LG TV no picture - Page 1
Forum section: electronics / repair

author: eli120
author_rank: Newbie
timestamp: December 29, 2021, 12:22:05 pm
content: Hi,My uncle gave me his TV (model uj620y 55") to repiar it.When I tryied to power the TV from the remote the logo shows for a brief second and disapper, then the standbay light blinks 3 times and that's all.I looked at the board and searched for an obvious problem(shorts, leaking caps, fuses, ect) but didn't find any.I measuered the voltages at the right connector and it seems fine(look at the picture below)Next I tried to measure the voltage to the LEDs output but it show 0V, then chacked the diodes before and they are fine.And also the voltage output of the transformer(65V AC but it show 0v) and the input(400V AC but it shows 0V)The Inductore and the transformer before I couldn't measure the voltage. but I know the bridge rectifer, the diodes around and the 2 big caps are fine.Also, I tried with LED driver bulb(dissambled from old bulb) and connect to the LEDs connector(via resistor and switch) and on 1 connector it shows 60V stady and all the lower strips were on.in the second stirp it didn't show any change in voltage(300V DC, no current) and no strips were on.So, based on the above, the problem is in the PS board or in the LEDs(or both)?Because I don't know if the PS needs load(all the strips) to working correctly, or maybe it protecting himself from defective load(maybe a shorted LED strip)I hope I was clear enough.Does anyone has suggestions?

author: m3vuv
author_rank: Super Contributor
timestamp: December 29, 2021, 12:40:14 pm
content: have you checked the esr on all of the psu/led driver board caps?,i just repaired an old hp 54503a scope that had a bad psu,all the caps looked ok but had pissed there pants,wasnt visible tho untill the caps were removed from the pcb tho,maybe worth pulling a few caps and checking for electrolye leakage as a lot of times its not possible to spot without actualy removing them from the board!.

author: eli120
author_rank: Newbie
timestamp: December 29, 2021, 07:06:11 pm
content: Quote from: m3vuv on December 29, 2021, 12:40:14 pmI checked a few of them(results in the picture)All the capacitnce seems fine, the big 2 caps(450v, 68uF) are reading 122uF, 0.22 ohm in parallel and I don't know what value it's should be(any ESR table ends before thos ratings)And I found 2 FET(Q12,Q13 were short- tf12t50p) but I thing it's because I made a mistake yesterday(I think I short with the probes by mistake), also the fuse was blown.Anyway, I don't recall those 2 FET were shorted before yesterday.(I will replace them in coming days- waiting for delivery)But, if I will replace those 2 transistors, should I get voltage in the LED output connector(when the LEDs are disconnected)? or just under load(all the LED's strips are connected and in good condition)Because I'm a little worry about opening the LCD panel to check each individual strip without the proper tools(suction cups mainly)thanks

author: MathWizard
author_rank: Super Contributor
timestamp: December 29, 2021, 09:20:52 pm
content: I worked on an LED TV with multiple voltage rails. I started from the plug, (with it unplugged) and worked my way in to find the low voltage Auxiliary voltage rail on the primary hot side, and then the Standby 5V rail for the main stuff on isolated sides.Then I found the SMPS for the high current LED drive. It was up to 250V with no load. I think it was meant to run around 90V at whatever current. I never load tested it, but the LED would not light up. And I had found the logic control to turn the light on/off and it still did nothing.I built a 30W resistor bank, but never tested it, but now I have better alligator clips, so I must do that.I'm saying find the Aux and Stby control voltages, then if they work branch out to other voltage sections, see where that goes.IDK how clean the voltages are supposed to be on these things, do you have an oscilloscope ?

author: eli120
author_rank: Newbie
timestamp: December 30, 2021, 12:11:06 pm
content: Quote from: MathWizard on December 29, 2021, 09:20:52 pmI'll check it in the coming days(right now I can't)And as far as I noticed the 3.3v(stand bay voltage) goes to the opto coupllers.Anyway, I'll wait a few days until I get the replacment FETs and check it out.Also, I don't remember I got output voltage to the LEDs(should be 60v-72v I think)And I don't have an oscilloscope, it's just too expensive for me right now to buy one(student)and thanksIf anyone has other suggestions to check please.

author: viperrepair
author_rank: Newbie
timestamp: February 10, 2022, 04:46:57 am
content: You have a burnt out led. Stops the whole TV from working.They are in series. Like Christmas lights.