Panasonic tx50DX700b blank screen with sound ok

Thread Starter

Acerman

Joined May 30, 2019
16
I am trying to fix this Panasonic 50 inch LED 4k smart TV. It has a blank screen with a blue hue but has perfect sound. I can change channels with the remote but there isnt even a 'no signal' on screen just a blank screen with normal sound. The screen panel is showing no signs of cracks or damage.

I have some decent knowledge of electronics and know how to use a multimeter but am by no means an expert.

I bought a donor TV very cheaply from Ebay which is exactly the same model. I have been attempting to troubleshoot using parts from the donor TV. To cut a long story short I have narrowed it down to a problem with the right hand LED driver board strip which is situated horizontally along the bottom of the screen panel. The reason that I think it is the driver board is that I have had 2 different tcons in the TV with no difference to the outcome. I have replaced power board, main board , lvds cable , tcon ribbons all taken from the donor TV. I have changed the power board to main board cable. I have also tried cleaning the pins on the tcon to panel connectors. No change

Today I compared some voltage tests on the smd capacitors on the right hand driver board on both the donor TV and the one I am trying to fix. There was a very different result. The donor TV is testing at 17.5 volts on the output side of the capacitor . The faulty model is showing virtually no voltage or 0.3 at the very most on the same side of the cap. The same result is achieved on all the larger sized smd capacitors on the right side driver board strip. AM I on the right tracks here? And if so, would removing the capacitors for testing be the way forward?
 

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dl324

Joined Mar 30, 2015
12,917
AM I on the right tracks here? And if so, would removing the capacitors for testing be the way forward?
It would seem so. Electrolytic capacitors usually fail open (I assume an initial short opens them); the one in question could be the exception.

Do you have schematics for the circuit in question? Going from pictures isn't a viable situation for members because we don't have a way (or, for most of us, inclination) to trace the circuit.

If the donor unit is working, you have an easy way to check for defective circuitry.
 

Thread Starter

Acerman

Joined May 30, 2019
16
Thanks for your response dl324. When you say that capacitors usually fail open , I assume that you mean that if a single cap was to fail it could affect and knock out the whole circuit. is that what you mean?
That would perhaps explain why almost all points on the right side board are showing very low voltage readings .

On this occasion Panasonic have used a third party manufacturer for the driver board (Innolux). I don't have the schematic for the board. I will try to locate one on the internet and post it if I find one. I don,t mind paying.

The donor TV has significant screen damage but when I first bought it , the lower part of the screen was normal. Unfortunately having been moved around once or twice the damage has got worse. You can , however see changes to the distorted screen pattern when for example pressing the 'home' button on the remote or by switching from TV to AV. All the same , I trust that everything works perfectly on the donor apart from the lcd screen as i tested it when I first bought it. As you say, ideal for testing purposes.
 

dl324

Joined Mar 30, 2015
12,917
When you say that capacitors usually fail open , I assume that you mean that if a single cap was to fail it could affect and knock out the whole circuit. is that what you mean?
The normal failure mode for electrolytic caps is that they become open circuit and no longer perform the intended function.

I surmised that it was a short condition that caused the cap to become opened. That short condition could take out components up stream from it. So it's possible that the short damaged components "before" and/or "after" it. Without having a schematic, we can't offer advice on what to check.
 

Thread Starter

Acerman

Joined May 30, 2019
16
Can you transplant the good screen to the donor tv, or vice versa?
Also have you tried all the different inputs (Sources)?
I like the donor idea.
There's more: https://www.justanswer.com/electronics/abl37-panasonic-th-42px70a-tv-when-turn.html
Good luck...…….
I don't think that it would solve it. The problem appears to be on the led driver boards which are attached to the screen panel by bondings which can't easily be removed from the panel. Switching the LCD would just give the donor a faulty screen panel. . However if I am wrong about the fault being on the driver boards, it may work. Thanks for your contribution.
 

