panasonic tx-55ax630b no backlight

[sordids999]

hello guys

has anyone come across this fault i`m leaning towards a faulty led but just wanted confirmation on this first

symptons:
The back light blinks once but still no picture or back light

voltages on the PSU

SRV 5.1v =4.9 volts steady
SRV 32v =0.0 volts but 32v once tv first turns on then drops to 0
SRV 16V =0.0 volts nothing even when tv is turned on
SRV VLED = 70v once tv turns on but drops to 32v then slowly decreases

thanks

[dick_barton]

Re: panasonic tx-55ax630b no backlight

Is that a pico fuse alongside SRV-16?

[sordids999]

Re: panasonic tx-55ax630b no backlight

I thought that myself and tested all fine

[budm]

Re: panasonic tx-55ax630b no backlight

It looks like you have 6 LED strings so it has 6 power MOSFETs mounted on the heatsink for the LED current return controllers.
1) TV off, black probe on the chassis, Red probe on one of the six black wire of the LED connector, turn on the TV and record the Voltage reading.
2) Repeat the procedure as in step 1 on the other 5 black wires.
3) Report all six readings.

BTW, please provide straight shot picture of the top and bottom side of the power supply board.

[sordids999]

Re: panasonic tx-55ax630b no backlight

P6 led connector has 7 black wires

black pin 1 jumps to 36v when tv turns on then decreases
black pin 2 jumps to 36v when tv turns on then decreases
black pin 3 no reading
black pin 4 jumps to 36v when tv turns on then decreases
black pin 5 jumps to 36v when tv turns on then decreases
black pin 6 jumps to 36v when tv turns on then decreases
black pin 7 jumps to 36v when tv turns on then decreases

pictures attached so im thinking its pointing to a bad led ?

[sordids999]

Re: panasonic tx-55ax630b no backlight

i cant seem to get the pictures to face upwards

[singingfish]

Re: panasonic tx-55ax630b no backlight

Same tv.
Same problem.
After trying changing the 3 boards I gave in and took it to a local repairer.

He found one faulty LED. he changed this out and the TV was fine for a week or so. Today
Apparently this TV will detect a return current outside the tolerances specified for the back lights, shut down the whole set as soon as it detects it and do a slow single red light blink to indicate this. Maybe something to do with the control needed for the fancy adaptive backlighting where it can dim areas for better blacks, I think?

Today the same fault is back. BALL ACHE :-(
I'm now assuming the back light strips just have so many full brightness hours on them that they're going to ping out one by one. I bought the TV as used, not working as a project.

So I'm going to source and replace all the LED strips.
Wish me luck with the big screen and not breaking it.

Matt

[Moreno83]

Re: panasonic tx-55ax630b no backlight

I got the same model and yes a single blink is a LED fault.

Im waiting for the leds to arrive. I bought just the led's and not the strips because with strips you dont know the history......
These models have a plastic lens on top of the LED , it is very easy to get it off and put it back on. ( clicking system ).

Ps , why would you have backlight full on? It is best to set it with every led tv on 50/60%.

[vinceroger69]

Re: panasonic tx-55ax630b no backlight

yes i would replace all the leds like above post who knows how long second hand strips would last unless your thinking of buying brand new led strips.

[LOPT]

Re: panasonic tx-55ax630b no backlight

Did any of you fix the problem by fitting new leds ?

To budm.. what was your verdict on the voltages the OP reported back to you..?

I have the 1 second Red blink. On power-up the psu gets switched off except for SRV- 5.1v.
Service Manual says 1 blink code can be P, A or Panel.
I removed the power board and on the bench fed the 3.3v feedback voltage and all voltages were steady and correct. As the backlight flashed on power-up i thought it must be a faulty A board so changed it. Now the set goes to standby on power-up whereas before it would just blink fast green then single red. I don't usually work on Tv's so any pointers for testing / changing leds would be welcome. Obviously i can discover by doing but advice is always welcome.
SM is very vague but mentions testing panel by pressing set & remote buttons to confirm or rule out a panel or A board fault. I tried but nothing happened..! There must be a better service manual somwhere!

Tony

[budm]

Re: panasonic tx-55ax630b no backlight

Those Voltage readings on the Black wires which are the Cathode return wires look to be perfectly match at 36V which does not make sense to me since we dealing with more than one string so I expect the bad string to show different Voltage reading, the red wires are all tied together to feed the Anode of the LED strings.
Do you have any LED tester to test each LED string?
What Voltages do you get if you perform the same test?

[LOPT]

Re: panasonic tx-55ax630b no backlight

Thanks for the reply and clarifying..
I have a lot of test equipment because i work with radio comms but i don't have a dedicated LED tester as i don't work with Tv's. However i did think last night that i would try to test those strings by supplying the necessary voltage directly from an external supply. I also thought i could do another test by supplying the 3.3v feedback voltage to the smps via a diode to see if i could force a start and maybe then see what the problem is either by magic smoke or partial backlight.
Also thought to temporarily disable the protection circuits signal but need to study the schematic more first..
Was busy all day so only just onto the forum now. Will test those voltages and let you know. I know they will only appear for a second before the sos takes over and shuts everything down.
I'm presuming supplying 36v to the strips in turn will be adequate ?

