LG 42PC51 plasma TV with sound but no picture

[MoNoImPoSm]

Hi all, I need some help with my LG plasma TV.
I have an LG 42PC51 plasma TV with sound but no picture.

The story:
The TV stopped turning on. I've checked the power board, there was no Va nor Vs voltage, at least 3 MOSFETs, a Zener diode, and a transformer were burnt. After replacing power board, the TV started to turn on for a second or less and then turned off. After disconnecting Ysus and Zsus power supply the TV turned on and there was a sound. I've checked main ICs at Ysus and Zsus and they were shorted so I've replaced them (only ICs). Now the TV turns on and there is sound but no picture. I'm not sure but it seems to me like the logic board is not firing up the Ysus board. The logic board is PDP 070209. There are four LED'S in a row (D1-4) and one separate (D15) on it. D1 is off, D3-4 are lit and D 15 is blinking.



Now my question is how to find out if the logic board or Ysus board is faulty. Any suggestions?

[dick_barton]

Re: LG 42PC51 plasma TV with sound but no picture

Have you tried the autogen test pattern screen. Disconnect the lvds cable to the main board and jumper the autogen pins as shown in your picture at the bottom right of the board. Power up and see if the test patterns are displayed.

[MoNoImPoSm]

Re: LG 42PC51 plasma TV with sound but no picture

Thanks for the reply. Yeah I've kinda tried that The TV is at my mother's in law home and I had no tools to solder the autogen contacts. As you can see there are no pins, just two small contacts with holes. Instead I've put a piece of wire into those holes but nothing has appeared on a screen. The wire was a bit loose thus I'm not sure if it was properly connected. I'm going to solder two pins and connect them with a jumper to be sure but I'm pretty sure that the result will be the same as previous. As far as I understand if the test patterns appeared this would mean that main board is bad (correct me if I'm wrong) and I don't think this is the case. As I've mentioned in previous post the power board, Ysus and Zsus boards were bad so my logic tells me that it's logic board or Ysus board is causing a problem.

[dick_barton]

Re: LG 42PC51 plasma TV with sound but no picture

Use a paper-clip. If the patterns appear then it would look like a main board fault. If no image then more fault finding.
Are you missing any voltages, for example VScan?

[MoNoImPoSm]

Re: LG 42PC51 plasma TV with sound but no picture

I'll check the voltages on the next occasion. There's one more thing I forgot to mention. The Vs voltage is 202 V (should be 190 V) and it's impossible to go below that level. That probably indicates that STK IC is not firing up.

[MoNoImPoSm]

Re: LG 42PC51 plasma TV with sound but no picture

So I've checked the voltages. All are present and mostly correct except the Vs voltage, that I've mentioned above and Power On voltage which should be 3,6V and is 4,5 V. Also the 5+ voltage on logic board at the top left corner as seen on the picture above is missing. The autogen test gave no result. I've also checked the screen in complete darkness on autogen test and the screen is completely dark. Any ideas or suggestions?

[Andrew F. Ali]

Re: LG 42PC51 plasma TV with sound but no picture

The 5V that is missing comes from the Y-SUS...Check if the Y-SUS Bd is generating 5V.

[MoNoImPoSm]

Re: LG 42PC51 plasma TV with sound but no picture

Andrew, my man. Lots, lots and lots of thanks for the most useful hint. I've checked the path of 5V on Ysus board and it turned out that there was a blown fuse. Should have missed it before. The TV came back to life.