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Old 01-17-2016, 10:29 AM   #1
Scotty16
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Default Panasonic TX-L42B6B – LED Single Blink

I was given the TV by a relative who had already put it outside in the rain ready for landfill, luckily it was screen side up & at an angle so I was able to fully dry it out (I think). Immediately on applying power the standby light flashes very fast (16 times I think) then goes steady for several seconds, during which time the screen flashes, trying to start, first showing four blank screens then a pause followed by another blank screen & then three screens on which I can see “No Signal” then another pause followed by another blank screen & three more “No Signal” screens. The standby light then turns red and starts flashing once every 4 seconds. That's how it stays and is non- responsive to the remote. From applying power to first red light is 10 seconds.

Finding no legend on the power board, and initially being unable to locate a service manual, I realised fault finding would be problematic. I foolishly took the rapid flashing as the blink codes I’d found mentioned on the net so, as it wasn't a lot of money, I purchased an "A Board" (TNP4G548) off EBay. Unfortunately this made no difference.

I then went back to Google and eventually located a SM from:-

http://elektrotanya.com/panasonic_tx.../download.html

(It’s for a TX-L42B6E (“E” at end) which is the European model)

Armed with the manual I measured the voltages:-

Connector P3
-----------------

8 24v Reads 24V for 10 seconds then falls to 0
7 24v Reads 24v for 10 seconds then falls to 0
6 GND OK
5 GND OK
4 BL_SOS Rises to 3.66V three times the settles at 0.88V
3 BL_ON Rises to 3.66V three times then settles at 0V
2 ???? Not Connected
1 PWM1 3.46V falling to 0V, then 3.46V falls to 1.1V then 3.00 falls to 1.0V then 0.08V


Connector P2
-----------------

14 GND OK
13 BL-PWM1 3.45V Three times then settles at 0.08
12 BL_ON 3.15V Three times then falls to 0
11 BL_SOS 3.66V Three times then falls to 0.88
10 KEY 3.25V
9 POWER-ON 3.25V
8 SUB-ON 2V for 10 seconds then 0V
7 GND OK
6 5vs 5.23V
5 GND OK
4 (FUNC ON) Not Connected
3 16V Not Connected
2 16V 15.43V for 10 seconds then falls to 0
1 16V 15.43V for 10 seconds then falls to 0


The manual on P22 says the single blink code means "BL_SOS - A Board Panel" (is that "A Board" or "Panel" meaning the screen).

I've not included any photos yet since I can't see any damage and, having the SM, I thought they might not be required.

Not wanting to pre-empt anything but I did find this on EBay:-

http://www.ebay.co.uk/itm/Panasonic-...8AAOSwdvpWDr2G

Which says it's a genuine Panasonic repair kit for the inverter board TNP4G549 which has:-

2x Transistor Q9804, Q9802 (Panasonic improved part) - Both B1CFRM000028
2x Capacitors C9807, C9812 (Panasonic improved part) - changed from 1nF to 2.2nF

It does say this is for two 50 inch versions of my TV (TXL50B6B & TXL50B6E) which have “No picture and sound, sometimes LED flashes 3 times”. It also says it only affects models prior to serial number 3330003 whereas mine is 3350550.

Before I noticed the serial number caveat I did try testing the two transistors mentioned whilst in circuit (never having tested transistors before). I read that you should select diode on the DMM so I did that (a diode symbol and "2K" on my meter). Using the guide on this site for “Troubleshooting 2 seconds to black - Testing mosfet.txt", I assumed the single connection at the top to be (1) and the other two connections (2) & (3) from left to right. I then got readings:-

(1) to (2) - 1.781 ohms on both (Black probe on (1))
(1) to (3) - 0.559 ohms on both (Ditto)
(2) to (3) - .869 on one & .868 on the other (Black probe on (2))

I’ve since read that maybe I shouldn’t test Mosfets like this but I’ve put the board back in the TV and it’s as it was. So no harm done, fingers crossed.

