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-   -   Panasonic TH-50AS700 - Can I rely on LED tester results? (https://www.badcaps.net/forum/showthread.php?t=87620)

senakahks 08-14-2020 04:10 PM

Panasonic TH-50AS700 - Can I rely on LED tester results?
 
Hi, I am new to TV repair and I have Panasonic TH-50AS700 TV to repair. It goes off when I change the HDMI cable or sometimes it goes off itself after some time, say half an hour. Once it goes off, the red bulb blinks continuously (should be a single blink) and can't turn on using the remote. When I turn off from the wall, it works again.

I tested LEDs without opening the case by giving supply of a Chinese LED tester via a ribbon cable. all inputs yields the same voltage. Does that say all LED strips are good?

petehall347 08-14-2020 04:48 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
yes if the voltages equate to the amount of lamps times each ones normal running voltage
example 20 lamps at 3v running voltage =60v . might not be dead on accurate reading on your tester .

Diah 08-14-2020 05:25 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
LEDs function and test depend on currents not voltage.... Panasonic sickness with backlight are the sockets of the strings. many i found they burned out because of the heat,

petehall347 08-14-2020 05:33 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
Quote:

Originally Posted by Diah (Post 981569)
LEDs function and test depend on currents not voltage.... Panasonic sickness with backlight are the sockets of the strings. many i found they burned out because of the heat,

i have only just got one of those testers and it works like i said . say i test one 3v led it reads 3v .. if i test a string of 20 it reads 60v ..if i test a string of 20 with 3 shorted ones it reads about 50v .

budm 08-14-2020 05:40 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
Quote:

Originally Posted by senakahks (Post 981547)
Hi, I am new to TV repair and I have Panasonic TH-50AS700 TV to repair. It goes off when I change the HDMI cable or sometimes it goes off itself after some time, say half an hour. Once it goes off, the red bulb blinks continuously (should be a single blink) and can't turn on using the remote. When I turn off from the wall, it works again.

I tested LEDs without opening the case by giving supply of a Chinese LED tester via a ribbon cable. all inputs yields the same voltage. Does that say all LED strips are good?

Does it stay on longer if you reduce backlights to 50% or less?
The LED tester you have probably puts out 30mA not in the hundreds as in operation in TV.
What inspection and Voltage testing have you done when TV works and when it does not work?
Can we see the pictures of the whole backside of the TV showing all the boards so we can see what we are dealing with?

neilc6 08-14-2020 08:26 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
1 blink code on a Panasonic is bad backlight so you likely have an intermittent LED in the string that is failing at higher current than your LED tester can put out and when it warms up. Take Budm's advice and lower the backlight level to see if it stays on longer. Also turn off any ambient light sensor setting that could crank up the backlight in a bright room.

senakahks 08-14-2020 10:00 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
1 Attachment(s)
It was in 60 Backlight and I increased it to 100. It went off in about 10 min. Now doesn't come on and red light keep on blinking.

An image of the boards of the TV we are working attached herewith.

What kind of reading should I take?

neilc6 08-14-2020 10:42 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
It looks like you caused an intermittent LED to fail completely by turning up the backlight level. Recheck your measurements with the LED tester. A one blink code can be confusing since it looks like its just blinking.

Diah 08-15-2020 09:35 AM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
Quote:

Originally Posted by petehall347 (Post 981572)
i have only just got one of those testers and it works like i said . say i test one 3v led it reads 3v .. if i test a string of 20 it reads 60v ..if i test a string of 20 with 3 shorted ones it reads about 50v .

in reality how would you know the string had 20 LEDs? mean you tear it down till the strings.. LED type and V if 3 or 6 or 4.7 V can know from the label no need this tester at least to me... i cant see the benefit of the tester.

as i said the currents play big roll. and in case if you have shorted LED with the tester other good LED at the strings will lid on ?? if yes.. mean with out tearing down you still have led come on so you cant see the one off... and if NO mean you need to tear it down and do test one by one to terminated the bad one.

petehall347 08-15-2020 02:43 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
Quote:

Originally Posted by Diah (Post 981707)
in reality how would you know the string had 20 LEDs? mean you tear it down till the strings.. LED type and V if 3 or 6 or 4.7 V can know from the label no need this tester at least to me... i cant see the benefit of the tester.

as i said the currents play big roll. and in case if you have shorted LED with the tester other good LED at the strings will lid on ?? if yes.. mean with out tearing down you still have led come on so you cant see the one off... and if NO mean you need to tear it down and do test one by one to terminated the bad one.

everyone has their own ways of doing things .
as i was always told voltage and current are related .

Diah 08-15-2020 02:56 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
dear friend... i know how much you are exact talking. .. thanks

senakahks 08-15-2020 08:48 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
Say if there are 4 output pairs from the PSU to backlight, if all pairs give the same voltage for LED tester, can't we predict that at lease at that particular moment strip sets are working equally? intermittent LED failure are a different case.

budm 08-15-2020 08:58 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
So at this point can you still get the TV to come on and stay on long enough for you to get to the user menu to be able to turn down the backlights level to as low as you can?

In post #1: When I turn off from the wall, it works again.

BTW, can I see good clear straight shots of the whole power supply/LED driver board?

neilc6 08-15-2020 09:35 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
Intermittent LEDs are tougher to track down with a low current LED tester. Clues are slower to turn on, flickering, or reading higher or lower voltage. As indicated before, a basic LED tester puts out around 20-30mA while your TV will run at 300mA or more.

senakahks 08-16-2020 07:34 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
Thank you for very useful explanations. I am planning to replace LED strips. Before doing that. I got reading from backlight connector using the LED tester (after disconnecting from driver).

1st pair: 28.1V
2nd pair: 28.2V
3rd pair: 28.2V
4th pair: 26.4V (about 2V difference)
5th pair: 28.2V
6th pair: 28.3V

The only issue is buying LEDs from this part of the world.

Can we make a dummy set of portable backlights (for different sizes) to make troubleshooting easy since replacing LED is a time consuming process?

budm 08-16-2020 08:18 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
That 4th pair looks suspicious. Try leaving the LED tester on that pair for a long time and see what happen to the Vf reading.
So the strip show about 28.2V Vf, driving the LED at 30mA will give 3V LED Vf of about 2.8V (or 5.6V for 6V LED), so the LED string has either ten 3V LEDs or five 6V (Dual LED) LED's.
If you have CV/CC bench power supply that can go up to 30V Min and min current of 300mA then you do true drive current through each LED string to test, start at 100mA first.

What is the LCD panel model number a shown on the sticker on the LCD panel.

neilc6 08-16-2020 08:46 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
I've found it difficult to find strips for Panasonic models. You can try Aliexpress but you need the part number off the strip. When I cant find strips, I repair the bad LED, move it to the edge in case of bright spots, and turn down the backlights but there is still a chance of repeat failures since all the leds have wear.

senakahks 08-16-2020 09:40 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
I opened the backlight and found that one LED is not working in 4th pair.

budm 08-16-2020 10:01 PM

Re: Panasonic TH-50AS700 - Can I rely on LED tester results?
 
So we can see the LED strips?
How many LED does it have?


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