Help Diagnosing Backlight Voltages (JVC LT-50C550)

[sickboy]

Wondering if anyone can help me understand the diagnostic process when deciding if the backlight supply is faulty or the backlights themselves.

This is a JVC LT-50C550 which uses a TV5001-ZC02-01 power supply. The symptoms are just a brief flash of backlights at switch on.

Voltages at switch on both white wires (to chassis ground) spike at 138v then settle down to 95v. Voltage on both red wires (to ground) are 45v.

Strangely there is zero volts when measuring between white & red wires on either or both pairs. Can anyone give me an idea what's going on?
Specific help on this particular tv is welcome, but I'm more trying to understand the thought process behind deciding whether the fault lies in the LEDs or the supply.

[neilc6]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

Most TV power supply LED driver circuits (Samsung is an exception) will detect over or undervoltage caused by shorted or open LEDs in a string, and shut down the backlight if a problem is detected. With your power supply, the leds have two wire pairs so likely wired as two separate strings. Googling LED strips for your model shows your TV has 12 strips of 6 leds each so total of 72 leds. Each string should have 36 leds. With 2.7V drop across each led (some use 6V leds), each string should have approx 97V of voltage drop which can be confirmed with an led tester.

I just got a UN58H5202 in and measured 79V on both strings with my led tester. The expected voltage is 113V based on number of leds so there are approx 12 leds shorted on each string just based on measurement.


https://tvledbars.com/product/replac...c14-01a-12pcs/

[sickboy]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

Brilliant, thanks Neil, so does that mean with each of my strings boosting to 138v, then settling back to 95v that my power supply is working correctly and I have an open or short circuit LED?

I usually just see no backlight and go straight for opening the screen and testing the led strips, but I'm trying to be a bit more methodical about it.

[sickboy]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

Anyone?

[dick_barton]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

If the set is not powering off, you could check to see if there's an image on the screen by carefully angling a flashlight on it.

[budm]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

What do you get if you only have one LED connected one at a time?

[sickboy]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

Thanks guys, I won't have access to the TV again till tomorrow but will let you know. Appreciate the advice!

[sickboy]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

What do you get if you only have one LED connected one at a time?

Hi sorry for the delay. There's no change with either one of the plugs disconnected, still spikes at 138v then settles down to 95v
Does this indicate that the supply is running correctly?

[sickboy]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

Update....

Decided to dismantle to screen to check the LEDS and managed to tear one of the tab bonds by accident... TV is now junk.

Just for the record though I went ahead and tested the strips, and every single one of them had at least 1 led not lighting up, some had as much as 4 or 5 failed leds

[neilc6]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

Normally in tvs with protection circuits, I usually find 1-5 bad leds. In tvs without protection circuits (Samsungs), 50 to 90% will be bad. Measuring with a led tester from the back, it is not uncommon to only measure 3V on a whole string.

Here is the UN58H5202 I mentioned before. There would be more LEDs out if they didn't cause the FET to fail in the power supply. Picture mode was set to Dynamic (100% backlight).

[elpiko]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

but what if you tested the strips and all lighting up correctly but you still get the volatge drop. even when the leds are not connected i get the voltage spike and then drops

[dick_barton]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

See your other post.

[Ukdigger]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

My TV shows the JVC logo for a couple of seconds and then no picture. There is no sound and I can't see anything on the screen using a torch in a dark room. When I test the LED strips with a tester they read 99.5 and 99.8. If I plug them back in and start the TV it works perfect all day untill it is switched off for a period of time and then it won't work untill I repeat the LED testing again. The markings on the main board say the LED curcuit should be 94v and I am only getting 92.2v. I would apretiate any help you could give me.

JVC LT50C550

Main board TV5001-ZC02-01

Voltage readings of LED strips 99.5 and 99.8


Voltage readings When working with disconected LED's

Con201
12
12
g
g
g
5
5
5
2
5
3
0.1
g

con5
92.2
4.8

con2
92.2
4.8

Voltage readings When working after testing LED's with LED tester

Con201
12.3
12.3
g
g
g
5
5
5
1.9
5
2.9
1.9
g

con5
117.5
13.4

con2
117.5
15

[dick_barton]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

.

[dick_barton]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

If there are 10 strips of 6 leds per strip then the driving voltage would be approx 60 x 3V = 180V or 90V if wired as 2 sets of 5 strips.
117V would seem quite high if that above was correct.

Edit
Looks like it's depandant on the screen model if it uses 10 or 12 strips of 6 leds?

[Ukdigger]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

Is there anyway to tell from the screen model number? V500HJI-PE8
MC5186082400761

[dick_barton]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

It look as though with that panel number its 12 strips of 6 leds
I think that you have an LED problem. The led tester power the leds with reduced current possibly warming up a bad joint so that when reconnected to the power supply all appears well.

[Ukdigger]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

Thanks, I didn't want to take the panel apart unless if the problem was elsewhere. It will be my first time doing this on an LED TV, I've only done 26 inch an LCD before. I have watched videos on Youtube and it look like a fun job. I only paid £30 for the TV so I'm not too worried about it if anything goes wrong, just got to be careful with the screen and its connectors.

[Shotgun25]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

It look as though with that panel number its 12 strips of 6 leds
I think that you have an LED problem. The led tester power the leds with reduced current possibly warming up a bad joint so that when reconnected to the power supply all appears well.

You are correct, I've got this TV in right now with failed and faulty backlight LEDs and the Panel is V500HJI-PE8 with 12 x6 LED per strip backlights.
The wiring for the LED strips appears to be 1 connector wiring supplies one side of the screen top to bottom, so 6 strips of 6 LEDs.

I've extracted the firmware with an external programmer from the TSOP48 NAND. Do you happen to know if the firmware for these SMART TVs contain anything that would need removing before sharing the firmware?

If it's safe to share as-is, where do you recommend I post it/which thread to add it to?

[dick_barton]

Re: Help Diagnosing Backlight Voltages (JVC LT-50C550)

You can share on the "EEPROM Dump Thread" which is the first thread on the forum.
If the file is too big to upload then you can Winrar it into smaller chunks.