[LOPT]

Re: panasonic tx-55ax630b no backlight

What Voltages do you get if you perform the same test?

Just got chance to take those voltages this evening...

There are 7 Black return wires on my TX-55AX630B
Measuring the peak voltage was difficult because shutdown was fast so the following may be averages..
They read as follows:-
25v
25v
<4v
25v
25v
25v
25v

All Red wires show 36v.

Supplying an external 3.3v to keep the smps alive worked but the PWM chip initiated a shutdown as i found out..

I have no idea how many l.e.d strips this set has but from what you say can i presume it has 7 and do those voltage suggest one is bad..?

Thanks in anticipation
Tony

[LOPT]

Re: panasonic tx-55ax630b no backlight

HI budm..

Do you have any thoughts about the voltage results you asked about..? (posted above)

Does the 3rd result show a bad string.. or is the <4v as expected..?

rgds

[LOPT]

Re: panasonic tx-55ax630b no backlight

I've now found and fixed the problem.

This models back-light has 7 rows of l.e.d.s
Each row has a left & right string of 7 l.e.d.s. So each row has 14 l.e.ds making a total of 98 for the complete back-light. The left & right strings are wired in series.
7 mosfet drivers power the back-light through 7 pairs of Red & Black wires.

If any Black wire return current is low it means there is a problem on that row (at least one l.e.d has failed).
If the l.e.d is open then the whole string will be down and so you will need to test each l.e.d individually. 20v is enough to test each string initially to find the faulty one(s). I set a power supply to 20v @ 150mA which is enough to light the good strings without blinding yourself.
Replace faulty l.e.d.s with 3v types.

Taking a large panel apart is not for the faint hearted so make sure you support the panel glass and LCD properly. Remove all the countersunk edge screws and then release the plastic surround which is held on with a multitude of catches that will catch again as they did with me so another pair of hands would be useful at this stage. I didn't have this luxury but managed anyway.
If doing this alone it's best to reduce the weight by removing all the boards first and the metal pieces where the stand fit to so you dont damage the pcb edge of the LCD.

Everything is glued in place. The back-light has a White thin plastic reflector glued to the metal chassis which needs careful removal using a sharp knife around the edges and some careful pulling.
Once removed the l.e.d strings can be tested in situ to find the bad one(s).
The strings are also held tightly in place with thin double-sided tape which will tear easily so take it easy and you wont need to renew it. Also be careful because the string pcb is very thin. You don't want to damage it.
Pry off the l.e.d diffuser with a knife.. fit a new l.e.d and before re-gluing the diffuser clean the old glue from both the diffuser and the pcb to ensure the diffuser sits flat otherwise you risk a bright spot on your screen.

The Red standby light 1 second flash every 4 seconds points to this fault although obviously a failed smps could do the same but in most cases suspect the back-light.

Tony

[zomi]

Re: panasonic tx-55ax630b no backlight

Best to redo all the leds, avoids running into the same issue again, mileage varies on replacing only the bad led.

[budm]

Re: panasonic tx-55ax630b no backlight

Good job finding the problem, sorry I was too busy to follow all the threads.

[LOPT]

Re: panasonic tx-55ax630b no backlight

Best to redo all the leds, avoids running into the same issue again, mileage varies on replacing only the bad led.

Yes i agree.. I've bought 100 but it's going to be a tedious job..

Good job finding the problem, sorry I was too busy to follow all the threads.

Cheers.. and thanks for the initial info which got me thinking on the right track

Tony

[BaHa]

Re: panasonic tx-55ax630b no backlight

Hey guys
I have the same problem as Tony!
I just wanted to ask if there is any easy way I could "fool" the board into thinking that there is a good LED strip in place of the faulty one?
I disconnected the faulty on from the connector and tried using 2 small pieces of 24V strips in series, but that is probably not enough current.
Any idea what sort of current is the TV expecting?
Just wanted to prove that this is the only issue before ordering LEDs and striping the TV's back light which is a pain...

Also, if anyone has a link for the LEDs, that will be great! (I'm in Ireland and only found them in AliExpress so far...)

Thanks!

[alfatv]

Re: panasonic tx-55ax630b no backlight

Hey guys
I have the same problem as Tony!
I just wanted to ask if there is any easy way I could "fool" the board into thinking that there is a good LED strip in place of the faulty one?
I disconnected the faulty on from the connector and tried using 2 small pieces of 24V strips in series

Thanks!

What type of 24V strips are you using?
If those are not LED strips from TV, they often use resistors to limit the current, this will not work in your case.
You need 14 x 3V LEDs in series, see post #15. The current is also mentioned there.
If you have LED tester, it's much easier to figure it out.