LG 65UH652T won't turn on

[spurf]

Re: LG 65UH652T won't turn on

This all is a little bit convoluted. From what I read your problem is you basically have no 13v. correct? What is the voltage DC on the main capacitor in standby and when you press the power button? And I really need to know what the part numbers are on those 2 controllers on the back.

I do have voltage on both the LED and 13V lines but they aren't stable. They start off below spec but rise to approximately the correct values if I use the 3V jig but they aren't stable.

The DC voltage on the large main capacitor fluctuates from 391.1 - 391.6 V DC and doesn't change when I press the power button.

What is the safest way to remove the lacquer? Scrape or solvent?

[spurf]

Re: LG 65UH652T won't turn on

I really need to know what the part numbers are on those 2 controllers on the back.

Here are the two controllers. I can't make out much on the square one unfortunately and this is the best photo I managed to get.

[spurf]

Re: LG 65UH652T won't turn on

I'm a bit stumped on what could be causing the the AC fluctuations on the lines that should be DC.

Does anyone have any idea which component(s) that would be malfunctioning to cause this?

[spurf]

Re: LG 65UH652T won't turn on

I picked up an LCR-T4 meter off eBay and have tested all the capacitors out of circuit and they all have capacities that correspond to their rating and have low ESRs except for the large capacitor. It's a 460V 120uF and has an ESR of 1 ohm at room temp. If I warm it up to 80 degrees celcius it drops to 0.6 ohms. I have tried warming it in circuit but the TV still doesn't power up. I'm unconvinced that this is the cause as if the capacitor was slowly degrading I don't see how it would work perfectly and then completely stop working.

I'm close to giving up...any suggestions?

[nomoresonys]

Re: LG 65UH652T won't turn on

well 2 of our best members, in the previous posts think it's probably backlight issue, so I would very carefully tear it down and check them out, very easy to break the panel, lots of how to videos on youtube.

[dskall]

Re: LG 65UH652T won't turn on

Without controller # I can't really help you. I suspect controller is not starting up. Need to check vcc to controller. I would also check diodes on heatsink. Still seems like a short that is shutting controller down. R_J would be the one who could help.

[nomoresonys]

Re: LG 65UH652T won't turn on

check R100, Q601, 602, probably need to carefully remove that white gunk to check them.

[spurf]

Re: LG 65UH652T won't turn on

well 2 of our best members, in the previous posts think it's probably backlight issue, so I would very carefully tear it down and check them out, very easy to break the panel, lots of how to videos on youtube.

I have tried powering the TV with the backlight disconnected and it exhibits the same behaviour. Does this exclude the backlights or do LGs refuse to boot up if the backlight is not connected?

[nomoresonys]

Re: LG 65UH652T won't turn on

i think the thing to do is powerup the powerboard as stand alone, with everything unplugged, then see if the voltages are as expected and stable.

[nomoresonys]

Re: LG 65UH652T won't turn on

i wouldn't be surprised if it's a bad cap or 2, don't know if I even remember seeing that brand before.

[nomoresonys]

Re: LG 65UH652T won't turn on

a couple things that can cause unstable voltages is bad/weak caps and bad voltage regulating diode/s.

[spurf]

Re: LG 65UH652T won't turn on

i think the thing to do is powerup the powerboard as stand alone, with everything unplugged, then see if the voltages are as expected and stable.

I've tried this (see my earlier post) and the voltages are not stable when the board is powered up with the jig. I'll test Q601 and 602 like you suggested. What is the best way to remove the white gunk? R100 tests fine

[nomoresonys]

Re: LG 65UH652T won't turn on

I guess just use an exacto knife and take your time.

[spurf]

Re: LG 65UH652T won't turn on

Sadly Q601 and 602 test fine ☹

[nomoresonys]

Re: LG 65UH652T won't turn on

well now that you have the gunk off, what voltages are you getting on those middle pins?

[spurf]

Re: LG 65UH652T won't turn on

well now that you have the gunk off, what voltages are you getting on those middle pins?

I get a fluctuating voltage between 37 to 38 AC and 0.6 and 1.8V DC

Testing both of them out-of-circuit the LCR-T4 identifies both of them as an N-E-MOS with C=1.97nF and Vt=3.3V

[nomoresonys]

Re: LG 65UH652T won't turn on

ok, need something consistent, can you start up the powerboard to where you have the 391 volts on the big caps, careful that's deadly voltage, then you can test with your meter to see where you have voltage on the hot side, put neg probe on neg. - leg of the bridge rectifier which is ground for measuring in the hot/primary side, has a - in front of it, it's the one on the end without the gunk on it, then measure where the voltage should be, so from the - of bridge to the + of big cap is probably 391 or so, thinking the middle pin of a transistor pin should be the same, might be able to trace the voltage from big cap on out but might be hard.

[nomoresonys]

Re: LG 65UH652T won't turn on

thinking you should find about 391 on one of the transformer legs as well.

[dskall]

Re: LG 65UH652T won't turn on

Q101 and Q102 are fets for 13v. D109 maybe snubber diode. What voltage do you get across it? C107 looks like start up cap. What voltage do you get across it?

[spurf]

Re: LG 65UH652T won't turn on

Well I was a bit careless and managed to create a short on Q601 which blew it and the main fuse. I decided to give up but found a replacement board with a later revision on eBay and will see how I go. Thanks for all the help and will update once I install it