LG 50PN4500-UA - has problems turning on

[ngth82]

Make: LG
Model: 50PN4500-UA.BUSLLJR

Symptoms:
Original owner says the TV doesn't always turn on. When you press the power button, the power LED blinks a few times. The panel glows, sometimes it turns off within 1 second. Pressing the power button two-three additional times seems to get it to stay on. Other times, the TV will turn on on the first try it shows the image for the selected input.

I tried searching this forum and online for other 50PN4500 TVs with similar symptoms, but i haven't been able to find much info that matches. A couple of search results are based from Mexico, and in Spanish.

All of the boards visually look fine. No bulged caps, no burnt components.

Debug:

1. I measured the Va and Vs voltages from the panel - they reach 55.38V and 205.4V, respectively, when the TV is ON.
   
2. The VS_ON signal ramps to 3.28V when you press the power button. This stays steady even if the panel appears to "turn off".
   
3. The T-Con has a 2-pin header location labeled "AUTO" with silk screen. I installed a 2-pin 2mm header here and installed a jumper. I removed the LVDS cable to the main board then plugged the set in. When the unit turns on, i see the standard LG test pattern on the screen. No bad pixels, no dead areas, no lines. The display looks fine.
   
4. For the ribbon / power cable between the power board and main board:
   - 17V is steady
   - 5.2V is steady (measured as 5.157V)
   
5. No visual damage to caps on the main power board, or the Y-sus boards, but i would suspect this is a potential reason why the unit has trouble turning on. I haven't had a chance to remove the caps and test them with my ESR meter yet. Any thoughts?
   
6. Looking online, there are a couple of videos on Youtube with people repairing the Y-SUS board, but the TV seems to "work" for the most part -- would bad IGBTs cause this problem?

https://www.youtube.com/results?search_query=50pn4500


Panel Sticker voltages:
Va = 55V - measured value is 55.38V
Vs = 205V - measured value is 205.4V

Please see attached pictures.

[ajshoe]

Re: LG 50PN4500-UA - has problems turning on

ii would start by freezing psb - see if u can get to not turn on repeatedly. no luck - try hair dryer on psb. most likely a temperature issue, cold solder joint, or bad xformer.

[tvtimmy]

Re: LG 50PN4500-UA - has problems turning on

Touch up as many solder joints under the power supply as you can. Dont bother with ic chips just yet.

[ngth82]

Re: LG 50PN4500-UA - has problems turning on

Good evening all -

I have not had a solid block of time to work on this TV over the past week.

I did pull off the (primary) power board from the middle-right of the unit and look at the backside solder joints under a microscope. They all look fine as far as I can tell. I did touch up the large connections to the transformers and IGBTs just in case though.

The same goes for the board at the far right - connections look solid.

I have not had a chance to look at the Y-sus, left, yet.

I'll find a new compressed air can this week and try to freeze the board. The TV definitely has an easier time turning on (e.g. first try most of the time) when it's already warm.

Cheers!
-Thomas

[Hamie]

Re: LG 50PN4500-UA - has problems turning on

So if you get test patterns and the TV doesn't turn off then you have a problem with your mainboard.

[ngth82]

Re: LG 50PN4500-UA - has problems turning on

So if you get test patterns and the TV doesn't turn off then you have a problem with your mainboard.

Hey there, I’m not entirely sure about that. I can definitely get the test patterns, but every now and then the unit still has problems turning on to generate the patterns. The LVDS cable is disconnectednand The PSon signal from the main board appears to be solid even when the panel doesn’t stay on.

I’ll hopefully have more time to look at it Monday.

[ngth82]

Re: LG 50PN4500-UA - has problems turning on

Lets close this thread.

I wasn't able to solve it, but i was able to sell the TV as-is earlier this afternoon for $80. Works for me.