I hope I’ve provided enough information to enable someone to kindly help me.
Thank you

Graham
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Old 01-18-2016, 09:30 AM   #2
Scotty16
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Default Re: Panasonic TX-L42B6B – LED Single Blink

Does anyone think it might be the Inverter board ? When Panasonic say that the panel is the fault I wonder if they mean when the screen incorporated the inverter instead of being separate board.
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Old 02-02-2016, 05:57 AM   #3
Scotty16
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Default Re: Panasonic TX-L42B6B – LED Single Blink

Thought I'd post again just to close this thread and to let anyone with the same problem know what happened (I hate threads which just end).

Well I got an inverter board and that made no difference so the service manual was right, it was the panel. To prove it I got busy with my screwdriver and got to the LED's :-



I was hoping to attach a video, which shows only the three middle strips flashing. The top and bottom strips are in series so that fits. Checked continuity and that was OK. Found this on Youtube which explains how to test the LEDs :-

https://www.youtube.com/watch?v=eEJoPCa7Kt0

Couldn't get piercing the strips to work so took off the clear plastic tops. Found that the very last one was faulty :-



Found a posting on the Web which said if one strip was gone to replace all the strips. The cheapest I found was second hand at £17 a strip, which for 10 strips (2 per length) is £170 !!!. So I removed all the panels and finally delivered the rest to the tip (working in my garden shed didn't give me much room, as a result of which I managed to rip off one of the ribbon connectors to the LCD screen so that decided it good and proper any way).

I read, on this site, someone saying the LEDs should fail in such away that the rest keep working. What about making them replaceable, from the back ? Each holder could have a small hole in it so you could easily tell which had failed. It might mean removing a panel or two to get to them but so what.

So it's back to my old faithful Sony CRT telly. Been waiting for HD to get good and it has with UHD. Do I now wait for the backlight being done away with by using OLED ? Youview built in would be very nice but the only models with it are Sony and I believe they disable the record facility when enabled.

Graham
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Old 02-24-2017, 06:58 AM   #4
drfoxx
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Default Re: Panasonic TX-L42B6B – LED Single Blink

Thanks for your post, it was helpful stopping me going down this route!
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Old 01-26-2019, 12:28 AM   #5
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Default Re: Panasonic TX-L42B6B – LED Single Blink

Succesfully repaired a TX-L42B6B by replacing the strips.
Salvaged the TV for 5€ and ordered the strips from China for 22€
The job was actually quite easy.
The LED driver board has two "circuits", the topmost and lowest strips are connected in series to the 1st and the three middle strips to the 2nd.
The middle strips did light up at first because the solder had cracked from one of the +/- connectors. Resoldered, reassembled and turned the TV on.
This will now spend it's new lease on life as my carage PC monitor.
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Old 03-20-2019, 11:46 AM   #6
alerton
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Default Re: Panasonic TX-L42B6B – LED Single Blink

just in case anyone is interested i have just been messing around with a TX-L50B6B which is basically just the bigger brother. it was given to me as not working, the backlights were just flashing on / off constantly & rapidly but there was no BL_sos / fault.

first thing i did was check the supply to the backlights, on this version there are 2 circuits each supplying 3 pairs of strips. with the BL disconnected the outputs were both fine and exactly the same but with the BL connected both the voltages went haywire. from this i presumed there was a BL issue (it has a LG LC500DUE display). took the panel apart and found 3 dead leds 2 from 1 circuit on 2 different strips and 1 from the other circuit.

anyway i found & ordered a new full set of led strips from china for around £27.

in the meantime since i had it in bits anyway i removed the faulty leds and bridged the gaps on the strip just to double check there were no other problems, it worked and all the other 57 leds fired up fine. so i 'rearranged' all the strips to spread the 'dead' leds as far apart as i could. i was going to have to build the screen back up anyway as i dont have space to leave everything in bits for 2 / 3 weeks waiting for the new Chinese strips to arrive.

now i regret spending the £27 on new strips because to my surprise the picture is totally fine with no sign of any dark / light patches which is what i expected. its been working for over 18 hours constantly so far and everything seems good. i have reduced the backlight level in the settings to around 50% so for now i think i will just use it as it is and when the new strips do arrive i will just keep them ready for if / when there are further led issues in future.

for what its worth i think this tv has a really good picture, reasonable sound and appalling set of connections, only 2 hdmi ??? what was they thinking of .

