Lg 50pk750

[humzilla]

My LG 50PK750 has been working great for 5 years.... Now I'm in withdrawal...

When I turn on the tv, the screen lights up and then goes dark. Using the remote or any of the controls on the tv cause the blue LED to blink like normal, but that's about it. I can turn the tv off and on but no video. I've pulled the power board and inspected the solder connections on the three power resistors. They didn't look great but measured 11 ohms as marked on the resistors. I re-soldered them just to make sure it wasn't a bad solder. Powering the tv back up, things are unchanged.

Inspecting the capacitors on the board, they all appear to be fine with no obvious deformation.

I'm hoping someone here might have some suggestions. I've enjoyed this tv up until now am hoping to bring it back from the dead.

[humzilla]

Re: Lg 50pk750 - screen flashes and then goes dark

Does anybody have any ideas?

I have measured all of the voltages coming off of the power board that typically fails and they all appear to be fine.

Any suggestions on where to look? I'm hoping I can resurrect this wonderful plasma tv.

[sordids999]

Re: Lg 50pk750

Posting pictures of the back of your tv showing the boards with clear pictures may help you more with fixing your problem

plus have you checked VS-VA voltages ?

[humzilla]

Re: Lg 50pk750

Okay. Here are some pictures of the TV's innards....

Powering the tv back up this morning, I noticed that the display is not going completely blank. There is still some "backlight." I know a plasma doesn't have a backlight, but the screen is still glowing some. When I power down the tv, this goes away. I am still not able to see anything further displayed on the screen.

I also measured Va and Vs...

Va = 132Vac
Vs = 437Vac

I'm only measuring these with a multi-meter so I can't see if there is any funny business going on.

[sordids999]

Re: Lg 50pk750

I also measured Va and Vs...

Va = 132Vac
Vs = 437Vac

are you measuring in AC because you should be measuring in DC black lead to chassis and red read on test point because those voltages seems really high

[humzilla]

Re: Lg 50pk750

Yep.... I was measuring in AC. In DC, I get the following measurements.

Va = 60V
Vs = 200V

[Andrew F. Ali]

Re: Lg 50pk750

Though I think your PSU Bd is not the problem and it is in-fact a MAIN Bd issue I'll share a method to 'self test' the PSU Bd.
1. Build a TEST LOAD by connecting two (2) 220Vac light bulbs in series so you now have two (2) wires to connect to a circuit.
2. Disconnect all jacks on the PSU Bd leaving the AC Input jack connected.
3. On the Jack P813, connect a wire across pin 18 (AC Det) and pin 10 (GND).
4. Connect another 100Ohm resistor across pin 15 (RL_ON) and pin 13 (STDBY 5V)
5. Connect a100Ohm resistor across pin 17 (M5_ON) and pin 14 (STDBY 5V).
6. Connect 'TEST LOAD' on Jack P812 across pin 1 or 2 and pin 4, 5 or 8.
7. Plug PSU Bd in the AC.
Test Load bulbs should light up and stay ON. If they do not stay on then PSU defective.
If they stay ON, measure Vdc across chassis GND (BLACK probe on DMM) and RED probe on pin 1 or 2.
Should get Vs=195-205Vdc

[Hamie]

Re: Lg 50pk750

Run test patterns by doing the following:

Disconnect the LVDS cable that connects the main av board to the control board (P31)

Connect the 2 holes beside the 2 resistors (r51 and r52) with the piece of wire, paper clip etc.

Power on the TV, if you get test patterns you have a faulty main board.

[humzilla]

Re: Lg 50pk750

I haven't tried the additional testing on the PSU board yet, but I did try to the jumper on on the control board (it was easier.)

The tv began go through a bunch of test patters... flashing the screen in different colors followed by displaying various boxes...

I'm guessing this means the PSU board is fine and that my problem is the main board. Do you guys agree?

[Andrew F. Ali]

Re: Lg 50pk750

Agreed

[humzilla]

Re: Lg 50pk750

Doing some searching on the web it looks like it's going to be pretty difficult to source the main av board.

Are there any usual suspects that I might be able to repair on this board or do you guys think I'm likely out of luck? Is it time to start shopping for a new tv?

[retiredcaps]

Re: Lg 50pk750

Doing some searching on the web it looks like it's going to be pretty difficult to source the main av board.

Check to see if this matches.

http://www.ebay.com/itm/171900735015

Usual ebay caveats apply.

[humzilla]

Re: Lg 50pk750

This board looks the similar but the part number on my board is P/N EAX61557902 (3). This board is P/N EAX61557905 (1). These sound like different revisions. Do you think the P/N EAX61557905 (1) would be a workable substitute?

[Andrew F. Ali]

Re: Lg 50pk750

If yo can post Pics of both boards we may be able to advise further. However, I am 95% sure it can work.....

[Hamie]

Re: Lg 50pk750

Most of the boards in the LG PK range are transferable, it shares the same y&z and cntl board.
As Andrew says this board should work fine.

[humzilla]

Re: Lg 50pk750

My TV is a 50PK750-UA.AUSLLR. The boards I'm finding appear to be for 50PK750-UA.AUSLHR. I'm not sure what the difference is between these two models but am suspecting it has to do with the revisions made on this board.

I know I'm being a bit paranoid here, but you guys think that the different control boards are interchangeable?

I've already posted a picture of my board and the e-bay pictures are pretty blurry but everything is in the right place. The boards appear to be nearly identical except for the revision difference.

thanks for your help and your patience... :-)

[humzilla]

Re: Lg 50pk750

I broke down and just went ahead and ordered the board. I finally received it today and (cue the church choir) the tv is now working...

Thanks guys for your help!

[vwlockboy]

Re: Lg 50pk750

Did the new board work for you? I have a similar problem but haven't started digging into it yet.

[vwlockboy]

Re: Lg 50pk750

Never mind I just realized you already answered my question...brain fart I guess

[frustrated plasma]

Re: Lg 50pk750

This was fantastic advice, and 2nd time this forum has helped fix my 50pk750 (had a loose ribbon cable a couple years back giving me gray lines).

I confirmed I have a bad main board via the 51/52 test. The main board seems to be sold out, anyone know of an alternate part # or have a spare they want to sell?

In the meantime, should I try baking the board to see if reflowing the solder may fix it? Nothing to lose, right? Or are there any other tricks to try first?

Thanks!