samsung un40h5203afxzc doesnt power up flashing power led

[ronl]

Hi Members I have a samsung un40h5203afxzc that.doesnt power up.power led flashes.I checked the voltages on the power board for the main board and all checks out. I'm hopping its not the backlights. It uses power board number bn44-00769c. I thought I would test back lights but pin markings on connector cnl802 is confusing. Pin1,2 = d+.pin3=s2-,pin4= s2+,pin5=n/c,pin6=s2-,pin7=s3-,pin8,9= d-. Power for backlights on power board are marked 151.8 v at 425ma.not sure if I should hook up external 151.8 v power to pin1 for + and pin9 for - to test backlight current draw? I saw a YouTube where tech disconnected main board cable and backlights came on.mine didnt.any thoughts? Or service info? Thanks,RonL

[ronl]

Sorry,pictures didn't upload.ill try again.

[ronl]

Hi Members.i tryed applying external power to led backlights and got no current draw.checked power board connector and found pins 3,4 and 6,7 have continuity between them.I installed jumpers on the cable going to the backlights and it lit up.saw current draw and brightness of backlights was changing even though applied voltage stayed constant. Next step is to disassemble and replace faulty led diodes. I do have some extra led strips I can use for parts.since this tv is a freebe,ill go further than I would with a clients tv.RonL

[ronl]

Hi Members.I carefully disassembled the tv to get at the backlight led strips.I applied external power and saw 9 leds not lighting up.I saw a YouTube where the tech used a simple paint heat and strip gun to remove the defective leds and good leds from a salvaged led strip.worked like a charm! I still have to scrape a bit of insulative paint on the led strip to enlarge the soldering pads so soldering in good leds will be easier.get to that after the weekend.I should have a working freebe 40" smart tv after this.my 1st time working with led backlights( most of my experience was with crt tech.).Having fun! RonL

[ronl]

Hi Members.I replaced all 9 defective leds with salvaged backlight leds from a junk set.I found it easier to use a heat gun to install leds instead of a soldering iron.all leds work now with my external power supply,but the strange thing is that the led array still draws too much current.at 151.8 volts,current draw should be 425ma.its around 600 ma. Of course,the power supply does not like that and pulses on and off.Is there big differences in voltage drop and current draw between different backlight leds?I assumed they were all approx 3 volt across them when conducting and drawing simular current? The original led backlights were from a 2016 tv and the replacements come from a 2020 tv.both Samsung and both have very similar looks and construction.any ideas? Thanks RonL

[kf4rca]

Excessive current is probably blew out the LEDs in the first place. I would put a current limiting resistor in series in the circuit until you figure it out.
I replaced some LEDs in a set once and as soon as I got it re-assembled, it blew another LED. Arggghhh.
There are TWO ways LEDs go bad. Open which gives no current. And short which gives higher current. Often, one will short and create higher current which, then, blows out another, causing an open LED.
Good luck!

[ronl]

Hi Kf4ca.You were right.I disassembled the set (much quiker the 2nd time)and applied my external power supply .1 led was not lighting.back to the heat gun lol.luckily I have more salvaged led backlights.I'm gonna order a cheap led backlight tester for future repairs. Thanks for the advice. RonL

[kf4rca]

Thanks for getting back with us. Often I'll find a set and it appears brain dead. But I discovered you could just plug a wire into the F connector and you should get off air audio if its not brain dead.
I wish the manufacturers would segment the LED circuits so if one goes out, they all don't go out. Remember the series Christmas light strings of old. You don't see those anymore. But I think OLED will replace LED.
A possible great source of LEDs are LED light bulbs. They appear to be the same color. They're usually discarded due to other circuit problems.

[ronl]

Thanks for the tip on sourcing led backlights kf4rca.i got busy with other tasks and its too hot in my area to use a heat gun to replace defective leds right now.ill get back to this project in a week or so. RonL