Last edited by alerton; 03-20-2019 at 11:58 AM.. Reason: poor spelling / grammar
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Old 03-20-2019, 01:50 PM   #7
Unspun01
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Default Re: Panasonic TX-L42B6B – LED Single Blink

It is unfortunate but this TV uses an LG panel with overdriven LEDs in the backlight. It suffers the very common problem of LED burnout in the panel backlight that many LG LED TVs get in the 42/46/50/55" sizes.

Requires complete TV "surgery", but not too difficult if you are careful to take proper and delicate care of the LCD panel itself during disassembly.

Working in a "garden shed" does not instill me with much confidence in the repair, but you should be able to fix it when your LED strips come in.

For future reference, you should not have to pop off the LED diffuser lenses to test the LEDs. This is unnecessary and you will have to re-glue them back exactly in place or else you may get weird light refractions.

You can test LED strips like this by gently scratching away the white coating on the LED strip board to expose some copper trace underneath and then use your digital multimeter to see if the LED tests ok. Some DMMs put out enough voltage in diode test mode to illuminate the LED slightly which will tell you if it is good or not. Any that measure as a short, or do not light (or provide reading at all) should be replaced.

The only thing you need to be aware of is where on the LED strip to scratch away the non-conductive coating for testing... The spots have to correspond to the + and - of the LED itself. Some LED strips have pre-exposed test points for just such reason, but I have not seen many of them that are convenient and often resort to scratching a small spot 1-2mm long for DMM probes.
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Old 03-20-2019, 02:51 PM   #8
alerton
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Default Re: Panasonic TX-L42B6B – LED Single Blink

Quote:
Originally Posted by Unspun01 View Post
It is unfortunate but this TV uses an LG panel with overdriven LEDs in the backlight. It suffers the very common problem of LED burnout in the panel backlight that many LG LED TVs get in the 42/46/50/55" sizes.

Requires complete TV "surgery", but not too difficult if you are careful to take proper and delicate care of the LCD panel itself during disassembly.

Working in a "garden shed" does not instill me with much confidence in the repair, but you should be able to fix it when your LED strips come in.

For future reference, you should not have to pop off the LED diffuser lenses to test the LEDs. This is unnecessary and you will have to re-glue them back exactly in place or else you may get weird light refractions.

You can test LED strips like this by gently scratching away the white coating on the LED strip board to expose some copper trace underneath and then use your digital multimeter to see if the LED tests ok. Some DMMs put out enough voltage in diode test mode to illuminate the LED slightly which will tell you if it is good or not. Any that measure as a short, or do not light (or provide reading at all) should be replaced.

The only thing you need to be aware of is where on the LED strip to scratch away the non-conductive coating for testing... The spots have to correspond to the + and - of the LED itself. Some LED strips have pre-exposed test points for just such reason, but I have not seen many of them that are convenient and often resort to scratching a small spot 1-2mm long for DMM probes.
whilst i agree that theses LG panels (vestel panels too) do seem to have longevity issues i wouldn't necessarily blame the manufacturers for 'overdriven LEDs'. there are settings in the menu to adjust the backlight. some would say that the LEDs should be 'limited' to extend their lifespan but you could say the same for anything, i cant see many Ferrari owners agreeing to having their cars revs limited in order to extend the lifespan of the engine or reduce the chance of them having a high speed collision, just because something can go at full speed / full brightness there is no excuse for using it at full all the time.
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Old 03-21-2019, 01:15 PM   #9
Unspun01
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Default Re: Panasonic TX-L42B6B – LED Single Blink

Respectfully, I disagree.

