Panasonic tx50DX700b blank screen with sound ok

Thread Starter

Acerman

Joined May 30, 2019
6
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
10,478
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
6
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
10,478
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
6
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.
 
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