42LG50 no backlight

[tom66]

Re: 42LG50 no backlight

Check the cables to the backlight are properly secured.

[Audioplus]

Re: 42LG50 no backlight

Will do, thanks Tom, will let you know late tomorrow!
Dan C - Audioplus

[dpal]

Re: 42LG50 no backlight

Be sure to check the fuse that member rb0746 posted on the first page. I have a bad fuse on the master inverter as he described. I replaced the fuse and the TV works. Time will tell. All my voltages seemed correct but I had missed the fuse .75A/76V fuse F1 color black not white.

[budm]

Re: 42LG50 no backlight

Yep, a lot of people overlook at this type of fuse thinking it is surface mounted resistor. This fuse is used a lot in Chi-Mei (CMO) inverter board.

[dpal]

Re: 42LG50 no backlight

Having no schematics for inverter boards tends to flat-line my learning curve on LCD TVs. This forum helps so much to keep me enthusiastic while trouble shooting.

[budm]

Re: 42LG50 no backlight

And if you want to see some pictures of what usually go wrong with TV's, Monitors, electronics, you can see them here for example:
http://s807.photobucket.com/albums/yy352/budm/SAMSUNG/
Main albums with sub albums on the right:
http://s807.photobucket.com/albums/yy352/budm/

[Solipsist]

Re: 42LG50 no backlight

I'm having the same issue as Torpex505. Plug the TV in, I get the red standby light. I press the power button and the blue light comes on, but no backlight on the screen. The flashlight test does show the menu on the screen, so it seems like just a backlight issue. Caps all look visibly ok. I also checked to make sure the 4 white fuses on the master invertor aren't open, but just noticed dpal's mention of the black fuse, which I'll test in the morning (near my bedtime here).
Torpex, did you ever find a resolution?

[dpal]

Re: 42LG50 no backlight

The black fuse was the issue. I studied this TV for a couple of weeks off and on overlooking that fuse. The voltage measurements going in and out of the master inverter was not enough for me to diagnose the problem area. I did replace the inverter caps as well and ended up using a pico axial fuse soldered carefully. It is still working great.

[Dgtech]

Re: 42LG50 no backlight

SOME SIMPLE BASIC ELECTRONICS ADVICE:

A fuse blows whenever it's max current rating is met by the circuit it is trying to protect. If a fuse blows in a circuit, it is because something in that circuit is drawing more current that it is supposed to from the power source. A lot of times this comes from a shorted component, or shorting component, that has a decreased resistance from it's original state causing a path for more current to flow to ground.

If you change this fuse out and find that the set works, the problem may still be there waiting for it's chance to take out another fuse. There are some instances where a fuse may just be weak. I usually see this in much, much higher voltage situations.

[Dgtech]

Re: 42LG50 no backlight

By the way, depending on what inverter board you have, there are some replacements on ebay that will save you a whole lot of time. You will find these by putting in the board numbers or the TV set model number into ebay search. I seen these boards range between $10 and $50. To be real with you - I only replace boards now for the most part. If I cannot find the bad component within 15 minutes, I replace the board. These boards are getting cheaper and cheaper.

[tom66]

Re: 42LG50 no backlight

Dgtech, not necessarily true about a fuse.

Fuses are thermal devices. They usually consist of wire that breaks when a certain temperature is reached. Now, they do not blow at their rated current -- in fact, often it takes two or three times that current to fail within one second. Fuses are rated as being time delay (they take around 30 seconds to one hour to blow), or fast blow (they'll blow within a few seconds at the rated current), as well as other categories such as anti-surge or very fast blow. BUT, if your fuse blows at, say 1 amp in 1 hour, it can also blow at 0.5 amps in 4 hours! It's an inverse square rule. (Note -- The characteristic tends to be inaccurate at low currents -- a 1 amp fuse will essentially indefinitely withstand half an amp or so. But it gives you a good idea about how the fuse behaves.)

In many cases, these fuses simply fail because they have run at a continuous current for too long and thermal fatigue has caused them to fail.

It is very rare for components to fail partially leaky, such that the fuse would blow now and again, and if they did, it would usually result in a lot of heat very quickly. For example, say the device did draw 1.5A at 24V continuously, enough not to blow the fuse. It would dissipate 36W of heat. For comparison, a typical small IC on that inverter can safely dissipate about 1W of heat before failure. So, if a fault were to cause 36W of heat to be dissipated, it would likely result in flames.

I do sometime board swap, but for something like a fuse, I'll replace that first and not worry about it. I might increase the current rating slightly; there is no harm in doing this, as the fuse is simply designed to prevent short circuits, not leaky current draw.

[Solipsist]

Re: 42LG50 no backlight

Thanks for the info. I somehow missed that the black fuse was the OP's issue.

I can simply put the leads of an ohmmeter on each side of the fuse while there is no power to test if the fuse is open or not, correct? I've been a bit hesitant to work near the inverter boards with the TV energized.

[tom66]

Re: 42LG50 no backlight

You should test the resistance of the fuse when the TV is off.

A good fuse will read less than 2 ohms on most multimeters (the 2 ohms account for the resistance of test leads.)

A bad fuse will read "OL" or a single "1" on the 200 ohm range. It may read a few hundred kohm or a few meg ohm but it is still blown.

[Solipsist]

Re: 42LG50 no backlight

So, the 4 white ones measure between .3 to 2 ohms, so they are good. That one black one, when I put the leads across it, starts around 15 M ohms and slowly goes down until it settles around 9 M ohms. Is this what you were saying would happen if it's bad?

[selldoor]

Re: 42LG50 no backlight

So, the 4 white ones measure between .3 to 2 ohms, so they are good. That one black one, when I put the leads across it, starts around 15 M ohms and slowly goes down until it settles around 9 M ohms. Is this what you were saying would happen if it's bad?

Yes it is bad.

[Solipsist]

Re: 42LG50 no backlight

Thank you. You guys have been very helpful.
The bad news is that I can't seem to source this board (1420H1-20D-Master) on ebay (or anywhere else through a simple google search). I'm only seeing the slave inverter board, not the master. I suppose I can try to source the fuse itself and perhaps get lucky that nothing else is shot.
DPAL - do you recall where you found that same fuse?

[tom66]

Re: 42LG50 no backlight

It is bad... but that is good . Replace the fuse. Only if the replacement blows should you look for a new board.

[Dgtech]

Re: 42LG50 no backlight

It is bad... but that is good . Replace the fuse. Only if the replacement blows should you look for a new board.

LOL, thanks for all of the technical aspects to engineering a fused circuit. I was trying to explain it in simple terms for the described situation to someone who may not have known. All that and we came to the same conclusion, replace the board if it blows again.

[Solipsist]

Re: 42LG50 no backlight

Since there are no real local stores that carry electronics supply/parts anymore and I don't really want to wait a week or more to order something with the chance it won't work (and not have a TV in the interim), I ran out to Radio Shack to see what I could find. Picked up an in-line fuse holder and a standard fast-blow glass fuse rated 3/4 amp. Since the fuse holder wires were a bit beefy for the surface mount soldering, I also purchased some 22 awg wire to transition from the lands to the fuse holder. Put it all in place and did a quick test before taping the holder inside the cabinet. I now have a functional TV again! And just in time for Dexter and The Walking Dead tonight. I'm considering ordering a few SMT replacement fuses for a permanent fix. I see Mouser has some that seem to fit spec in stock.
Thanks for everyone's input on this problem - it's much appreciated.

Oh, and pics of the jury-rig:

[tom66]

Re: 42LG50 no backlight

Well, since you know the holder works... you can switch to using a proper fuse. Nice rig!