Maybe I shouldn't blame the manufacturers for "overdriven" LEDs, but I certainly blame them for cutting corners.

For example, instead of installing only 10 LEDs per horizontal strip and driving them harder for more light output, it would be more responsible to put 12 LEDs a bit closer together on the same strip and drive them less aggressively for the same light output. It would also improve backlight uniformity.

Another reason to blame the manufacturer is the poor heat sinking of the LEDs. The real-estate available to these LED strips tucked away inside a large flat-screen panel is practically infinite. The larger a TV is, the more space there is. Why make people suffer with thin strips which are poor heat sinks instead of a 2" wide aluminum substrate? They could use clips or screws instead of crappy two-sided tape (which acts like an insulator and doesn't promote heat transfer). It would be a superior design, with no perceivable weight increase but it would cost a few extra pennies. It would also be less prone to damage also during removal or installation than someone prying or bending a stubborn strip that is tightly adhered with tape.

Have you ever seen a complete hack try to replace tightly taped LED strips? They strip gets all bent out of shape and half of the led diffuser lenses pop off... Not a pretty sight...

Also, I often find the LEDs that have failed are the LEDs that are suspended above a bend or crease in the back of the panel metal so there is physically no way for the heat from the LED to be transferred away to the heatsinking of the strip and ultimately to the back of the panel where the heat should end up in a good heat-sinking design.

The LED ends up suspended over an air gap and cooks itself prematurely.

I understand the need for bends or creases to reinforce the mechanical strength of the panel to resist twisting, but Jesus it's a bad idea when considering thermal performance of the LEDs.

A user should not have to worry about having their backlight setting at the max or min and also have to be concerned that the max setting will destroy their backlight... No margins here... Manufacturer fault plain and simple.
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Old 03-21-2019, 01:58 PM   #10
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Default Re: Panasonic TX-L42B6B – LED Single Blink

I blame the consumer, You all wanted large, cheap, flat panel tv's and you got them.
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Old 03-21-2019, 02:19 PM   #11
Unspun01
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Default Re: Panasonic TX-L42B6B – LED Single Blink

Yeah, I hear you. That's actually funny! (no sarcasm)

But really, I can't blame the consumer either. Not all of them anyway... (Anyone who buys cheap junk brands gets what they get)

But what consumers really want is what was promised by the manufacturers... An energy efficient TV, with 100,000 hours LED life MTBF.

I am quite happy with my older CCFL backlight LCD TV and my Plasma TV. Both are marvels of technology and very reliable.

However, when manufacturers started touting very efficient TVs, with a lifespan of 100,000 hours (yes, one hundred thousand hours), everyone wanted one. And for good reason.

But How many LED backlight TVs have we seen that are nowhere close to these promised numbers?

Exactly. Not one.

5000 hours, 7000 hours? Still nowhere near the promise of 100,000 hours.

I have seen people spend thousands of dollars on LED TVs from "supposed" good name brands like LG and Samsung, only to have those TVs fail in weeks, or months if they make it past the warranty period at all...

There is no excuse for this.

Not everyone was looking for the cheapest crap they could find. In fact, many people thought by spending lots of money they were avoiding such crap. But the truth is LED TVs are a boon for manufacturers, because they can both upcharge consumers and cut corners at the same time thereby boosting their own profits.

LED TVs are a travesty, and I blame the manufacturers.
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Old 03-21-2019, 03:59 PM   #12
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Default Re: Panasonic TX-L42B6B – LED Single Blink

We now have side lit sets. As manufacturers should have realised by now, LEDs fail regularly. IF they wanted us to repair these sets, it would have been an easy matter to incorporate a removeable panel to get to the side leds without removing the screen. I havent seen a set with this feature but Ive seen plenty of these sets in landfill.
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