LG 50PV400 Have Sound but No Picture

[sickofit]

Ok,about 2 weeks ago the tv all of a sudden had a dark thin vertical line from top to bottom,almost in the middle,just a little more to the left.It is a bedroom tv so didn't much care,as was still watchable.

Well last night we were watching it and all of a sudden the display went dark,like it was turned off...but the sound was still working...uh oh....turned it off,left it for a minute,tried it again..same thing...sound works but nothing on the screen...all buttons work as usual,can turn vol up\down..tv turns on\off ok...just no picture..tried dif input,same thing.

Doing some research this sounds like a bad y-sus or bad y-buffers,correct..??...I've got the back off the tv,and have used my DMM to test the fuses on the y-sus board,and the fuses on the PSU board,all ohm out fine.

Can someone please chime in with some more tests i can perform before ordering some boards up...this has 2 buffer boards connected to each other with a small ribbon cable...is there a way to test the buffer boards

[Hamie]

Re: LG 50PV400 Have Sound but No Picture

Is it worth fixing? Because either way the original black line will remain.

Anyway, check voltages

Vs and va located on the powersupply.
Vsc and -vy test points located on the ysus.
And zbias located on the z-sus.

There might be test points marked + and - for these voltages to be read, or sometimes they are measured accross the legs of a clearly marked resistor.

They should match the panel sticker.

[Hamie]

Re: LG 50PV400 Have Sound but No Picture

[sickofit]

Re: LG 50PV400 Have Sound but No Picture

Wow...Thanks for the quick reply.......Ya,I always like to try and fix whats broke,kind of a dying art these days.

Ok,have done some readings and this is what I have found.

Power supply Vs tag reads 203v,I get 205...Va tag reads 55V,I get 55.2

Y-Sus board VSC tag reads 150V,I get 191.3.....-Vy tag reads 190V,I get 190.2

Z-Sus board Z-Bias tag reads 130V,I get 129.5


So the only reading that appears to be off is the VSC on the Y-Sus board,and while keeping the meter probes on and trying to use the VSC adjust did nothing,maybe flick down to 191.2 but doing the whole sweep of the adj screw,really did nothing to change the voltage.

Any ideas where to go now..??

Is there anything to be learned by the blinking green light on the small main board...maybe error code wise..??...It will blink...pause..blink fast once or twice...then start over..

Thanks for the help guys,really appreciate it.

[Hamie]

Re: LG 50PV400 Have Sound but No Picture

Can you disconnect the buffer boards from the y-sis and check the vsc, -vy voltages again.

[sickofit]

Re: LG 50PV400 Have Sound but No Picture

Ok,i'll get at that soon and report back....glad I found the training manual on this site......as with this set I cant just unhook them,it says in manual they have to be completely removed or damage will result.

[sickofit]

Re: LG 50PV400 Have Sound but No Picture

Ok......now I think we're getting somewere..!!..

With both side Y-buffers totally removed,I got a reading of VSC-155V(remember I was playing with adj),and I was also able to adj the voltage this time,so put it to 150V where tag said it was supposed to be.

-Vy stayed the same at 190.2V tag says 190V

So do you think its safe to say I need to order up a set of Y-Buffers..??

This site rocks..!!!..

[Hamie]

Re: LG 50PV400 Have Sound but No Picture

Plug in the top buffer and turn on the TV? Do you get an image on the top half of the screen?

If not reverse the process and run it with only the bottom buffer.

[sickofit]

Re: LG 50PV400 Have Sound but No Picture

I know nothing about this stuff,i'm learnin.....but the training manual says to never run set with just P121 disconnected,which is the ribbon cable connecting the 2 upper\lower buffers,and also never run the set with P213 disconnected which is the ribbon cable going from y-sus board to lower buffer,and P221 which again is the ribbon cable going from the bottom buffer to the top buffer...says damage will occur.Here's a paste from the manual.Its stated in a couple other spots through the manual NOT to run the set with those just disconnected,to remove the y-buffers completely,and in order to run with just one in,I have to have one of the cables unhooked it says not too.

