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Old 02-21-2018, 04:02 PM   #1
finntricky
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Default Panasonic TX-48CX400B low LED volts

Hi guys
hav'nt been around for a little while, but just got this panny/vestal in.
tv turns on and can see pic with torch, and it did have a brief led flash, but no longer,
17IPS20 power board
now longer flashes the LEDS on boot, checked volts, and LED voltage is steady at 79v, it did max at 180v once while testing, is it possible the the driver chip has give up ? how can i test it, all other voltages seem to be correct,
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Old 02-21-2018, 10:25 PM   #2
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Default Re: Panasonic TX-48CX400B low LED volts

So if the TV is off, put the LED connector back in place, and then you attach the meter probes to the negtive leg and positive leg of either one of the two 56uF 160DC cap and then turn on the TV, you do not see the Voltage jumps up 10 180V any more?
If you did have the LED connector in place when you get that reading of 180V, it is telling me that you have open circuit in the LED string some where.
When the LED power supply does not see the load the output will go up to the max which will then trip the over Voltage shut down then all you get will be the feeding Voltage that feed the boost converter circuit, in this case, it is around 80VDC.
It can also be that the BL-ON is not present to turn on the backlights driver circuit.
So lets verify for sure if the boost converter is working or not.
There are many threads about this power supply board but there are many version of this 17IPS20 PCB which can be stuffed in may ways to make a bunch of different PCBA's.
That barcode is the number of the PCBA.
Is it 23253638? or 23253538? I have a hard time reading all the numbers on that barcode.
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Last edited by budm; 02-21-2018 at 11:18 PM..
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Old 02-21-2018, 11:41 PM   #3
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Default Re: Panasonic TX-48CX400B low LED volts

thank you, i did'nt even think about getting BL ON from main board, so i will check that.
the LED plug was in while testing, but still remained at 79v with no increase of volts,
had a read of the threads on here and also watched a few vid's on you tube,
before asking here for some inspiration, but i think something has sorted and can't find it at the moment, but i'll see if i have the signal from the main board when i get home tonight so i can rule that out.
i can't believe i did'nt think of checking it.
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Old 02-22-2018, 11:15 AM   #4
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Default Re: Panasonic TX-48CX400B low LED volts

Quote:
Originally Posted by budm View Post
So if the TV is off, put the LED connector back in place, and then you attach the meter probes to the negtive leg and positive leg of either one of the two 56uF 160DC cap and then turn on the TV, you do not see the Voltage jumps up 10 180V any more?
If you did have the LED connector in place when you get that reading of 180V, it is telling me that you have open circuit in the LED string some where.
When the LED power supply does not see the load the output will go up to the max which will then trip the over Voltage shut down then all you get will be the feeding Voltage that feed the boost converter circuit, in this case, it is around 80VDC.
It can also be that the BL-ON is not present to turn on the backlights driver circuit.
So lets verify for sure if the boost converter is working or not.
There are many threads about this power supply board but there are many version of this 17IPS20 PCB which can be stuffed in may ways to make a bunch of different PCBA's.
That barcode is the number of the PCBA.
Is it 23253638? or 23253538? I have a hard time reading all the numbers on that barcode.
23253538 is the correct one
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Old 02-22-2018, 12:43 PM   #5
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Default Re: Panasonic TX-48CX400B low LED volts

checked what i think is the BL_ON off pin 5 and get 3.28v when i turn the set on, still only get 79v at LED plug, but im getting 390v on the 3 main caps, 47uf 450v
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Old 02-22-2018, 01:21 PM   #6
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Default Re: Panasonic TX-48CX400B low LED volts

checked what i think is the BL_ON/OFF on pin 5 power board to main board connector and get 3.28 volts on turn on, and holds steady,
the 3 big caps rated 450v gives me 390v, so if i have the right pin, and the 3.28v is right, then the LED circuit is being told to turn on, but still only get 79v at LED plug, even when plug connected,
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Old 02-22-2018, 02:10 PM   #7
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Default Re: Panasonic TX-48CX400B low LED volts

It would appear that you have faulty LED/s backlight strips.
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Old 02-22-2018, 02:21 PM   #8
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Default Re: Panasonic TX-48CX400B low LED volts

sorry ignore as wrong psu number

Last edited by vinceroger69; 02-22-2018 at 02:23 PM..
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Old 02-22-2018, 02:30 PM   #9
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Default Re: Panasonic TX-48CX400B low LED volts

So far the PFC Voltage booster is working because you are getting 390VDC on the main filter caps which also means the PS-ON is present.
The key point right now is that we are not 100% sure if pin 5 of the connector is BL-ON/OFF or not.
Can you identify the PS-ON pin.
And If I remember correctly, some of these VESTEL boards use ACTIVE LOW for the control signals, but no matter what it uses, you should see the signal toggles from one state to another state when TV is toggle on and off.
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Old 02-22-2018, 03:23 PM   #10
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Default Re: Panasonic TX-48CX400B low LED volts

i used this sch, not R9 version but R6, as that is all i could find, pin 5 volts are 0v when tv is off, and 3.28 when on, the problem is with these boards is there are not any volts or markings on them to say what they are or should be
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Old 02-22-2018, 06:11 PM   #11
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Default Re: Panasonic TX-48CX400B low LED volts

