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LCD Backlight question (Samsung)

Hey All,

I know a few of you work on repairing LCD tv's and before I toss this set out (it's personal, not customer), I was wondering if anyone ran across a problem where for lack of a better explanation, the backlight for the LCD panel is drawing excess current, even though there is no visual problem with the backlight. The set still has a bright picture, isn't shading one side over the other (or is it top and bottom?) the color temp seems normal all the way around.

It's about a 10 year old Samsung SyncMaster P2570HD.

The long story, over the summer I noticed the smell of "something buring" in the office the set was parked in but couldn't tell where it was coming from. A couple weeks later, about 20 minutes after turning the tv on, the picture went out, the set was still on, still had audio but solid black.

After searching around for disassembly help (snaps mostly, no screws) the problem was obvious, a (probable 2W) resistor was charcol black along with the circuit board and foil traces underneath. The resistor measured around

400K still and as with other Samsung monitors/tv's, the board was populated with those 820uf/25V caps that always seem to fail.

So I replaced all the caps, took a guess at 390K for the resistor, patched the foil traces and fired it up. Worked perfectly.

But, in about 15~20 minutes, the smell came back. The resistor was overheating again.

Off to search for a schematic, which does seem to exist at serveral of the sites, but none of them match. The power supply module is a IP-58155A but all the schematics I found for that part number is actually for a IP-54155B, not the same animal.

But they do show R301 (the resistor that overheats) as a 390K.

Anyway, in the search for more schematics one hit returned an Ebay auction for the exact power supply module (IP-58155A), $35 with free shipping, sold american.

The module came a few days later and appeared to be brand new, really. No indication it was ever used or repaired.

Put it in and as you can guess, 15~20 minutes later, that burning smell came back.

Looking at the foil traces, one side of it (R301) is directly connected to the connectors for the lamp feed, where neither of the online schematics shows that. With the two connectors for the lamp assembly disconnected, the resistor is cool as a cucumber with the set running (duh).

So that is what leads me to asking if the lamps can possibly be drawing excessive current, even though they seem to be fine with brightness and function?

I mean I know the repair would require replacing the whole panel which isn't worth it but I'm curious if there is anything else in the panel besides the lamps that can cause that resistor to overheat.

Normally something that large (2W) would require a dead short (or close to it) for it to turn to charcoal like the original.

Any guesses?

-bruce snipped-for-privacy@ripco.com

Reply to
bje
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Have you done anything with the settings? Most TVs come from the factory on "display mode" with all settings at maximum. Which, of course would max out the power-supply.

Find the "Settings" mode and crank down the brightness and balance the colors to something more realistic.

Note: Depending on the design of the unit, you will have to reset each time there is a power-failure of more than a very few minutes. They should be checked every so often, anyway.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

Hmmm, I dunno about this, it's one thing to max out a power supply and another to cause parts to smoke and burn. Me thinks Samsung would have a few lawsuits to deal with just because someone enjoyed "vivid".

The set has had a hard life and there is really no tears here. Like I mentioned, it's been in use for the past 10+ years, usually on for 12-15 hours a day (usually background noise).

The only reason it's not in the trash yet is 1) it's a native 1080p in a small package (25") which doesn't seem to be made anymore and 2) it has just about any video input you need, hdmi, dvi, vga, component and composite.

The sad part about all of this is I replaced it with a 32" Sanyo that Microcenter was blowing out for $49.95, less than the parts I spent trying to fix the Samsung.

-bruce snipped-for-privacy@ripco.com

Reply to
bje

That would be the fallacy of "Leaping to Conclusions", with a smattering of "begging the question" thrown in.

Try the settings, and see if that does not cool things down. If not, you are no worse off. If so, enjoy it for a few more years.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

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Bad lamps. Most CCFL inverters shut down when a defective lamp is encounte red and I am very surprised this one doesn't. It won't be a shorted lamp bu t a lamp(s) that are weak and require more voltage to deliver the current t he inverter transformer is designed to deliver.

That board has a single output transformer, but it has two secondaries. A way to check this is to turn the TV on and put your scope probe *NEAR* the output harnesses or connector and watch the waveforms. Use a high vertical input setting and a fast horiz setting. Make sure to place the probe in th e same relative position to the connector on both sides as the physical pla cement of the probe changes the amplitude greatly.

Don't connect the probe electrically to the inverter unless you were planni ng to buy a new scope anyway...

Now, one of the two waveforms will be much higher than the other, maybe twi ce as high. The actual amplitude isn't important because you're only lookin g for differences between the two sides, not taking p-p measurements.

Mark which side is higher, then see if you can switch the two harnesses and try again. You'll probably have to flip the board over to do this so make sure you put down an insulator between the board and TV frame because thos e heatsinks could be at hot ground potential.

If the side that was higher is now lower and vice versa, you have an open l amp(s) in the display. You might be able to find CCFL tubes on line to rep lace them.

Reply to
John-Del

Thanks for passing along the info and it makes sense to me.

I didn't know the bulbs could go bad like that (even though when the set is on, the picture looks quite good with brightness and color temp).

I really doubt I'll proceed any further, I tried that lamp replacement once with a laptop and it didn't go well. Having all the different video inputs is handy but that really is the only thing going for it.

But since vga and dvi connections are becoming rare what is the point.

thanks again,

-bruce snipped-for-privacy@ripco.com

Reply to
bje

FWIW, lamp replacement on that display would be much easier than on a laptop display.

The big thing is to avoid damaging the ribbons between the LCD and the address board. They're easy to damage but also easy to not damage if you're just aware of them.

Relamping that old boy may allow another 10 years. Every TV hitting the market has a shorter life than the one it replaced. That's a fact.

Reply to
John-Del

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