Y-Drive Boards work as a path supplying the
Sustain and Reset waveforms which are made in
Y-DRIVE UPPER (TOP) Y-DRIVE LOWER (BOTTOM)
Sustain and Reset waveforms which are made in
the Y-Sustain board and sent to the Panel
through Scan Driver IC's.
The Y-Drive Boards receive a waveform (Y-Drive)
developed on the Y developed on the Y-SUS board then selects the SUS board then selects the
horizontal electrodes sequentially starting at the
top and scanning down the panel.
Scanning is synchronized by receiving Logic scan
signals from the Control board.
The 50PV450 uses 12 Driver ICs on 2 Y-Drive
Boards commonly called “Y-Drive Buffers” but are
actually Gate Arrays connected to 1080 horizontal
electrodes across the panel.
This model also does something new, Monitor 5V is
sent to the Lower Y-Drive where the low voltage
Data Buffer are located.
Also, The upper Y-Drive receives FG10.9V and
regulates it down to FG5V for the upper and
routed down to the lower Y-Drive buffers.

The upper Y-Drive is responsible for driving the upper half of the panel's horizontal electrodes with
Y-Scan signals through the Panel's Flexible printed
circuits. (540 horizontal electrodes).
The Upper Y The Upper Y-Drive is also responsible for developing Drive is also responsible for developing
the FG5V operational voltage for both Drive boards.
It receives FG10.9V from the Y-SUS on P111 pins
11~12 and routs this voltage to IC191 which regulates
it down to 5VFG.
The Upper Y-Drive then delivers the 5VFG to all the
buffers for their low voltage signal processing circuits.
The 5VFG is also sent down to the lower Y-Drive via
P121 pins 21~30 to P221 pins 1~9 for the lower Y-Drive buffers. Can not read pins because they are
covered in silicon.

Warning: Never run the Y-SUS with just P121 disconnected.You must remove the Upper Y-Drive board completely.

The Lower Y-Drive is responsible for driving the bottom half of the panel's 540 horizontal electrodes
with Y-Scan signals through the Panel's Flexible
printed circuits.
It receives FG5V from the Upper Y-Drive on P221
pins 21~30 from P121 pins 1~9 for the lower Y-Drive buffers low voltage signal processing.
Another new development is that the lower Y-Drive
board receives Chassis Ground and M5V P213 pins 14 and 15 for the Data buffers. The Y-Scan logic
signals are also related to chassis ground and the
Data buffers distribute the
Y-Scan logic data to all the Buffers, (Gate arrays) on
the upper and lower Y-Drive boards.
These signals are routed to the Upper Y-Drive
through P221 to P121 on the upper. Can not read
pins because they are covered in silicon.

Warning: Never run the set
with just P213 disconnected.You must remove the Lower
and Upper Y-Drive boards
completely.
Never run the set with P221 unplugged unless you remove
the Upper Y-Drive board
completely.

[Hamie]

Re: LG 50PV400 Have Sound but No Picture

Usually you can run the set with just one buffer.
You probably have faulty buffer IC.

You can check them, think this on one of tom66's videos.

https://m.youtube.com/watch?v=51SZsfH60EY

When doing this test you keep both your upper and lower buffers connected by the ribbon cable in the middle.

[Terryfictv]

Re: LG 50PV400 Have Sound but No Picture

Can you send a pix of the bottom of the z sustain board ?

[sickofit]

Re: LG 50PV400 Have Sound but No Picture

Ok,bringing this post back from the dead,lol,haven't had much time lately,but am ready to do some more testing.

I've checked the y-buffers for shorts and don't appear to have any,I also ordered a tested set from ebay,with no change when installed,still no picture at all.

VSC on the Y-sus board is supposed to read 150v as per sticker,but with either set of y-buffers I have,hooked up,the VSC jumps from 150v to 190v and is not adjustable with them hooked up.If I unhook the buffers,the VSC drops to the right 150V,and is adjustable at the pot....Which led me to believe the y-buffers were bad,but it acts the same with both sets of buffers I have.

Also have followed 5v from p supply to each board,as well as VS,all read good.

I also tried unhooking the LVDS cable from the control board and jumpered the auto_gen ports on the control board,powered on,and have no test patterns.

Where should I go from here..??