I have that diagram also but after looking at you board and the schematic, they are really that close to what you have on the board.
But one thing you may be able to do is to look at the LED driver IC to see if it uses the same one as used in your board or not, the schematic show U500 as MP3394, if it is then you can check the VCC pin and the EN (BL_ON/OFF) pin to see what you have.
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Old 02-22-2018, 09:38 PM   #12
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Default Re: Panasonic TX-48CX400B low LED volts

BTW, you need to check those two diodes by the two 56uF 160DC caps that I had you measure the Voltage earlier, check the RESISTANCE of those two diodes to see if they show low resistance readings in both direction or not, if those diodes conduct both ways then you will not get the Boosted Voltage.
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Old 05-27-2018, 08:45 AM   #13
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Default Re: Panasonic TX-48CX400B low LED volts

Main caps 47uf 450v reading 327volts
CN4 readings
Stby =1.94 volts
+5stby=0.00 volts
12v =12 volts

Very strange this one I can't seem to control the tv with remote
And voltages on 56uf caps voltages was good but after removing the psu now no reading
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Old 05-27-2018, 11:20 AM   #14
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Default Re: Panasonic TX-48CX400B low LED volts

Sorry ignore first reply from me had issues with button and remote to turn on tv which has been resolved
Exactly same results as finntricky posts and brief flash of backlight once turned on plus voltages dropping on led pins
So I'm 99% sure its a bad led but I'll update this post once I've stripped it down

Thanks
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Old 05-28-2018, 08:06 AM   #15
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Default Re: Panasonic TX-48CX400B low LED volts

With these large screens where once I get in to replace LEDs, I like to replace them all with new ones and so do not have to worry about LED failure in the near future
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Old 05-30-2018, 04:50 PM   #16
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Default Re: Panasonic TX-48CX400B low LED volts

Update ##
Bad led inside the screen and omg what a nightmare
The plastic supports that hold the screen are supposed to just twist but are glued !!!
And everyone one of the 66 led lenses fell off maybe too much heat dried the glue up !
So here I am now one by one glueing them back on so see you next year
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Old 01-16-2019, 08:34 AM   #17
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Default Re: Panasonic TX-48CX400B low LED volts

Bringing back an old thread because it has the same panel and TV model as I have here.

Just to confirm, PSU is a 17IPS20 - 23253538
TV Model is TX-48CX400B

Initially it had no backlight, I stripped the TV and checked all LED's in the screen illuminated as they should using an external PSU. Both 'groups' of LED's ran fine at around 90 VDC each.
I eventually replaced the U500 (MP3394S) which did seem to cure the fault for a very short while, the backlight fired up and worked for a minute or so them again it went out. I spend a while doing cold checks based on a variation of info I found online but to no avail, I assumed there was some other fault elsewhere on the PSU that took out U500 so I gave up and ordered a replacement PSU.
The replacement PSU arrived, I fitted it and sure enough, the same problem as when I first changed U500 on the original board, it fires up for a minute or so then the backlight dies.
I went on to measure the voltages to the backlight, As the set if starting up (No backlight yet) I get around 80volts to each backlight 'group' this then rises to around 120volts as the backlight fires up. After the backlight dies, again it drops to 80volts.
What I've noticed, while keeping U500 cool with a stream of cool air the backlight stays on, however after doing various tests and measurments eventually without cooling U500 failed again, I was back to no backlight at all. I changed back to the original PSU with the new U500 (MP3394S) and again, the backlight would come on but after a short while (U500 heating up) the backlight would go off again.
Now, I measured the current to each group of LED's in the backlight, at 90volts (Using an external supply) one grouup pulls around 80mA and the other group around 150mA. Raising the voltage up to 120 volts as to match the TV's internal supply both groups pull in excess of 300ma. This I suspect is why the MP3394S is failing? They are rated at 200mA as far as I can see.
Does anyone have any idea where I should look next as this is taking more time and U500's than I can afford to throw at it.

Thanks.
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Old 02-24-2019, 09:49 PM   #18
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Default Re: Panasonic TX-48CX400B low LED volts

its not normal for the mp3394s to heat up if you have the schematics try to check all components surrounding the ic. also check the OVP circuit if you have an led backlight tester with current display let it fire up all the LED backlight from 80volts to 120vdc and let it lit for about 30min to an hour to isolate the problem that the LED backlight is not the culprit. my experience is 80vdc to 120 vdc current read out of 300mA max. it can pump up to 48 watts thats ok. there is always a possibility of shorting led when it reach the normal voltage so this test will surely catch it.check also if it has mosfet connected to the IC. hope you fix it just try and don't over see the obvious.
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Old 05-27-2019, 07:42 PM   #19
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Default Re: Panasonic TX-48CX400B low LED volts

Hi Finntricky

Did you find the fault on the 48cx400b?
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Old 05-29-2019, 10:40 PM   #20
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Default Re: Panasonic TX-48CX400B low LED volts

Quote:
Originally Posted by thuss View Post
Hi Finntricky

Did you find the fault on the 48cx400b?
sorry thuss, can't remember if i fixed it or sold as parts,
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