Help fixing LG TV eax40097901/14 power supply board - Page 1
Forum section: electronics / repair

author: Liteace
author_rank: Regular Contributor
timestamp: April 02, 2020, 10:38:25 pm
content: Hi ALL, need help please, I have here my LG32lg3000 tv it has not worked for a while, plug into mains, hit switch, red led with come on. press remote, nothing happens, the red led flashes and switches off.I thought now is the time to try fix it, I have checked the normal capacitors, changed a couple of them, the rest look ok, Ive got constant 5.2 volt from the 2 pins, when I short those with power on pin Im getting nothing from tuner or backlight supply.I have checked T501, I think that is OK as I have 12v on drain of mosfet Q205 Ive also checked power to PC503, with the 5.2v and power-on pins open, ive got 5.2 volts on pins 1 and 2 of PC503, when I join 5.2v and power-on pins, pins 1 and 2 go low on PC503 as Q202 is pulling it to ground competing the circuit, with my limited knowledge I think that part of it is working ok, what I cant understand is what is happening on the switched side of PC503Could one of you guys have a look and tell guide me through the next test step pleaseThanks

author: PKTKS
author_rank: Super Contributor
timestamp: April 03, 2020, 11:54:32 am
content: I've seen lately some posts like yours...Asking help with somewhat limited skills.Nothing wrong with that.. the problem is...These PSUs require a level of skill above the medium.You need to check several "test points" as youalready pulled the board out just a matter oftesting ( WITHOUT ANY POWER !!! ) MOSFETsand main switching parts.With power and VERY MUCH NEEDED EXPERIENCEyou need to check the most important board outputs.With the schematic the tests will put a road mapof following procedures.Much like just a faulty MOSFET or other simplecomponent prevents the PSU starting.These PSUs are not entry level stuffAND THEY REQUIRE CAREFULLY HANDLINGVoltages are high enough to cause damage.Paul

author: Liteace
author_rank: Regular Contributor
timestamp: April 03, 2020, 12:01:07 pm
content: Thanks for the reply, Ive learnt the hard way many years ago what main voltage and charged cap can do so Im extra carefulI know what 80% of the components are on the PCB and am OK'ish with understand their operation but where there is PNP's and NPN's mixed inline I get a little confused thats why Ive asked for help on the next step of the test, I do have this and been trying to study but Im stuckThanks

author: PKTKS
author_rank: Super Contributor
timestamp: April 03, 2020, 12:07:48 pm
content: Quote from: Liteace on April 03, 2020, 12:01:07 pmWith experience you learn how to test things even wo schema.You don't even need to understand the circuit to test it.But YOU DO NEED A PROPER VERY GOOD UNDERSTANDINGOF HOW TO CHECK EACH PIECE.You must check the most critical parts like MOSFETs and BJTsas well as diodes and bridges- all OFFLINE with no powerand using proper instruments...All this requires a decent level of practice and time to master.Very first step.Second step is to test the expected OUTPUT valuesTable that and do a proper evaluationPaul

author: Gyro
author_rank: Super Contributor
timestamp: April 03, 2020, 12:11:22 pm
content: Do you have access to the service manual or only the schematic?The first step would be to see if the LED behaviour appears in the fault-finding tree. It could be an indication of backlight failure for example.

author: Liteace
author_rank: Regular Contributor
timestamp: April 03, 2020, 12:24:26 pm
content: I dont think its back light failure, it look to me as T101 transformer is supplying the 24v and the 16v, Im not getting any output from any of those pins, nore the 12vI dont have a service manual for this only the schematic I posted, when main on/button is pressed on the TV the red led comes on and stays on indicating stand by mode, when an number key on the remote is pressed (this used to switch on to that channel) now the led flashes 3 time and extinguishesI have checked with the power board plugged into the tuner board and when a number button is pressed pin on/off gets pulled to ground so I think the tuner board and the remote part of the TV is working as it shouldThanks

author: PKTKS
author_rank: Super Contributor
timestamp: April 03, 2020, 01:57:42 pm
content: With the board out - stand alone only.Check all critical components prior to go deeper.You need dead sure they are okPaul

author: fzabkar
author_rank: Super Contributor
timestamp: April 04, 2020, 02:31:14 am
content: The always-ON 5.2V standby supply is working. The 12V supply is switched on by the 24V supply. (edited)When you short Power-ON to 5.2V, this should cause Vcc to appear at C510/Q504-E.You should now see the PFC supply switch on. C600 (1uF, 400V) should measure 340VDC, and C608/C609 should be significantly higher, say 400VDC. This is the APFC voltage.U102 should then switch on and produce 24VDC and 16.5VDC on the secondary side.Take care, otherwise a different kind of corona will kill you.

