Dark Laptop Screen

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I'm working a HP G61-429WM that has a dark screen.  It's a backlight  
problem, as a flashlight shows screen detail, and video to an external  
monitor is fine.

The problem would manifest itself within a few seconds of startup.  I  
replaced the inverter, and the system ran for several hours with good  
video and backlight.

Once the customer got it back, the backlight has been sporadic.  It will  
typically work for several minutes for it goes out.  It will randomly  
come back in, and back out, without any apparent physical stimulus.

Anyone care to set odds on "second bad inverter" vs "bad lcd panel"?

LCD panels for this model are sort of expensive--$100, and I would just  
as soon not spend that on a guess.

Re: Dark Laptop Screen

Grinder wrote:
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But this is the "roulette" of DIY inverter replacement.
The inverter characteristic (load line) must be matched
to the load. Wouldn't it be nice if the manufacturer
sold the right part, instead of some jobber ? If you
buy a "compatible" replacement, you don't really know how
close it is to the correct item.

When the tube first starts running, the "striking" voltage
is around 1000V. The conductive plasma inside the tube
draws a bit more current when warm, and in response, the
measured output voltage on the inverter is perhaps 700V.
The 700V would be the "burn" voltage, the long term voltage.

The circuit is not "tightly regulated" on purpose,
as like a car engine, you want it to "start when it is cold".
Now, when the tube is hot, if it draws too much current,
the overcurrent in the inverter gets triggered. Tubes
likely draw current in response to the physical dimensions
of the CCFL tubes. So matching "big" tubes to "small" inverter,
or vice versa, would likely lead to premature failure or
undesired optical characteristics.

Since the circuit is high voltage, a shield could have
shifted inside, after the unit was re-assembled. The CCFL
is capacitively coupled, with a relatively small capacitor.
(27pF capacitor rated for perhaps 2kV). What that means
in English, is small capacitances waved around the
circuit, like a foil near the CCFL tube, can make
a difference. In some cases, they touch the foil to the
outside of the tube, as part of the circuit. You just
can't trust these guys - put things back *exactly* the
way you found them (minus the dead bugs and dirt). Because
otherwise, you might never figure out what trick they're

A dude from sci.electronics wrote a book about CCFLs and
inverters. Which is amusing just for the seat of the pants
approach. I guess the work was commissioned by Linear. This
might give you a feel for how twitchy this stuff can be. It
might also give you some idea how you make measurements. This
is one circuit type, where naively grabbing your multimeter,
can result in a fried multimeter (and the wrong answer). You're
dealing with 1000VAC at 25KHz, and if you're lucky, you get
burned instead of shocked :-) The only problem with being
shocked, is getting thrown across the room... and hitting
something solid. The 25KHz is selected to make the CCFL
inaudible, rather than protect you from a shock hazard.
I'd probably want around 100KHz, to keep it in "skin effect"
country. The higher the frequency of the high voltage,
the shallower the skin penetration (so it doesn't
hit any nerves and you don't feel it).



In summary, spending $100 for the whole assembly,
means you don't have to read my post :-)


Re: Dark Laptop Screen

Once upon a time on usenet Paul wrote:
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LOL! I read your whole post and I don't have the problem. ;-)

As usual I found it very informative. Keep up the good work!


"Humans will have advanced a long, long, way when religious belief has a  
cozy little classification in the DSM."
David Melville (in r.a.s.f1)  

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