LG 42PG20 sound no picture

[martino]

Hello all,

My next patient on the table.
Set powers up as it should,led stays blue sound works but no picture.
Got the manual from tom66 archive.
It tells me to unplug all cables from mainboard,plug Tv in and short ex auto gen on the logic board..
Did that,screen stays dead,no test patterns etc...
Measures all voltages,they are all present,VA and VS etc are all in spec...
It does have 3 caps bulging on PSU but all 5v etc are there...

So,with no test patterns etc I'm kinda stuck on what to think,is the logic board bad? Or did the Y-board die without shorting out?
What would be your suggestion to test/try???

[martino]

Re: LG 42PG20 sound no picture

Little bump

[martino]

Re: LG 42PG20 sound no picture

Put TV back on the bench,anyone have suggestions???

[tw2005]

Re: LG 42PG20 sound no picture

Put TV back on the bench,anyone have suggestions???

If all the voltages are present I'd be thinking the logic control board. Any evidence of life from the sustains like heat or does the screen energise at all when power is applied?

The LEDs on the control board , what did they do?

[martino]

Re: LG 42PG20 sound no picture

The logic board has 3 LED's ,the top one D103 is lit solid,other two are off....

I will take some pics of some connectors tomorrow morning (3.30am here),cause on the Zboard,there is a plug 4 that goes to the logic board,that has 15v printed on it on pin 1,2&3 but I only measure 5volts there....
And on the Yboard there is a plug P901(going to Zboard) that has Zsus,I measured 20 volts and not sure if thats correct(seen a post where a guy had 81volts on that)

Screen does not energize at all,not even for a split second.Didn't have it on long enough to notice any heat on cooling blocks etc....

But I will take pic's in the morning when my head is not so sleepy lol...

[tw2005]

Re: LG 42PG20 sound no picture

yeah, not that I know much about these but seems a shame to have an unfinished project.

[Alastair E]

Re: LG 42PG20 sound no picture

-Vy present on Y-SUS board?

If not, disconnect Buffers. Present then?
--If so, replace buffers.

If not, Check transistor by small yellow transformer on bottom of Y-SUS board, its an IGBT or MOSFET, cant remember which.

My bet is on the buffers....

[audiofreak19]

Re: LG 42PG20 sound no picture

First thing I would change the capacitors that are buldging and go from there.

[tom66]

Re: LG 42PG20 sound no picture

I just had one of these with a bad buffer board.

Check -Vy/Vsc on Y-sustain. If it's jumping about or low, pull the buffer, if it returns after pulling buffer, replace or repair the buffer. On mine the top IC had failed. A new board was £18 so I just used that.

[martino]

Re: LG 42PG20 sound no picture

With bufferboard CONNECTED:
Vy and -Vy around 7 volts.
Vsc and -Vsc 6.81 volts.

With bufferboards DISCONNECTED:
Identical as with buffers connected.......


***If not, Check transistor by small yellow transformer on bottom of Y-SUS board, its an IGBT or MOSFET, cant remember which.***

Not sure which transistor you mean,is it a smp or a bigger sized one(without heatsink)

[martino]

Re: LG 42PG20 sound no picture

Picture added

[tom66]

Re: LG 42PG20 sound no picture

There's a repair kit for your symptom. Involves replacing a 14 pin IC, a diode, and a couple resistors:

IC905
D702
R711
R750

http://www.ohmsupplies.co.uk/YSUS-RE...000-NO-PICTURE

[martino]

Re: LG 42PG20 sound no picture

It's save to say those Vy readings are not good?
What voltages should I expect to see on a average Vy?

I'm going to look at that kit Tom and try to measure around that IC etc, would be nice if that's the problem

I'm having a hard time finding parts some times. Like yesterday I found a couple fets bad on a Panasonic SN board. DG501rf etc. cannot find specs or anything grrr... The Panasonic guys in my neck of the woods just completely refuse to order components for me

[SlickClick]

Re: LG 42PG20 sound no picture

First thing I would change the capacitors that are buldging and go from there.

I agree or in the least find out if these caps are part of the 15v circuit you are only getting 5v on...

[tom66]

Re: LG 42PG20 sound no picture

-Vy & Vsc are on panel

n.a. / -Vy / Vsc / n.a. / Vzbias

from memory.
But typical values -180V -Vy and 140V Vsc from the one I worked on just recently, since sold.

[martino]

Re: LG 42PG20 sound no picture

Soooooooooooo,I measured around a bit,trying to force myself to repair at component level as much as possible,but just looked at shopjimmy....
A substitute board is $28 bucks..!!!!! Can't let that slide

Anyway's,before I order,quick tip how to measure bufferboard for shorts???

Tom,I looked at that repair kit,and it has D702,r711 and r750 in it,I checked all those and surounding components and they check fine....

[tom66]

Re: LG 42PG20 sound no picture

Here is my short detection trick and it is for the same board.
https://www.youtube.com/watch?v=Iw0_...4&noredirect=1

For Samsung board but adaptable to LG board:
https://www.youtube.com/watch?v=51SZsfH60EY

[martino]

Re: LG 42PG20 sound no picture

That's awesome Tom,same TV...!!!!

Ok,buffers are ok....

Now,see attachment,since I'm getting 7 volts on Vy etc,I poked around where my voltage is disappearing,I found this guy(see pic) that has 186volt on its center pin,but 0 volts on the two outer pins! That doesn't sound good to me...
It's Q503 which is QPF 5N5OC.

How I would love to have a diagram of this board

[martino]

Re: LG 42PG20 sound no picture

forgot pic

[tw2005]

Re: LG 42PG20 sound no picture

It's save to say those Vy readings are not good?
What voltages should I expect to see on a average Vy?

I'm going to look at that kit Tom and try to measure around that IC etc, would be nice if that's the problem

I'm having a hard time finding parts some times. Like yesterday I found a couple fets bad on a Panasonic SN board. DG501rf etc. cannot find specs or anything grrr... The Panasonic guys in my neck of the woods just completely refuse to order components for me

RF or RP? which SN TNPA5066?

SMD or TO-220P

http://www.ebay.com/itm/DG501RP-Manu...item1e85538f9d

May also get away with 2PG009, 2PG011,DG502. They're all the same current, 40A, 510V for the 501,009. 540V for the 011 & 502 as a guide. very hard to get exact data on the DG series,