LED TV Back light inverter

You need a new Google

Sold out though, and so is everywhere else I have found

Have you actually checked that the LED strips themselves are not shorted? I tend to find that is the most common failure mode - one goes short and then brings down the whole inverter PSU.

I normally test each in isolation with two or three PP3s connected in series, and a current limiting resistor. Power each strip in turn and you often find one that does not light.

(sometimes close visual inspection of the LEDs will show one that looks overheated - just taking it off the PCB strip can often get a strip working again (with a slightly dim area on the screen))

Good shout.

I think I had ruled the strips out as I'm sure the board shuts it all down with the LED strips disconnected from the inverter.

I'll go back and have another look

What size resistor do you normally use

cheers

They quite often shut down when they detect a load that is too far out of expected load. So open circuit will often do it as well.

I normally go for something to limit current to something to say 10mA. So a 1/4 watt 2k7 should be ok if using 3 batteries.

This site has some pictures of the unit.

File Type: jpg IMG_2650.jpg (506.3 KB, 3 views) File Type: jpg IMG_2651.jpg (462.3 KB, 3 views)

It doesn't particularly look like an inverter. The inverting is done on the PSU, creating DC voltages. The signal BK_ON (IMG-2650) might be a PWM signal so that the backlight intensity in all zones changes at once. I'm not a TV repair man, so this is a guess.

It could be that the mainboard isn't sending BK_ON for some reason.

Paul

That's quite handy - it looks like yo can make contact with all the strips independently from that connector without needing to take the actual screen apart to get at the backlight strips.

LED driver would probably be a better term

(Inverters were typically found on older style LCD backlights that used CCFL tubes, and typically needed >= 600V)

It looks like it outputs 24V to drive a string of LEDs - so possibly 6 LEDs per strip.

Possibly...

Looks like it is a top bar with many LEDs on it - presumably split into groups

Do we think given the markings on the board that I go between LED1+ and LED1- then LED2- then LED3-, then onto LED2+ and LED4- then LED5- then LED6-

I'll report back

Pin layout for interest

LED2+ NC LED6- LED5- LED4- NC NC NC LED3- LED2- LED1- NC LED1+

It's a bizarre wiring plan.

We are given eight wires for six groups of LED.

Your guess is a good one. I think, just based on "wire posture" in the overview picture, that the end wires are returns for their "group". So LED1+ suits three strips of LEDs. and LED2+ suits the other three strips of LEDs.

Apparently there is some sort of booby trap with those LEDs.

I usually stop at taking panels like that apart :-) Because I don't have the bomb squad training to know what awaits inside... I've ripped the lids off a few integrated circuits in my time.

Paul

Sounds plausible.

I have taken a few panels apart - you normally have to start from the front and and work your way down through the layers. Normally disconnect the flexi connectors from the tcon boards early on. Take off the front bezel (screws into the edge normally), then the protective front layer (if there is one), then the LCD itself etc. Then you go down through layers of diffusers, and a nice freznel diffuser, a semi opaque acrylic sheet or two, and eventually you usually get to the metal backplate with a white reflector covering the inside of it, and the LED "lamps" (usually a surface mount LED with a lens bonded to the front of it) poking through holes in the reflector. Normally you need to get the reflector out to be able access the LED strips which are usually stuck with double sided tape to the inside of the metal box. Quite often there are a bunch of plastic clips that need some combination of pushing and twisting or sliding to get out of the metal, and to free the reflector. (typically quite fragile and easy to snap - but then again - not that critical if you lose a few in the process.

Couldn't get any sort of light out of it, nor could I find any configuration of wiring to actually pull any mAs so I might go back to looking at the LED driver board and lift a few capacitors to see if any have gone bad.

When you plug the board in there is a good flash and arc so am wondering if that is more likely the problem.

The back light is a 53GT-AZ420V600 - a quick google shows the thing. It sits at the top of the panel and presumably shines down, there may be one at the bottom, but it isn't obvious.

Not essential that I fix it, but the bit's between my teeth now.

With the board unplugged and using a torch the rest of the TV seems to be working (no Arial plugged in, but I can change channel and see the number on the screen or bring up the menu)

Thanks for all the suggestions so far

There is apparently a board here for sale at £7:

There's a picture here. The connector has four wires. There seem to be two connectors, one with a red dot, one with a black dot.

It might be edge lit with just one strip.

You have the option of testing the LEDs one at a time. My digital multimeter, on the "Diode" range of ohms, puts out around 2.7V or so at a few milliamps, and I could use that to light one of my white LED indicator lamps (not a high-power LED). The polarity is marked on the PCB, so you should be able to get the polarity right on the first try. You would test the LED strip, with the power connectors disconnected.

Paul

All sold out unfortunatly

Might have to dismantle further to test the LEDs.

Had a check of the board tonight, no dodgy Capacitors, but one Diode was shot and the other although kind of working was quite high resistance on the conductive path. Ordered a couple of new ones, but wonder what other damage has occurred, they are fairly high power (3A) diodes (SB3B0).

Looking at options if I test the strip and it is OK, but I can't get a new driver board I might have a punt at something like this

Thanks to everyone who helped.

The issue was the driver board, unable to get a direct replacement so had a punt on a universal board from ebay £4.50ish. had to swap a couple of wires round to match the sockets, but it's up and running now.

Quite pleased!

Well done!

Cheers, T i m

Did you check Ebay for a used spare? I've fixed a couple of TVs economically doing that. New boards were prohibitive.