Panasonic TH-65FX800A backlight LED's working but picture is very dark

[Bazfletch3]

Hi all

While I have quite a decent background in electrical & electronic troubleshooting & repairs, this is my first time stripping down an LCD TV for repair.... I have inherited a 3yr old Panasonic LCD which has a picture, but it is quite dark (too dark to be watchable - but still quite easy to see; I didnt need a torch to see it).

I started by checking and trying to adjust all of the picture and backlighting levels, but that made little to no difference. I also did a factory reset.

From there I envisaged that the backlighting wasn't working, however after disassembling everything I can see the backlighting is working - although I suspect not correctly. This particular TV has a solid glass backing panel, and the backlighting consists just of 2 separate LED strips which run along the bottom edge.
I have got hold of a service manual (which is for the D variation, not the A, but I believe they are the same; just the Indian market version rather than my Australian one) and checked the voltages noted for the test points on both the P & A boards, and everything checks out fine.



My suspicion is that the backlighting strips are fine, but are not getting sufficient voltage to illuminate brightly enough? Looking through the circuit diagram (below) I can see that each of the 2 strips is fed as 4 individual circuits; my next thought was to check the voltage outputs of these 8 feeds to see if they are consistent, which I am presuming they should be? From here I am thinking that it could be a fault of some kind with the P (power supply) board - I have found a couple available on the second hand market. Does anyone with many more levels of experience than me have any suggestions as to whether my approach so far seems valid, and or any other suggestions for me?

Many thanks

Barry

[JohnCT]

Your TV uses a common voltage source (VLED) for all four channels of your back lighting circuit, and four individual "sink" type drivers for each of the two sections of the two LED strips in the display screen. Even if that voltage checks normal, it won't tell you if the LEDs are actually being driven hard enough to produce a normal picture.

The regulation of the supply to the LED strips is not based on voltage but current. If you can open one of the cathode lines and check the current we can get a better idea of what's going on. The common VLED voltage could be fine but if the controller IC isn't pulling the low side of the strips down, there won't be voltage across the strips to provide enough brightness. 50 ma should provide enough brightness for a good picture.

If you're getting 50ma or more, you probably have an issue with the display itself, although there are often other issues seen on the screen when a bad display causes brightness problems (uncommon as it is), such as uneven brightness, lag in images between scene changes, etc. Dark images with good backlighting could also be a bad tcon (if it has one) or main board.

If you're getting somewhere in the 20ma or less range on any of the four channels which is what I think you will find, your current supply isn't providing enough voltage drop across the strips to provide enough brightness. This can be caused by a problem right at the LED controller, but it could also be an issue from the main board not commanding more brightness from the back light circuit.

BTW, do you know the operational history of that TV? IOW, was it running absolutely fine one day and then gone suddenly dark, or did it start getting dark in sections and the darkness spreading until it became unwatchable? The answer to that will help diagnosing the issue.

John

[nomoresonys]

Can you post a good focused picture of each board and one of the whole back as to see how everything is connected.

[Bazfletch3]

Thanks for the fantastic info JohnCT I havnt had a chance to get back to it today but I will check the currents and see whats happening. The TV came from my in-laws; long story but it was a warranty claim that was written off when it failed after 12 months - around the same time that Panasonic pulled out of the Australian TV market. The in-laws got a full credit refund and bought a Samsung TV instead. They were watching it one day and it just went dark. They left it for a few hours turned off and then restarted it and it worked for a session, but since then it has been consistently dark and hasnt alterned.

nomoresonys I have some photos which i took previously - hopefully they are focussed enough (can take more when I get back to if anything is unclear) (In the overall picture the T-Con board sits under the central metal cover at the bottom)

[JohnCT]

Before you do anything, unsnap those two ribbon cables and take a look at them closely to make sure there's no arcing or even melting of the ribbons.

They were watching it one day and it just went dark. They left it for a few hours turned off and then restarted it and it worked for a session, but since then it has been consistently dark and hasnt alterned.

That's good news. I've actually seen some TVs come in where sections of the LED strips start failing one by one and the customer doesn't bring them in until it's unwatchable - kind of throws the diagnosis off. Between its age (under a year) and your description, it's unlikely you have to go inside the screen unless there's something really strange going on - like a burned strip that might be grounding itself to the frame. A quick test of unwanted leakage to ground (not 100% conclusive) is to remove the ground screws from the power supply board, lift it up and put a box or book under the board to float it above ground, then plug it in. If the picture becomes bright, you have a grounding issue on one of the LED strips inside the display. The good news is that this is actually uncommon.

Since the screen is evenly lit (although evenly dark), you only have to open one line to measure current. Once you get a current reading, we can figure out where to go next.

John

[Bazfletch3]

Before you do anything, unsnap those two ribbon cables and take a look at them closely to make sure there's no arcing or even melting of the ribbons.