Thread Starter

Acerman

Joined May 30, 2019
16
Here is an update. I have been trying without success to get hold of the schematic for the LED driver boards which are made by Innolux. Without the schematic, it seems that people on here can't really help. My only thought is maybe to test the smd capacitors on the driver board by lifting one side with a soldering iron. However I have very limited experience with smd soldering and am worried about lifting the tracks and therefore wrecking the driver board.
 

dl324

Joined Mar 30, 2015
12,917
My only thought is maybe to test the smd capacitors on the driver board by lifting one side with a soldering iron. However I have very limited experience with smd soldering and am worried about lifting the tracks and therefore wrecking the driver board.
That's a recipe for disaster.

Is the donor board compatible with yours?
 

Thread Starter

Acerman

Joined May 30, 2019
16
The LED driver boards are identical on both the donor and the one I am trying to repair.
Yes. There is a photo of part of the driver board at the start of this thread. I have been testing both to compare results . Voltages are very different on the faulty panel.
 

Thread Starter

Acerman

Joined May 30, 2019
16
Are you saying that you aren't able to compare the voltages on the good board with the bad one and identify any defective components?
Unfortunately I havent been able to pinpoint the exact component. All the smd caps are showing zero volts on the bad board and about 17 on the donor. There are lots of discrepencies and lots of differences.And so pinpointing a single component or even isloating one part of the board has been dificult. . I have tried searching for a short to ground. More than one component read a low resistance value. I tested one of the pins on the chip on the driver and it appears to show a short. I wonder about that chip . It has 4 pins .
 

dl324

Joined Mar 30, 2015
12,917
Unfortunately I havent been able to pinpoint the exact component. All the smd caps are showing zero volts on the bad board and about 17 on the donor.
Are all of these capacitors on the same electrical nodes?

When multiple components are on the same node and something is holding that node near ground, a very sensitive digital voltmeter can sometimes be used to locate the component holding the node low. A more appropriate tool in this case is a current tracer (e.g. HP547A). This tool sold for several hundred dollars in the late 70's, but you can sometimes find them on eBay for dimes on the dollar.
 

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Thread Starter

Acerman

Joined May 30, 2019
16
Are all of these capacitors on the same electrical nodes?

When multiple components are on the same node and something is holding that node near ground, a very sensitive digital voltmeter can sometimes be used to locate the component holding the node low. A more appropriate tool in this case is a current tracer (e.g. HP547A). This tool sold for several hundred dollars in the late 70's, but you can sometimes find them on eBay for dimes on the dollar.
Thanks Dennis. What do you mean by electrical nodes? You may be able to see some of them on the first photo on my thread at the top. There are about 6 larger smd caps fairly well spaced apart from each other going all the way across the driver board. The driver board itself is long and narrow .
Thanks for the tip about using a current tracer. I am happy to try one or two things and who knows I may pinpoint the problem. I suspect the little 4 pin chip that I mentioned.
 

dl324

Joined Mar 30, 2015
12,917
What do you mean by electrical nodes?
Electrical nodes, or nets, are electrically connected and are considered to be at the same potential (this ignores interconnect resistance and is usually okay). The voltage measuring method I mentioned uses the fact that the interconnect does have resistance and that whatever is holding the node near ground will be at the lowest voltage of any object on that node and current into that point will be the highest.
Thanks for the tip about using a current tracer. I am happy to try one or two things and who knows I may pinpoint the problem.
Good luck finding one. I checked eBay and no listings came up.

To use the voltage measuring method, you need a volt meter that can measure millivolts, or fractions of millivolts accurately. Your garden variety DVM won't be good enough.
 

Thread Starter

Acerman

Joined May 30, 2019
16
Electrical nodes, or nets, are electrically connected and are considered to be at the same potential (this ignores interconnect resistance and is usually okay). The voltage measuring method I mentioned uses the fact that the interconnect does have resistance and that whatever is holding the node near ground will be at the lowest voltage of any object on that node and current into that point will be the highest.
Good luck finding one. I checked eBay and no listings came up.

To use the voltage measuring method, you need a volt meter that can measure millivolts, or fractions of millivolts accurately. Your garden variety DVM won't be good enough.
Thanks for your help.
 
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