42″ LG LED TV Repaired
Just 2 weeks ago my friend Chafik who owns his own Business in my Hometown (a Repair & ICT Shop) phoned me if I could help him with fixing a big LG 42 inch TV. The LG model 42SL9500-ZB was made in 2010 and had modern LED Backlight but wouldn’t go on.
And when I looked at the Boards I found a broken Fuse Fast 8A smd with subscription F101 on the LED Backlight Board.
This board (previous photo) was mounted in the middle on the back of the TV and covered by a metal plate. Under this Board, that was mounted with 6 screws on 6 higher positions pressed in the back plate, there were 3 cooler pads sticked on its bottom to help removing any generated heat onto the bottom metal plate.
After I had replaced the Fuse, I tested the LG with a protective 75 Watt Light Bulb in series with the Power input wires.
And the TV almost worked because I could see the Startup sequence when the Light Bulb went on and off fast in a kind of certain code after the standby button was pressed. But the TV didn’t come up. So we imagined that the Lightbulb took to much power away and we tried the TV again but now without the Lightbulb.
Now the LG worked again, so I went home thinking it was fixed.
But later my friend phoned me that the fuse on the backlight board must have gone bad again because the TV had worked but afterwards the Backlight was off again.
So he took the Backlight Board of the Backplate and brought it to me to have it inspected again. And as he suspected the F101 F8A Fuse indeed was broken again.
Because the TV only refused working when the Backlight Board was attached, the cause of the problem had to be on this Board only.
I took my digital Meter and measured on Diode test the Semiconductors on this Board. There simply had to be a bad component or a short current that caused the Fuse to blow again. There were 3 almost identical PWM controlled Power sources on Board that all had special 100V 40A N-Channel Mosfets.
They all measured okay except one Mosfet Q403 that had a low resistance between Source and Drain of about 34 Ohm. And as we know a Mosfet normally always has 2 conditions in which it can be set. Active (ON – About 0 Ohm between Source and Drain) or OFF (Open switch between Source and Drain, meaning a high resistance of about 1500 Ohm or more).
After I removed this Bad Q403 N-Channel Mosfet a type 40N10-25 the Backlight Board was tested again. And the Fuse F101 was replaced. And Yes Indeed, the TV worked again without blowing up the Fuse and without protective mode from the TV Power Supply itself. But of course the Backlight intensity was weaker nevertheless it worked again. Next Photo shows the now replaced new Mosfet. The Backlight Board number was KLS-420ELD Rev. 0.6.0.
Above photo the Power Board that is mounted on the left in this LG TV. Next photo shows the TV board itself with input selector and all its Input that is mounted on the right.
And previous photo finally presents this LG in working glory again!
So if you ever fix anything, keep in mind that a Fuse nomally never goes broken without a good reason! In this case the unthrustworthy Special Super Mosfet 40A 100V!!. See following Datasheet:
Until another time another repair.
Albert van Bemmelen, Weert, The Netherlands.
Please give a support by clicking on the social buttons below. Your feedback on the post is welcome. Please leave it in the comments.
P.S- If you enjoyed reading this, click here to subscribe to my blog (free subscription). That way, you’ll never miss a post. You can also forward this website link to your friends and colleagues-thanks!
Note: You can also read his previous repair article below:
https://www.jestineyong.com/tektronix-2440-oscilloscope-repaired-part-4/
(192)Dislikes
(2)
33 Comments
Leave a Reply
Yogesh Panchal
October 19, 2015 at 2:33 pm
Sir, Good job . Missed Mark with arrow on pics for defective area and components.
Albert van Bemmelen
October 19, 2015 at 5:01 pm
Hi Yogesh, Mosfet Q403 is clearly visible and described on the Backlight Board with the text Q403 beside it, and also Mosfet Q404 is mounted above Q403. An arrow wouldn't give any new information.
Tito Kanshulu
October 19, 2015 at 4:11 pm
Good job sir,indeed the blowing of a fuse in any gadget must give us questions.
Albert van Bemmelen
October 19, 2015 at 5:26 pm
Yes indeed Tito, although there are situations were the Fuse is just gone bad or from inferior quality. And in such cases there is no other cause than the Fuse itself. But because the Fuse in this repair was replaced already and went bad again it was obvious that there had to be another cause! And if the cause, in this case the Mosfet Q403, behaves erratic it is harder to detect it. Since it still could be working perfectly over a short period of time before collapsing between Source and Drain again. As it had in this example.
Robert Calk
October 19, 2015 at 4:28 pm
Good job, Albert. Yeah, a TV made in 2010 is not old enough for a fuse to get old and weak.
Robert Calk
October 20, 2015 at 8:40 am
Also, I answered your comment you made of the false claims about the Zen50. You may have missed it.
https://www.jestineyong.com/which-resistor-to-use-with-this-led/
Albert van Bemmelen
October 20, 2015 at 2:56 pm
I forgot to look there Robert. But I will see what you had answered at that Post. Thanks.
Corriete
October 19, 2015 at 7:10 pm
goood but but i agree with the previous comments.I am also interested in the area where you placed the light bulb. a photo with marking would be appreciated. thanks continue to share sir.
Albert van Bemmelen
October 19, 2015 at 11:23 pm
The light bulb is always connected in series with the Power cord were the European 230V AC (or American 110/120V AC) input goes into the TV. So logical in series with the External connected Power Cord. (Or were logical the Power Fuse or the real Power Switch interrupts the Power input). The light bulb prohibits the TV from directly short circuiting on the Power Line in case of TV problems. When there is a short circuit in the TV the Bulb will continuously light up. Else the bulb will dim and the TV will normally work. The higher the Wattage needed for the Device under test, also a higher Wattage Light Bulb probably will be necessary in order to startup the Device under test.