That's good news. I've actually seen some TVs come in where sections of the LED strips start failing one by one and the customer doesn't bring them in until it's unwatchable - kind of throws the diagnosis off. Between its age (under a year) and your description, it's unlikely you have to go inside the screen unless there's something really strange going on - like a burned strip that might be grounding itself to the frame. A quick test of unwanted leakage to ground (not 100% conclusive) is to remove the ground screws from the power supply board, lift it up and put a box or book under the board to float it above ground, then plug it in. If the picture becomes bright, you have a grounding issue on one of the LED strips inside the display. The good news is that this is actually uncommon.

Since the screen is evenly lit (although evenly dark), you only have to open one line to measure current. Once you get a current reading, we can figure out where to go next.

John

I checked both ribbon cables and they looked pretty good to me - nice and clean and all the contacts looked in good condition.

I also managed to check current readings on 2 different lines, however I only read 0.5mA (on both). Assuming my readings are correct (Im pretty sure they were well connected and stable), theyre obviously incredibly low - but enough to light the LED's?

I also isolated the power supply board from ground, and that made no difference.

Would you have any suggestion from this point as to whether it would likely be a power supply board issue (which contains the LED drivers), or a main board issue? Or any way to try and confirm further?

Cheers

Barry

[Bazfletch3]

JohnCT and for what its worth, the TV is actually 3 years old now - because its been sitting under a bed for the last 2 years forgotten about!

[JohnCT]

I checked both ribbon cables and they looked pretty good to me - nice and clean and all the contacts looked in good condition.

I also managed to check current readings on 2 different lines, however I only read 0.5mA (on both). Assuming my readings are correct (Im pretty sure they were well connected and stable), theyre obviously incredibly low - but enough to light the LED's?

I also isolated the power supply board from ground, and that made no difference.

Would you have any suggestion from this point as to whether it would likely be a power supply board issue (which contains the LED drivers), or a main board issue? Or any way to try and confirm further?

Cheers

Barry

0.5ma would not be enough to light the LEDs. I would think (guess really) that 5-10ma are about the minimum for lighting the LEDs enough to see a picture, even a dark one. Are you on DCA on your meter? Can you see the backlights lit from behind the TV? There are usually small openings where you can see the backlight leak through from behind with the rear cover off the TV.

Is there a schematic for this TV on line?

John

[Bazfletch3]

0.5ma would not be enough to light the LEDs. I would think (guess really) that 5-10ma are about the minimum for lighting the LEDs enough to see a picture, even a dark one. Are you on DCA on your meter? Can you see the backlights lit from behind the TV? There are usually small openings where you can see the backlight leak through from behind with the rear cover off the TV.

Is there a schematic for this TV on line?

John

Its only a cheap UNI-T meter, but its definitely DCA - was a pain in the butt to get those measurements as I ended up taking them between the ribbon cable traces and the terminal header. I might have another look at them later to triple confirm - but thats why I checked 2 separate lines...

You can certainly see the backlight, as the construction of this model has a solid piece of glass as the chassis, with the white "diffuser" panel simply fixed to it with adhesive, and the LCD panel fixed in front of this (all just with double sided tape). I still have the LCD panel loose fitted so I can gently lift it up easily.

I have the service manual for it - its too large for me to attach directly here, but I have it saved in a sharepoint folder as per below., schematics are from page 35 onwards.

https://fletcherconsult-my.sharepoin...D1axw?e=udoNyI

Cheers

Barry

[Bazfletch3]

Hi all

I finally had some more time to do some further investigation - disassembled everything again so I could get to things fully. My TV has 2 separate LED strips, each with 92 LEDs. Looking at the circuit diagram, and by testing, each of the 92 LED strips has 4 individual strings of 23 LED's. I managed to light each string individually by jumpering the different pin combinations through. In each case when I checked current readings they were somewhere between 0.4 and 0.7 mA as it "warmed up", and then settled around 0.5 to 0.6. The LED's were all lit, but to my eye I cant really tell if they were "bright" or not... Each string was getting in the vicinity of 116VDC.

My knowledge of circuits is lacking at best, but a token google search seemed to suggest that almost any flowing current would light a modern LED in at least some fashion.....

Does anyone have any further suggestions on anything else that I might try or test? Ive found a retailler that has both the main and power supply boards available second hand, and which will accept them back (less a 25% restock fee), which Im tempted to try next - I figure in all likelyhood it must be one of those. Of course its possibly only a single component failure on one of those boards...

[Bazfletch3]

Hi all

Just to tidy this up, and to potentially help someone else in the future..... I went ahead and ordered a new power supply board, and that fixed my problem.

Im just waiting on some 3M double sided tape to arrive so I can properly put everything back together. Can anyone suggest what I should use to fully remove the old double sided tape residue from the back of the LCD panel? and what process I should use to clean the back of the PCD panel and the backlight diffuser so that I dont have any marks inside the screen?

Cheers

Baz