Mismatching Current In New Edge LED Necessitated Bypassing In SONY KD-43X7000E
This TV was picked up by me on the way back from hospital as the customer was very regular and close to me. The complaint reported was restarting. I checked this in their house before taking the TV out and confirmed it. He carried the TV from the first floor and kept it inside my car.
I took up the job on priority as such complaints were very interesting for me and I was curious to know what went wrong with the TV and why was it restarting. The TV used an external 19.V Adaptor. I cleaned up the inside thoroughly.
My techie friends advised me to check the inverter section as backlight issues would be the prime cause for restarting in this model of TV. I checked the inverter section on the PS Board which consisted of two LED Driver ICs, the number of which was BD9423EFV. I downloaded its datasheet and studied its functions very well.
As you will notice from the above, this IC worked with (pins 19 & 20) SDA data & SCL clock coming from the µController.
The LED outs come from pins 11 to 16. I checked the VCC to the IC and found 19.5V present. The two sets of Mosfets were ok. I also checked other components for any shorts or open and did not find any. The voltages were building up to around 24V initially which should go to around 48V. The LEDs were fed with positive potential commonly and controlled by the ground provided by these two ICs.
In order to know what was happening in the backlight, which is under the panel and diffuser/reflector sheets, I dismantled the whole TV. I inserted a lamination sheet of suitable size under the panel and stuck it with paper tapes on two edges, after carefully detaching its sticky hold on all the four sides of the seats. Then kept a thermocol on top and held it gently tilting it to a 45 degree and carried it to my son’s bedroom (he was away and out of town) supported by my belly so as not to rub or touch any of the doors, and kept it on his bed! I might have looked like a tiger carrying it’s pray up a tree! (LOL) Unless we take due care of the panel while carrying, it will crack like a crispy potato chip as it is very, very thin and brittle. We should avoid applying pressure directly at any place by our hand or fingers. The larger the panel, the riskier would it be.
The whole operation looked like a mini excavation work to pluck the roots of any vegetable like yam. (LOL) I enjoyed every bit of this complexly delicate work!
Finally the two strips of the edge LEDs were removed. It consisted of 32 LEDs split by 8 LEDs in series, requiring four external connections, out of which the positive was common. Let us have a detailed closer look:
I applied power keeping these two LEDs externally connected and checked what was wrong with it. One strip was glowing very well, while the other was dim. I interchanged the strips to check whether the strips were having the problems or the driver ICs. It confirmed that the strip was defective. Let us have a look:
The inverter worked as the T-con board was connected though panel was not. This is yet another lesson that I learnt in the process.
As the replacement strips were not available in the local market, it took a couple of days more to receive it from a distant supplier. I connected the LED strips externally and checked it and allowed it to run for about ten minutes before fixing.
After undergoing the laborious and tedious job of assembling the TV back, for which I had to repeat my belly dancing carrying the panel back, I applied power and got disappointed to note that the TV was restarting again! On discussion with my techie friends in the group, almost all opinioned that the LED strips available in the market might not match the original OEM quality and we need to remove the protection pin, before which all the outputs would have to be shorted to ensure even distribution. So, I removed the PS board and first of all ‘stitched together’ the LED minus input pins on the PCB, which could be done easily because of a test point etched on the PCB. Let us have a look:
I could easily do it with the recently acquired cordless pointed tip soldering iron, picture of which is reproduced from the Amazon site:
Next step was to isolate the feedback from pin 2 (fail report) to the µController. Detecting the track was very difficult in the double sided PCB, as it goes to the other side and returns several times. So I used a thin hair like wire and inserted into the hole which was connected to pin 2 of one IC and put a red mark on the other side with a sketch pen. Then traced it’s another dip along the line and did the same thing. Let us have a look.
The pin number two of both the ICs were jointly coming to base of one NPN SMD transistor, collector of which was traced to pin 12 of the interconnecting wire to the mother board. This helped me understand a few things. The error in the form of voltage, if generated would bias the transistor which would conduct and short the pins to ground, as the emitter of it is grounded. So, the sensing pin of µController is held high and the protection is triggered when it is pulled down. This much functionality was understood by deduction without even checking alive. How absurdly simple, like how Watson tells Sherlock Holmes! (LOL)
So I took a sharp surgical knife and cut the track of 16K resistor the other end of which went to base of transistor through a resistor.
Then powered the TV and found that it got released and started working with perfect backlight. Now the question was whether any damage would take place because of the removal of the protection? Well, we have to keep our fingers crossed, as many techie friends opinioned that it would work for sufficient time as LED strips were replaced. Anyhow, I put the TV for test for several hours, before fixing the back cover and again subjected it to prolonged testing. It was found working very well!
Mission accomplished with a lot of lessons learnt, which culminated in a knowledgeable satisfaction getting added to its collection!
This article was prepared for you by Parasuraman Subramanian from India. He is 74 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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You may check on his previous article on One Side Dark Problem Solved In Samsung Back Screwless LED TV Model UA43NU7100K
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Waleed Rishmawi
December 14, 2024 at 4:55 pm
very interesting surgical repair that really got my attention because I never faced such an issue when dealing with back light replacement. I did not even know that there was a feed back decting a faulty (not according to specs) back light strips. thanks for the new information, i have leaned a lot today. have a blessed day
Albert van Bemmelen
December 14, 2024 at 5:27 pm
Your tv repair articles are a priceless contribution for all us hardly experienced tv repair engineers. But that is probably why you also use unknown items like using a thermocol ? (some kind of protective not sticking plate on top with the screen at a 45 degrees angle?).
I however still won't repeat any of the here professionally carried out adjustments you have done in this repair because most of it sadly went over my head!
Mark
December 14, 2024 at 5:30 pm
Great job Parasuraman,
It's great to have others to get advice from and bounce ideas off