I just got a sexy, slim, and altogether awesome LCD screen from someone on Freecycle, who said it just turns off after a while. Hm. Well, I brought it home, hooked it up, and it seemed to work great, except for one problem… the brightness control didn’t do anything.

Tore it apart and on the power board (with the inverters integrated), I found one NASTY sight – there was one chip with an explosion-mark on it (like it caught on fire), and the one next to it also looked Xtra Crispy™ but not quite burned. Obviously we have a problem. I looked on the other side of the board and instead of brown like the rest of the board, it was BLACK in that area. Nice and crispy.

I Googled the part number and found that the chips are dual channel (N and P-channel) 35v “enhancement mode power MOSFET” chips. Couldn’t find any sources for that chip, or even a cross reference, so I started browsing chips. Combined SO-8 package and dual-channel, and got a short list of possibilities, all but one are not in stock.

http://www.mouser.com/Search/Refine.aspx?N=4932268+4294741803+4294689029&Ns=P_SField|0

So the question. Would that one 60v chip work in place of the original 30v? Isn’t that just a maximum rating (like a capacitor) or is the circuit actually designed to drive these at those particular voltages?

i just asked him on irc. he says from brief research he’s guessing it wouldnt work. something about switching speed dropping with voltage.

Holy ****, a useful reply… honestly wasn’t expecting that on this forum. Log in to see images!

(It makes sense too)

Well, right now I’m fighting another problem. I pulled off the fried old chips, but forgot that surface mounted **** usually has ****ing dobs of SUPER GLUE under them… serving what purpose, I do not know. But I pulled two pads off the board I need to reconstruct when I get the new part. Meanwhile I was expecting to use two backlights on the lower part of the screen, which should work just fine (yay, LCD screen on my workbench again, instead of a 15-year-old piece of **** CRT!).

Problem I encountered… after putting the screen back in and powering it up, no backlight at all. Euh. I found that one of the pads that got pulled off interrupted a circuit, so I just bridged that back together, hopefully it works. If not, well, ****.

Oshti—...

Yep, the bridge worked… partially. I don’t exactly understand how this ****ing thing works, but I think it was monitoring the outputs of the chips (the drains of the chips are connected together) and maybe the burned-but-not-fried chip was still working, but just very hot from handling the load of the second, dead, chip (which would explain why it shut off after a while, so they said).

Now when I turn it on, the backlight comes on for a second and turns off (leaving the screen still on). It does this quite reliably like clockwork. Turn it on, light comes on, light turns off. Cycle it. Repeat. So to make it work while I wait for a replacement chip (then cross my fingers that I’ll have the soldering skills to put the new chips in…Log in to see images!

I can also source around a little more, for another shop that actually has it in stock…

what the **** tubsweetie i thought you were banned

Yeah… but if that control IC still works (see pic – there are two more of those same MOSFETs below the pic), then I think I might err on the side of “maybe for once, I can just replace the blown part, and the magic smoke will be re-contained” Log in to see images!

And yeah, I’ve got a cold cathode case lighting inverter here… and it seems to work great. What I don’t understand though… why are there four plugs? There can’t be four backlight tubes in this relatively small screen, are there? Or… please don’t tell me… they’re not cold cathode tubes?

The crispy:

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http://hostfile.org/IMGP0161.jpg

The hax:

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http://hostfile.org/IMGP0162.jpg

edit: Euh, it stripped the links from the images… (added links to full images)

Seriously. Friend nao. I’ve never seen anyone else online that even actually knows what I’m talking about, let alone could actually contribute to a meaningful conversation on a higher level and make me feel stupid (in a good way). Can you believe I’m also unemployed? Log in to see images!

Well, I’m now working on putting that inverter into the case with a little power-switch hanging out the top (at least temporarily)... and I’ll power it off the 12v from the power supply. I couldn’t find any circuit that’s actually switched with the backlight (so it’ll control the power by itself), so it’ll have to do. Better than nothing, but not as good as I had h—...

You know something? I just had an idea. How about bridging the second (working) set of ICs to the dead pair so it’ll see voltage and not shut down? I mean, I won’t plug the backlight into that side (so it won’t have the additional load)... hm. It’s worth a try…

edit: while the soldering iron’s heating back up again… grumble grumble. Here’s the whole board, as it is right now (note the ****ty, poorly soldered tiny bridge wire where that toasty chip used to be)

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http://hostfile.org/IMGP0163.jpg

Yeah, but I was sure to wire it right… since they’re in a mirrored sort of configuration, the wires need to cross each other – the chip on the bottom right is the same as the chip on the upper left, and vice versa.

Wired it up like that, and guess what? No dice. Same ****. So, hesitantly, I tried plugging in one of the upper bulb’s wires. Same ****. Plugged both of the upper ones in and unplugged the lower ones (so the lower is now only powering the upper). Yes, same **** (it only powered the upper light for a second, then shut off). Plugged them both in and plugged the power in. It came on and stayed on until I unplugged it a couple seconds later. So yep, it was “working” (limping along) on that one remaining IC. Now I just need to replace them. *facepalm*

I never accept defeat.

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http://hostfile.org/IMGP0164.jpg

Just gotta wait til December for that ****ing IC to come in… meanwhile, I’m using that case lighting inverter to run two of the bulbs (one top and one bottom). It’s a slightly dim, but quite usable picture. And I have to use those two wires sticking out the top to turn the backlight on and off. Not bad for a free monitor, methinks.

I suppose, but I can’t stand using that wire to turn the light on and off. Chances are, knowing me, if the MOSFET replacement doesn’t work, I’ll just hammer away at other solutions until I have a solution that works perfectly.

Kinda like I did with another dead LCD monitor I repaired that had a dead internal SMPS… <.<

edit to add:

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http://hostfile.org/IMGP0165.jpg