LG47LW450U Not powering up correctly

[nogger]

Hi all, i normally repair PSU units (all types) on the bench & rarely get to see a complete TV but this week someone dropped an LG LED tv in that they said wouldn't start up. Not having much experience of LED TV drive circuit's i would appreciate some advice.

The PSU number is; EAY62169801 (pics attached) & i couldn't find a circuit anywhere (typical LG) . I did find a training manual for the 55LW5600 model which showed almost identical PSU (even down to the component refs). Has anyone a circuit diagram for this PSU

Ok, i found that on power up, the standby LED is on, & the 3.5v standby is ok. On startup, the LED goes to white then goes off & the 12v & 24v come on ok. The voltage to the LED Drive though is only at 35.9 volts (screen never illuminates) & this is where i am unsure. The training manual says (for the 55LW5600) that the Drive voltage should be 56 volts going to 63 volts. The 35.9 v that i am seeing seems way too low to me (it never changes even with the LED strips unplugged). Also i monitored the DRV_ON pin from the mainboard & it never ever went high to 3.5v just sat there at almost zero. I forced the DRV_ON by connecting to the 3.5v line & the Drive voltage never changed from 35.9v

To be honest i'm not even sure if there's a fault on the PSU & the main board because when i go to switch the TV off the front panel LED goes to RED but then immediately goes white & then off as if it's switched itself on again, strange. Also , at no time do i seem to get a DRV_ON signal from the mainboard.

Anyone any thoughts ? is the drive voltage way too low at 35.9v , anyone got a circuit for this PSU ?

Any assistance appreciated.

Thanks
Graham

[nogger]

Re: LG47LW450U Not powering up correctly

Anyone ?

[budm]

Re: LG47LW450U Not powering up correctly

The LED drive Voltage may have drop down due to the protection circuits kick in due to open connection to the LED backlights.
You need to put the meter to connector and see if the Voltage did jump up high (>50v) and then drop down to 35V or not.
Bad LEDs or open connections are the common failure in LED backlights TV.

[nogger]

Re: LG47LW450U Not powering up correctly

Budm. the voltage is exactly the same when the LED's are connected. It's always 35.9v regardless & never peaks above that.

[budm]

Re: LG47LW450U Not powering up correctly

Can you check DCV between chassis and the LED+ pins on the connectors P202, 203 when power switch is activated?

[nogger]

Re: LG47LW450U Not powering up correctly

Budm, thanks for replying.

I'm pretty sure i did this already & the voltage just goes to 35.9v but i will check this in the morning again.

Graham

[nogger]

Re: LG47LW450U Not powering up correctly

I checked this again & confirm that regardless of whether the LED's are connected or not the Drive voltage on switch on goes to 35.9v & never goes higher.

Graham

[nogger]

Re: LG47LW450U Not powering up correctly

Anyone any ideas ?

Graham

[nogger]

Re: LG47LW450U Not powering up correctly

I just powered up this PSU on the bench & again forced the standby & LED Drive signals high & this time the LED drive went to 63 volts for approx 4-5 seconds before dropping so it would seem the PSU is ok. When i popped it back into the TV & leaving the main board disconnected i again forced the Standby & LED Drive signal high & i got backlights on !

I guess this means the PSU is ok then & it's possibly a main board issue. I need to check the LED illumination to see if there's any uneven illumination but i guess if not then it's a mainboard issue ?

More testing & updates to follow

Graham

[nogger]

Re: LG47LW450U Not powering up correctly

I popped the PSU back into the TV & removed the link forcing standby on but left the link forcing LED_DRIVE connected. Powered up & again backlights came on. The drive voltage peaked at 63volts & dropped back to 52v & the LED's stayed lit. Next i took readings from chassis gnd to each of the metal rivets at the top of the PSU (see images) which represent the test points for the end of each diode chain. (El62 to El69 & El70 to El77) & therefore the voltage drops across each chain. I measured the voltages at each of the rivets (see attached ph0to of test results written on the page 56 of the LG LED training manual mentioned earlier). You can see that the voltages after the LED's came on are all around 1-1.3 volts with the highest being 1.42. According to the training manual the voltage drop across each diode is 6.14v so the voltages look good to me , anyone think different ?. If i had a faulty LED open circuit then i would think the voltage would be zero as they are wired in series & if shorted then a diode drop higher ?

Next i removed the link forcing the LED_DRIVE on , switched on & again no backlights, LED Drive reading 35.9 as before . I monitored the Drive_ON signal coming from the main board & it NEVER comes so i am beginning to think this is a mainboard issue . I noticed that when i apply power , the front panel LED faintly glows RED, fades away & then glows bright RED again before fading to WHITE & then BLACK at which poiint the TV is switched on. If i touch the touch btton to switch it off it goes back to RED , then WHITE then goes off again so it isn't6 actually switching off properly & keeps trying to start up.

Guys, your thoughts /opinions on the above are welcomed, anyone else think it's mainboard & from the readings above do you agree the LED's must be ok ?

Graham

[nogger]

Re: LG47LW450U Not powering up correctly

No One ?

Well i have ordered a main board anyway so fingers crossed .

Graham

[emilios87]

Re: LG47LW450U Not powering up correctly

Hello, i have the same problem, did you fix it with changing the main Board?

[ajlockhart]

Re: LG47LW450U Not powering up correctly

Graham, did you manage to repair the TV?

[nomoresonys]

Re: LG47LW450U Not powering up correctly

Sometimes a duck is a duck, if the mainboard isn't sending drive_on signal, probably need a new mainboard.

[Andrew F. Ali]

Re: LG47LW450U Not powering up correctly

I think what is happening here is what we have been seeing regularly with these LGs. The BGA chip is not making proper connections on the Main Bd. and so removing the Main Bd completely from the TV and using a Hot Air station at 350C for 10 mins at a moderate air flow of 4 on the speed control, heat the BGA chip and let it properly cool on its own. Adjust the air flow sufficiently such tht other SMD components do not blow off the board. This will fix your TV. BTW I have done this process about 4 or 5 times already and no call backs thus far, some since 2 yrs ago. Good Luck.

[H-B]

Re: LG47LW450U Not powering up correctly

Hi,
Please read the info from this guy! Super!

https://alpengeist-tvrepair.blogspot...-clicking.html