lg 50pa650t no picture

[thuss]

Hi

My mates tv went on the blink its a lg 50pa650t.

The tv comes on but no picture. The voltages seem fine

I have removed the upper buffer board but still no picture.

I have read in various threads that it could be damaged screen ribbons.I cant find any evidence of this.

Any ideas could it be both buffers and y sus gone?

thanks in advance for any help.

[rccrasher65]

Re: lg 50pa650t no picture

Check your voltages with a Digital volt meter. check VS and VA. Then check VSC. on y sus to see if they are to the panel label specs.

[thuss]

Re: lg 50pa650t no picture

I got the following voltages va 53, nc 206, vs 206, and on the vsc with the buffer boards removed I get 2.5v, vy 2.2.

I assume the ysus is blown?

[thuss]

Re: lg 50pa650t no picture

Sorry I rechecked the ysus voltages on the resistors and I get the following vsc+282, vsc-132 on the vy+321.9, vy-132.1 I think the ysus is ok? (Thats with the buffer boards disconnected)

[RCA2000]

Re: lg 50pa650t no picture

I got the following voltages va 53, nc 206, vs 206, and on the vsc with the buffer boards removed I get 2.5v, vy 2.2.

I assume the ysus is blown?

Remember--you must measure VY and VSC to FLOATING ground, IIRC--VY--190v. or so-- is usually measured across a resistor. If it is low- with the buffers connected--it is VERY likely one of the buffers is bad.. If still low with then unhooked--Sustain board problem.

[thuss]

Re: lg 50pa650t no picture

Sorry I dont really know what you mean by a floating ground? But I did get the following reading across a resistor i.e vsc+282, vsc-132 the following readings across a second vy+321.9, vy-132

[RCA2000]

Re: lg 50pa650t no picture

PLasma tv sets use a "floating ground" supply--for some voltages that drive the panel. They are generated from the Y-sustain board--right on it. These drive the buffers.

I need to see a picture of the Y sustain board up close--and I can guide you on the floating ground test.

[thuss]

Re: lg 50pa650t no picture

one more question if anybody can help the VZB = 149.8 V (Measured at R412) on the z sustain board on the one side of the (-)resistor I'm getting a reading of 0 whilst the on the vzb+ i'm getting a reading of 149.8 is that to be expected?

[tom66]

Re: lg 50pa650t no picture

Yes. (On LG all panel voltages are measured across a marked component, red lead one end, black the other)

The Vscan (Vsc) is relative to the -Vy base, but measuring across resistor takes this into a account (on an LG at least)

Vsc is scan supply for buffer boards. If this is low with both buffers removed the y-sus may be bad.

[thuss]

Re: lg 50pa650t no picture

I have read the resistors incorrectly. I used the chassis of the tv for my black probe and then with the red probe I touched the resistors and read the voltages.

I thought it was never a good idea to have the black probe and red probe on either side of the same component I will try again Thanks

[tom66]

Re: lg 50pa650t no picture

I thought it was never a good idea to have the black probe and red probe on either side of the same component I will try again Thanks

No, just a different way of testing useful for some types of components.

[thuss]

Re: lg 50pa650t no picture

sorry about late reply I have done what Tom66 has suggested and I get the following with everything connected i get vzb(z sus) 137v, vsc 140 and -vy 190.

But when I disconnect the ysus I get vzb 150, vsc 150 and -vy 190.

I have changed the buffer boards but still get no picture do you think its the screen? The buffers are new.

I dont get any glow or anything on the screen . But when I disconnect the ysus the tv led light works as it suppose to and responds as it should.

But when I reconnect the ysus the tv tries to stay on but after a few second goes back onto standby.

Also when I disconnect the upper buffer I still do not get half a picture. could it be the tcon board? any help would be appreciated

[tom66]

Re: lg 50pa650t no picture

Did you check -Vy and Vsc on your new board and compare them to readings before, also test them with and without buffers installed.

[thuss]

Re: lg 50pa650t no picture

Yes sort of. The old y sus had the same readings more or less (+- 5 volts). But when I attached my old buffers the voltage would drastically reduce but when I remove the old buffers the voltage would come back, I suppose one possibility both ysus boards are faulty or screens faulty?

[tom66]

Re: lg 50pa650t no picture

Can you perform this test on your buffer boards?
https://www.youtube.com/watch?v=51SZ...Y&noredirect=1

[thuss]

Re: lg 50pa650t no picture

yes I did this before I installed them I was getting no beeps I will check again thanks maybe not so new boards

[thuss]

Re: lg 50pa650t no picture

yes I did do the tests again as per the above link that you posted. Just to get any beeps on continuity I had to 60 M ohms.(there was no continuous beep on any connector block)

strange I'm getting beeps all down the 4 connector blocks but I have read its usually the top chip that blows

[thuss]

Re: lg 50pa650t no picture

I thought I let you know the ysus was playing up even though the voltages on the resistors were within range.

I replaced the upper buffer tv works fine apart from one black horizontal 1/2mm thick line across screen in line with the top chip, top buffer board.

[tom66]

Re: lg 50pa650t no picture

I replaced the upper buffer tv works fine apart from one black horizontal 1/2mm thick line across screen in line with the top chip, top buffer board.

So, bad panel.
The TV will stop working soon as the buffer chip will be over stressed. You will find the buffer board blown again.
Sorry, don't think it can be fixed.

[Alastair E]

Re: lg 50pa650t no picture

Not always a bad panel where just 1 row of pixels out....

I had a Samsung 50" just recently. It was the actual plug where the ribbon attaches. The connection in the plug was damaged and a very careful 're-alignment' (Bending) completely cured the problem...

Check carefully with a magnifying-glass all the connections on that plug and even follow the dead pixel line from the pixel line along the glass through to the orange mylar and then into the plug/buffer-board...

A Break in the panel or the mylar orange tab conductor will be easily seen as a black/burned or injured spot on the conductor affected.

You'll have to de-case the panel to check properly, suggest you draw a line on the panel with chinagraph pencil while set de-cased and on, so as to see which line the affected one is while set is off, and safe to closer examine.....

--It worked for me tracing out that 50" samsung....