG47LC7DF Fuse Blows

Hello killian6pk,

Does this one need to be replaced also?

Yes, most definitely.
If they are reasonably priced, then it wouldn't hurt to replace them all.
Reason: They degrade after a time due to their internal molecular structure changing every time it has to deal with in-rush currents.
Regards,
Relayer

[budm]

Re: LG47LC7DF Fuse Blows

Sounds like the MOSFET for PFC is shorted out, which in that case the PFC IC should be replaced at the same time.
Which MOSFET is it in the pictures? Designator of the MOSFET?

[killian6pk]

Re: LG47LC7DF Fuse Blows

The mosfet is Q103 on heat sink close to small yellow transformer. In an earlier post on this thread Relayer had a picture that shows the heat sink this component was on. There is a small diode next to that heat sink, that I pulled one leg up and checked. It was ok. Where is the PFC IC located? Do I need to continue checking other components or does that one mosfet blowing protect the others? Also I am having trouble getting the solder to melt on this board. My iron is variable temp and is set on 800 deg. F., what temp do you use when working on this type board?

[vinceroger69]

Re: LG47LC7DF Fuse Blows

add some new solder to the part you want too remove it usually helps and sometimes the pfc ic is mounted on the hot side on the underside of the psu.

[killian6pk]

Re: LG47LC7DF Fuse Blows

Bud
I found a small ic on the back side of the board very near the mosfet that blew. It is a four leg per side flat rectangular surface mount chip. Does this sound like what you are talking about? Location on board is U101. The numbers on it are
line 1 - 000
line 2 - oh48
The o looks like a lower case letter o instead of a 0 like on the line above it. Did a little looking on Mouser and found several pages of this type of chip (all kinds of leg configurations). How do I measure it to determine if it is good or bad?

[killian6pk]

Re: LG47LC7DF Fuse Blows

I have looked thru the Service Manual parts list and can't find a U101 on the list of parts. How do you determine the specs for the IC if it is bad so that you can order another one?