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Old 12-04-2019, 01:45 AM   #1
Bloopeta
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Default Panasonic TX-55CX400B backlights flashing on and off

Hi peeps, have a Panasonic TX-55CX400B here, guy I got it from said he tried to turn it on other day, and no backlights.

When I received it last night, I plugged it in, and it came straight on and appeared to be working fine, so knowing this would only be temporary, I left it running for a good few hours, and they eventually went off. I tried turning it off and back on again, and this time the backlights started flashing at a steady continuous pace. So I turned it off and left it overnight as it was getting late.
This morning when I turned it on, same problem, backlights flashing, so I stripped it down and tested the LEDs and 1 was dead. So changed the dead LED, thinking that would solve the problem, but it hasnt.
So now even with the bad LED replaced, turn it on, and backlights flash in a constant uniform manor, only difference now is, they are all illuminated. Also you can hear a noise from the powerboard, that runs in time with backlights flashing. However if I unplug the backlights from the powerboard, and switch on, this doesnt happen, it comes on and holds steady.

Ive included a pic of underside of PCB, cant see any dry joints etc, but I have included a close up of a section of the board that caught my eye, just would like peoples opinions on that section please?

Anyone came accross this before? Or could point me in the direction of where to start with diagnostics now please?
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File Type: jpg 20191204_075459.jpg (594.4 KB, 16 views)

Last edited by Bloopeta; 12-04-2019 at 01:59 AM..
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Old 12-04-2019, 06:10 AM   #2
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

The joints in the R551 area look a little funky??
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Old 12-04-2019, 06:14 AM   #3
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

That cap by C514 looks a little suspicious.
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Old 12-04-2019, 06:24 AM   #4
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Check all your diodes.
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Old 12-04-2019, 11:12 AM   #5
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Do you have backlight tester?
If you do, what is the Vf of each LED string?
BTW, the bad LED, was it shorted or open?
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Last edited by budm; 12-04-2019 at 11:23 AM..
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Old 12-04-2019, 12:51 PM   #6
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Quote:
Originally Posted by budm View Post
Do you have backlight tester?
If you do, what is the Vf of each LED string?
BTW, the bad LED, was it shorted or open?
Ive ordered one a couple of weeks ago, bit still hasnt arrived yet

And the bad LED was shorted

Last edited by Bloopeta; 12-04-2019 at 12:55 PM..
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Old 12-04-2019, 12:53 PM   #7
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Quote:
Originally Posted by nomoresonys View Post
Check all your diodes.
Ive tested all diodes with DMM, and this one D516 is raising suspicion, on diode test mode its reading 020 both ways :/ and beeping both ways.
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Old 12-04-2019, 12:58 PM   #8
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Cane we see the bottom side of the board so we can see how that diode is connected in the circuit.
Try lifting one leg off the board and re-test the diode.
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Old 12-04-2019, 01:04 PM   #9
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Quote:
Originally Posted by budm View Post
Cane we see the bottom side of the board so we can see how that diode is connected in the circuit.
Try lifting one leg off the board and re-test the diode.
Here are the pics, ill lift a leg and retest and report back
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Old 12-04-2019, 01:20 PM   #10
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

That diode is part of RCD (Resistor/Capacitor/Diode) Snubber network.
The solder joint on the leg of the cap that is connected to the Cathode of that diode D516 has ring around the joint, that should be re-soldered, look at other solder joints as well for that kind ring forming on component legs.
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Old 12-04-2019, 03:21 PM   #11
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Quote:
Originally Posted by budm View Post
That diode is part of RCD (Resistor/Capacitor/Diode) Snubber network.
The solder joint on the leg of the cap that is connected to the Cathode of that diode D516 has ring around the joint, that should be re-soldered, look at other solder joints as well for that kind ring forming on component legs.
I have lifted 1 leg of said diode, and getting the same reading as before, 020 and beeping both ways. So that will need replacing?
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Old 12-04-2019, 05:29 PM   #12
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

What are the numbers printed on it?
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Old 12-04-2019, 05:39 PM   #13
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Quote:
Originally Posted by budm View Post
What are the numbers printed on it?
HER207

I have replaced it, resoldered the area you pointes out, and went over board and touch up any points that looked remotely like needed it, and hey presto, we have functioning backlights! Gonna leave it now as its 23.27pm here, and ill get it all resembled in the moring.
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Old 04-25-2020, 11:55 AM   #14
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Backlights working but sometimes the backlight will go off
17ips20 low voltages across most caps
Led voltages upon turning on the tv will jump to 155 volts but does not drop
Led - 10v led + 155v led - 10v led + 155v
Any help please

Last edited by sordids999; 04-25-2020 at 12:15 PM..
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Old 04-25-2020, 11:59 AM   #15
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Pictures of the psu
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Old 04-25-2020, 12:05 PM   #16
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Sorry hopefully better pictures
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Old 04-25-2020, 12:20 PM   #17
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Possibly an intermittent LED in the backlight. Did you try turning down the backlight level? Not sure if that model has local dimming as well.
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Old 04-26-2020, 02:34 AM   #18
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

The strange part is if I swap the 17ips20 which as you can see in the picture has taken alot of heat in the top right with another 17ips20 but slightly different I'm now getting a dark patch in the middle of the screen so its possibly pointing to a bad led inside the screen
It's a shame because I cant find any new sets of these leds anywhere
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Old 05-13-2020, 01:43 PM   #19
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Hi guys sorry jump on this thread but it's the only one with my TV
(Tx-55cx400b)
I'm waiting for a few bits to arrive to test properly, the screen was blank so I've done the strip down and checked and I have 3leds out (fingers crossed that is the only fault)
But whilst I'm waiting for items to arrive I've just remembered that some of the spacers/risers/standoffs where brittle and broke so I'm now looking for those, but what the 🤬 are they called🤔 (see pic) please help me.....
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Old 05-13-2020, 09:49 PM   #20
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Default Re: Panasonic TX-55CX400B backlights flashing on and off

Where have they broken ? If its just the attaching lugs i just use dab of hot glue for sticking them down.
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