Humberto
October 20, 2015 at 1:29 am
Albert, a great repair was done for you. Good quality in your photos, and explanations were great. Congratulations and keep up.
Albert van Bemmelen
October 20, 2015 at 5:00 pm
Thanks Humberto, I understand you meant to say that the repair was done by me instead of for me? Saying for me would imply that someone else repaired my TV. And it wasn't mine of course but of one of my friend Chafik's customers. After this repair he also had another TV, a Sharp from a costumer that was an easy repair. It wouldn't startup because of a bad bulging 2200uF 16V capacitor on the PSU board. After replacing it by a new 3300uF 16V electrolyte the Sharp worked like new again. But because it was a very easy repair I didn't make an article of it. I do believe it still has to be interesting somehow.
Robert Calk
October 20, 2015 at 6:32 pm
I'm the same as you. I fix some stuff that is so simple I don't write an article either. I fixed a 9 LED torch light yesterday. It had bad solder joints, and one LED somehow was installed onto the PCB backwards - simple repair.
Humberto
October 22, 2015 at 1:01 am
Sorry for the missunderstanding Albert. I know you really repaired the device.
Albert van Bemmelen
October 22, 2015 at 3:20 pm
Don't worry Humberto! Your English is much better than my Spanish.
Me no hablo espanol.
Humberto
October 23, 2015 at 11:01 pm
Ok Albert, no problem.
Ok Albert, no hay problema.
reza
October 20, 2015 at 10:53 pm
hi Albert van Bemmelen
thank u for sharing this article.
Albert van Bemmelen
October 21, 2015 at 7:52 pm
Thank you Reza. These repairs are easy. Only the special replacement Mosfets are rather costly. (If I'm correct they almost cost 5 Euro and the same amount or even more for Shipping from the UK => 1 Euro is about 1.135 US Dollar).
Andre Gopee
October 21, 2015 at 10:36 pm
Good Article Mr. Albert. Gives me inspiration in electronic works
Albert van Bemmelen
October 22, 2015 at 3:24 pm
Thank you Andre, I appreciate it!
mahmoud
October 22, 2015 at 5:49 am
Hi Albert this was agood repair by u and very useful for me hope after this we see the best article frpm you thanku.
Albert van Bemmelen
October 22, 2015 at 3:25 pm
You're Welcome Mahmoud, glad you liked the article!
Albert Hoekman, Holland
October 22, 2015 at 10:23 pm
Hi Albert, Good article and a good description of the way you solved the problem. A coïncident is I just got an LG 26LC2R with chassis LP61A on my desk. I dit not yet open it but the problem is a very slow starter (just like me, LOL). I manage this is a problem of "pregnant" capacitors.
If it's another problem I think I write an article for this site of Jestine Yong.
Albert van Bemmelen
October 23, 2015 at 5:15 pm
Hi Albert, It's always those short living Electrolytes. (Dutch: Elco's).
I've had that problem in a monitor too before. I still use this monitor after having replaced several of those capacitors.
Albert Hoekman, Holland
October 23, 2015 at 9:19 pm
Indeed this was also a problem of bulged capacitors. I do have to order this because I have lots of Elco's for PC motherboards but not for TV's. Can you tell me where I can order these Elco's (1000uF 35 Volts), except that horrible Conrad company? I don't want to wait for a week or so. You may e-mail to ruimzien at upcmail dot nl.
Albert van Bemmelen
October 23, 2015 at 9:28 pm
I found 10 pieces of them radial for only $1.99 US dollar on Ebay.
I hope that you are able to pay with Paypal. Look here:
http://www.ebay.com/itm/35v1000uf-Electrolytic-Capacitor-10x20mm-10pcs-/261838519021?hash=item3cf6cabaed:m:mNwsEV3kNPHWHmBfxz9ET0A
I will add your email address to my contact persons list, if that's okay.
Good luck with fixing your Monitor!
ronald zablon
July 30, 2016 at 9:11 pm
good job. i like it.
Biswajit Roy
September 10, 2016 at 11:39 am
good
mahmoud_tajpour
September 16, 2016 at 3:35 am
HI dear Albert before this I did not pay attention to the RDS of MOSFET transistor that it is very important but now I learned from your article may give more explanation about RDS and ho it works?
thanks a lot
Albert van Bemmelen
September 20, 2016 at 7:06 pm
Glad it was an helpful repair Mahmoud_tajpour. Indeed the RDS(on) value can always be found in the Datasheets and tells us how much DC Power we lose in the Mosfet Resistance when source and Drain are closed. And we also can calculate and measure what voltage we will lose over the RDS(on) when it conducts the DC Current.
bert taban
February 23, 2017 at 8:13 am
it s very informative...good job sir. two thumbs up!
Jasley
October 21, 2018 at 9:03 pm
Hi Jestine. Im from Malaysia. Your blog is so helpful that help me repair faulty monitor last week. Now I like to repair LED tv. Can I know where can I get good main board for my tv?. It was LG Led tv model 42LE7500
Jestine Yong
October 22, 2018 at 9:17 am
Hi Jasley,
It's hard to find mainboard for LED TV. You can try the local authorized service center and also from online website like aliexpress and ebay.
Jestine
Jagan
July 24, 2021 at 12:16 pm
In this inverter board red led blinks...this means any fault in board or bulbs ? Because I had facing problem in this LG42SL90QR
Stand-by mode...