LG 42LG50FD-AD Intermittent NoGo

[con244]

Hi I have a LCD tv that works sometimes and doesn't other times. I have attached some pics. When I power the tv up the red stand by light comes on, when I switch the tv on the light flashes 3 times and then the blue on light comes on. Sometimes the screen lights up but more often it just stays off. I have checked for standby 5V and have it at the connector that goes to the large board from the PS at all times even when the tv does not start. No 24V or 12V to be found. When the set did actually power up I checked for all the voltages and all was as it should be. I suspect a bad joint some where and have already done a bit of re soldering on all boards to no effect. Does anyone have any ideas what could be the problem or where I could start looking? Could dodgy caps be the problem? Do they cause intermittent problems like this? I appreciate any help you may be able to give me. Hopefully the pics will be of some use.
Thanks
Con

[con244]

Re: LG 42LG50FD-AD Intermittent NoGo

I let the set sit for a couple of hours, not powered. On powering up the set came alive. I left it on for about 5 minutes then switched it off at the on/off switch. It would not switch back on. From memory, when I used to repair audio gear, silicon generally fails as the temperature drops and caps fail with heat. Is that right? If so perhaps it is caps after all. Any ideas? Should I just shotgun the seven caps in the secondary side?
Con

[budm]

Re: LG 42LG50FD-AD Intermittent NoGo

Look for PS_ON signal pin on the power supply board which is generated by the main board, it should be >3vdc which will turn on the PFC, 12V/24V power supply. See if the signal is there or not when the TV is not working.

[con244]

Re: LG 42LG50FD-AD Intermittent NoGo

Thanks for your reply budm, after leaving it for a short while I measured 4V on the pin that has POWER ON written next to it when pushing the on button. The tv came on, I also measured 12V and 5V where they should be on that connector. I am using the manual that has been posted here. I have reposted it. The connector designation is shown as P1100 on the manual but I think it is P200 on my board. After a while I turned the set off and remeasured the voltages. There was no voltage at the POWER ON pin or the 12V pin. The 5V standby was still there. There is a blue LED on the T board that comes on when the set works and then slowly fades after about 30 or 40 secs. Don't know if that is significant?
Thanks again
Con

[budm]

Re: LG 42LG50FD-AD Intermittent NoGo

That POWER ON (4v) will only be there when the TV is powered up, that is why it is called POWER ON, it is for turning the main power supply (12v/24v) on, in standby (or TV off) the main power supply will be off to save power. The standby power supply is always on and running the minute the TV is plugged into the outlet.
So just monitor that POWER ON pin when the TV stops working again.
Please read the link under my signature to get basic understanding how typical LCD TV works.

[con244]

Re: LG 42LG50FD-AD Intermittent NoGo

I think I wasn't clear with my last post. When the button was pressed the 4v came up and when the tv was turned off the 4v disappeared. Whenever I tried to turn the tv on after that there was no 4v. After leaving it for a while the 4v would come back when the button was pushed. I did read your post, perhaps I'll read it again, I do appreciate the help you are giving me,
Thanks
Con

[retiredcaps]

Re: LG 42LG50FD-AD Intermittent NoGo

1) The main board controls the POWER ON voltage.

2) The picture of the main board is too blurry and fuzzy to see anything.

3) On the main board are a number of voltage regulators. See the our links/FAQ to see how to test. Report your findings in the exact format as shown in the examples.

4) When the TV is working, trace the 4V voltage on the main board to see where it originates from. A component in that path could be bad or failing. Another possibility is a bad solder joint.

[con244]

Re: LG 42LG50FD-AD Intermittent NoGo

Hi I left the set unplugged over night. I plugged it in and checked for 5V standby and it was there. Then I checked for the 4V start up (actually 4.45V) voltage after pushing the on button and it was there as well. The tv started with no issue. I left it running for about 20 minutes and turned it off. Tried to turn it back on after 1 minute- no go. The stand by and startup voltages were both there 5V and 4.45V respectively. I tried to find any bad joints and have reworked several suspect ones. I feel like this is a going bad component that is ok when it is cold but starts acting up when it has warmed up. I have tried to trace the 4V start up voltage but as it is there all the time I don't see any reason to doubt that circuit. Can I supply any pics that may be of use?
Thanks Again
Con

[con244]

Re: LG 42LG50FD-AD Intermittent NoGo

Sorry about that blurry pic hopefully either of these will be better! Do you need the other side as well?

[con244]

Re: LG 42LG50FD-AD Intermittent NoGo

Will check the voltage regulators. I will check them when the set is working and when it is not to get comparative results.

