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MobilePro 790 - No Backlight

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bitannihilator Page Icon Posted 2020-05-09 7:28 PM
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I just bought a 790 for parts for an 880 upgrade, when I powered it on it seems that the only issue is that there is no backlight on the LCD. Ive tried swapping in a known good panel and inverter board from a 900 but that will not light, but the panel and inverter from the 790 will light and display fine in my spare 900. I'm looking at the board and the resistors below the display connector appear to have a few missing compared to the photo of the 780 board I found on an archived copy of hpcnec. Here's a photo of my board:



Could these missing resistors be why the backlight isn't working or is it likely something else?

Thanks!
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Rich Hawley Page Icon Posted 2020-05-10 6:04 PM
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Those "missing" resistors are not missing. They are simply not used.

Am I to understand your goal was to have the proverbial 890? And it is the 880 that doesn't light up?

There may be nothing at all wrong with the inverter board… could be something as simple as the ccfl in the lcd panel is bad.

Did the unit light up before you got to tinkering?
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bitannihilator Page Icon Posted 2020-05-11 9:16 PM
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Okay, that's good to know. It's probably something deeper then.

Yeah, my goal was the 890, which ended up working perfectly as expected.

It's the 790 that doesn't light up. I tried swapping in a panel that I know works and has a good ccfl and inverter from a 900, just swapping the ribbon between the 2, and that doesn't light up either. I still get image but no backlight. Interestingly, the panel/inverter that was originally in the 790 lights up and works perfectly in the 900 so I don't think its an issue with the panel itself.

Thanks!
Matt
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C:Amie Page Icon Posted 2020-05-13 2:15 PM
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Perhaps the issue is on the signalling, the 790's not able to set the line to high to enable the CCFL. Could be a short somewhere.
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Rich Hawley Page Icon Posted 2020-05-13 3:53 PM
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Sounds like a motherboard problem since you've ruled out everything else…
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bitannihilator Page Icon Posted 2020-05-14 3:59 PM
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Quote
C:Amie - 2020-05-13 2:15 PM

Perhaps the issue is on the signalling, the 790's not able to set the line to high to enable the CCFL. Could be a short somewhere.


That's where my mind is going to as well. Is there any kind of pinout for the LCD ribbon or possibly a board schematic to trace back everything?

Thanks,
Matt
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