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Strange Thinkpad T41 Video problem

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Here are the facts:
My TP T41 LCD is dark. Shine a light on it and you see dim video.
When plugged into an external monitor I have a Great picture. (Indicates a good video chip)

I took the entire panel off and put in on another T41 and it works great. (indicates a good LCD, inverter and backlight)

Re-install on original laptop and it's dark again.

When I hold my frequency meter near the inverter there are no oscillations, on the working system I measure 68.2 Mhtz.

If I manually operate the lid close button, both the LCD and external monitor go dark. I release the button and the external monitor returns to a nice bright picture. The LCD returns to the very dim picture. (indicates a good lid switch and proper sleep functions)

I'm STUMPED. Anyone have an idea?

Thanks!

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If you're also swapping the cable that goes from the lid to the motherboard, it sounds like your motherboard is no longer providing either the +5v for the inverter, or it's not sending the signal to turn the inverter on. I've seen it happen on a compaq before where the MB would no longer send a power-on control signal to the inverter any more.

That's what I concluded as well. Problem is, I have no idea how the 5volt is supplied.

There is only one cable to the display so the 5 volts is supposed to be on a pin in the connector... but which one. (The cable was swapped with the lid)

Thanks for your reply.
Cecil
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plasmastorm said:
Inverter or back light fault. Most likely the back light which means a new screen.

try www.rolta.co.uk.

I probably fix around 5-6 machines a week with back light faults.




If you would've read the original post, he swapped the entire lid to another laptop and it works, so that proves his LCD, cable, inverter, and blacklight tube are all good.




And yes, the +5 is a certain pin on the cable. Just look at the inverter, it should only have 4-6 wires going to it, and looking at the traces, it's easy to tell what does what. Usually the +5 pins on the inverter lead to a small surface mount fuse before going to the chopper. The grounds are pretty obvious. And 1 or 2 pins will have a very small trace that usually leads to the PWM/Oscillator IC, those are the logic turn-on signal line(s).

mavroxur said:
If you would've read the original post, he swapped the entire lid to another laptop and it works, so that proves his LCD, cable, inverter, and blacklight tube are all good.




And yes, the +5 is a certain pin on the cable. Just look at the inverter, it should only have 4-6 wires going to it, and looking at the traces, it's easy to tell what does what. Usually the +5 pins on the inverter lead to a small surface mount fuse before going to the chopper. The grounds are pretty obvious. And 1 or 2 pins will have a very small trace that usually leads to the PWM/Oscillator IC, those are the logic turn-on signal line(s).



I agree, the 5 volts should certainly be on the cable attaching to the inverter board. But.... the cable to the board is a flat ribbon with 24 conductors. (the inverter also has 7 LED's for power, charge, wireless, sleep, caps, num and disk). I have a schematic for the T41 but it doesn't include the display circuitry.

Probably should just junk the thing but... I'M STUBBORN!! :-)

Cecil

eudaly said:
I agree, the 5 volts should certainly be on the cable attaching to the inverter board. But.... the cable to the board is a flat ribbon with 24 conductors. (the inverter also has 7 LED's for power, charge, wireless, sleep, caps, num and disk). I have a schematic for the T41 but it doesn't include the display circuitry.

Probably should just junk the thing but... I'M STUBBORN!! :-)

Cecil





If you're stubborn, i'm assuming that you'll be getting after it with a DVM and getting back to us with the results of what you found :-)

mavroxur said:
If you're stubborn, i'm assuming that you'll be getting after it with a DVM and getting back to us with the results of what you found :-)



yep... the old eyes make it difficult but i'm a tryin'

well... I found the problem. It was a small fuse (about the size of a black pepper flake) on the motherboard. I took a fuse off of another motherboard but lost it in the carpet before i could get it installed. I successfully suppressed the urge to throw things and put the carcass in a box under the bench. (That's the first stage of throwing it away).

Well, fuses generally aren't black. If it looks like this....

http://commons.wikimedia.org/wiki/File:SMD_Transistor.j...


It's not a fuse, it's either a transistor, FET, or something similar.





If it looks like this

http://www.littelfuse.com/data/en/Product_Photos/Littel...

or looks similar, it's a fuse. A fuse will only have 2 leads. If it has more, it's not a fuse. The reason I mention this is because SMT fuses generally aren't black, and not the size of pepper flakes. SMT transistors are though.

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