[con244]

Re: LG 42LG50FD-AD Intermittent NoGo

OK here we go with the regulators from the main board:

IC903
AZ1085S-ADJEI
827J84
SET WORKING
PIN 1 0.02
PIN 2 1.27
PIN 3 3.28

SET NOT WORKING
PIN 1 0.02
PIN 2 1.27
PIN 3 3.28

IC904
AZ1085S-3.3EI
802K06
SET WORKING
PIN 1 0.0
PIN 2 3.30
PIN 3 5.07

SET NOT WORKING
PIN 1 0.0
PIN 2 3.30
PIN 3 5.07

IC905
AZ1117D-3.3EI
830N99

SET WORKING
PIN 1 0.0
PIN 2 3.30
PIN 3 5.05

SET NOT WORKING
PIN 1 0.0
PIN 2 3.31
PIN 3 5.03

IC907
AZ1117D-1.8EI
824J30

SET WORKING
PIN 1 0.0
PIN 2 1.80
PIN 3 3.31

SET NOT WORKING
PIN 1 0.0
PIN 2 1.80
PIN 3 3.31

1C908
AZ1117D-1.8EI
824J30

SET WORKING
PIN 1 0.0
PIN 2 1.81
PIN 3 3.31

SET NOT WORKING
PIN 1 0.0
PIN 2 1.80
PIN 3 3.31


IC909
AZ1117D-3.3EI
830N99

SET WORKING
PIN 1 0.0
PIN 2 3.30
PIN 3 5.05

SET NOT WORKING
PIN 1 0.0
PIN 2 3.31
PIN 3 5.05

IC910
AZ1117D-2.5EI
826N80

SET WORKING
PIN 1 0.0
PIN 2 2.51
PIN 3 5.05

SET NOT WORKING
PIN 1 0.0
PIN 2 2.51
PIN 3 5.03

IC101
6E
833

SET WORKING
PIN 1 3.28
PIN 2 0.0
PIN 3 3.29

SET NOT WORKING
PIN 1 3.28
PIN 2 0.0
PIN 3 3.29


STANDBY VOLTAGE 5.45V
START VOLTAGE 4.45V

I am not sure that IC101 is a voltage regulator. I could not find any reference anywhere to this one. It has what looks like a degree sign in front of the first line.
To my untrained eye there only seems to be a very small difference between to working and not working voltages. What do you guys think?

Con

[cadiman]

Re: LG 42LG50FD-AD Intermittent NoGo

This power supply has three 35v 3300 caps.

There is one 35v 1000uf cap.

There are two 10v 3300 caps.

I would start with those first. (10v 3300)

[Glitcher]

Re: LG 42LG50FD-AD Intermittent NoGo

Usually don't bad caps only work once they are heated? (assuming they are not all the way gone yet) And once they are left turned off and cold again they won't work again until it is warmed up or heated since the ESR temporarily goes back down with heat.

[con244]

Re: LG 42LG50FD-AD Intermittent NoGo

Thanks Glitcher and cadiman. Nothing to lose as far as I am concerned. It's Sunday here, I will pick up some 3300uF10V caps tomorrow and see how they go.
Con

Cadiman
I could only find 2 x 2200uF 10V caps would they be the ones?

[budm]

Re: LG 42LG50FD-AD Intermittent NoGo

"I left it running for about 20 minutes and turned it off. Tried to turn it back on after 1 minute- no go. The stand by and startup voltages were both there 5V and 4.45V respectively" If that is the case, the main board is still functioning since the PS_ON is still present but the TV still will not come on (the power supply will not start up with PS_ON applied). So we will need to look at the power supply circuits next.
You can try heating the power supply board with hair dryer for a few minutes and see if the TV will start up.

[con244]

Re: LG 42LG50FD-AD Intermittent NoGo

Budm I think it's the opposite problem it works when it's cold and doesn't when it's warmed up. I have found that if I turn it off when it's warmed up it won't start again until I leave it unpowered for at least 45mins. Maybe I should invest in some freezer spray?
Con

[koka0011]

Re: LG 42LG50FD-AD Intermittent NoGo

check Hi-V transformers in Inverter board.Replace all tha caps in secondary side of the power supply.

[budm]

Re: LG 42LG50FD-AD Intermittent NoGo

You can use canned air duster upside down for cold spray.
I am still a little confused since on post #4 "After a while I turned the set off and remeasured the voltages. There was no voltage at the POWER ON pin or the 12V pin. The 5V standby was still there" You do not have the voltage on the POWER ON pin.
But right you do have it but the TV will still not turn on?

[con244]

Re: LG 42LG50FD-AD Intermittent NoGo

You can use canned air duster upside down for cold spray.
I am still a little confused since on post #4 "After a while I turned the set off and remeasured the voltages. There was no voltage at the POWER ON pin or the 12V pin. The 5V standby was still there" You do not have the voltage on the POWER ON pin.
But right you do have it but the TV will still not turn on?

Sorry I was a little confused myself. It's been a while since I've had my nose in any circuitry. The way it stands now is, there is standby power at powerup and startup power at turn on. I feel like the problem is on the PS board. It is caused by heat because it starts ok when the set is cold. Just need to narrow it down some more. I will get some freezer spray tomorrow. But I will keep fiddling with it til then.
Thanks
Con

[con244]

Re: LG 42LG50FD-AD Intermittent NoGo

Tried cadimans suggestion. Replaced 2 x 3300uf10 caps. There was an improvement so shotgunned the rest of the nearbycaps on the board. The 3 x 2200uf25 were soldered into the board via some metal sleeves. I seem to remember these things being a problem on some of the audio gear I used to fix so soldered the legs to the pads as well as the sleeves. Seems to be working ok now. Will soak test for longer time and see how it pans out.
Con