author: Liteace
author_rank: Regular Contributor
timestamp: April 05, 2020, 08:18:16 pm
content: Quote from: fzabkar on April 04, 2020, 02:31:14 amThank you very much for the detailed info, much appreciated.Now Ive manged to go a step furtherI have checked output of Q504 by measuring across C609, I am getting 12.2 volts when power-on and 5.2V are bridgedC600 = 336VDCC608/C609 = 394VDCI checked inputs of U102 pin 12 and Q114 (C) with power-on and 5.2V bridged and have a healthy 12v thereI have also checked pin 14 (out) of U102, there is no voltage on that pin, anyone know if there is supposed to, and what is should be.I have also checked Q205, I do have 12v on the center pin D, am I correct in thinking this get switched on by being grounded from the 24v output of T101 and as U102 is not switching on T101 thats why Im missing the 12v on pins 5&6 of P201 connectorThanks for all your help so far

author: fzabkar
author_rank: Super Contributor
timestamp: April 05, 2020, 08:51:15 pm
content: Yes, the gate drive for Q205 comes from the 24V rail. If the 24V rail is missing, then the 12V rail also shuts down. (I have edited the error in my previous post. Sorry.)Check D252/D251 and D201 for shorts.Assuming there are no shorts on the 24V and 16.5V rails, then the problem must be in the area of U102 and Q110/Q111.The OUT pin of U102 would be pulsed at high frequency (PWM).

author: Liteace
author_rank: Regular Contributor
timestamp: April 06, 2020, 12:51:52 am
content: Im think Im slowly getting there, I have checked, without removing the D252/D251 and D201 and I think they are OK, I will remove and double check, reason being I see there's plenty of caps and resistors between outputs and ground on theseI checked Q110, on center pin D Ive 396 volts, nothing on Q111 as it looks like Q110 has to switched on 1stI also see that pin 15 on U105 switches/controls Q110 and pin 11 of U105 switches/controls Q111 what output should I be seeing from these, would it be correct to called them control pinsThanks Again

author: fzabkar
author_rank: Super Contributor
timestamp: April 06, 2020, 02:47:39 am
content: You won't be able to see much with a multimeter. You would need an isolated oscilloscope for that.This is the datasheet for U102:https://www.onsemi.com/pub/Collateral/NCP1396D.PDFIIUC, T101 and C114 form a resonant circuit. On the positive half cycle, Q110 (high side MOSFET) switches on and Q111 (low side MOSFET) switches off. Current then flows into pin 6 of T101 and out of pin 2, thereby charging capacitor C114. On the negative half cycle, Q110 switches off and Q111 switches on. C114 then discharges into pin 2 of T101 and out of pin 6.If U102 senses a fault via its slow fault (#9) or fast fault (#8) pins, it shuts itself down and stops outputting pulses to the MOSFETs. That's why these kinds of PSUs are not straightforward to troubleshoot. That is, you don't always see the problem that causes the PWM controller to shut down because the problem goes away when the controller is off.

author: Liteace
author_rank: Regular Contributor
timestamp: April 06, 2020, 02:35:21 pm
content: I think I have found the problem, I checked these with an old 20 volt laptop charger/supply.The mosfet on the right is Q110 and I think is working properly as it switches on and off OK when I apply a little through skin voltage to the gateOn the left is the Q111, that is behaving completely differently, it does not want to switch offI will replace both of them as they are both out and see what happens, a MASSIVE thank you for all the help and info

author: fzabkar
author_rank: Super Contributor
timestamp: April 06, 2020, 07:57:56 pm
content: Beware of fake MOSFETs. Good luck.

author: Liteace
author_rank: Regular Contributor
timestamp: April 16, 2020, 12:30:01 pm
content: I managed to get to the component shop, they gave me 2 of these, the old ones are more plastic, these new ones wont be a problem where they bolt to the heatsink, causing shorting or anything?here's the heatsink, the back of the board and also the heatsink its connected to the fixing screw grounds in the cornersThanks

author: fzabkar
author_rank: Super Contributor
timestamp: April 16, 2020, 10:26:05 pm
content: Normally you would need a thermal pad and insulated mounting hardware, unless the heatsink is electrically isolated from the PCB.

author: Liteace
author_rank: Regular Contributor
timestamp: April 17, 2020, 11:51:39 pm
content: Thanks, I got the mounting hardware to insulate the new mosfets form the heatsing, tested with a meter to check they was insulated and all good, switched on TV, pressed remote and it done exactly the same as before, 3 flashes from the LED then nothing, so I'll try and check ins/outs of that NCP1396A the best I canThanks again