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Old 02-15-2015, 09:22 PM   #53
phenol
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Join Date: Feb 2015
City & State: Vienna
My Country: Austria
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Post Re: A guide on how to troubleshoot 2 seconds to black

Excellent tips, could save a lot of money and time.

Quote:
Originally Posted by retiredcaps View Post
<snip>
5) Bad CCFL
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Pay ATTENTION as there is a VERY HIGH VOLTAGE (600V to 1000V)! No multimeter is required for this test.

The CCFLs light up the panel and they can be bad due to age, bad solder, bad wiring connection or a variety of reasons. The easiest way to test for bad CCFLs is to have a least one other good CCFL. If you see a pinkish/redish hue, this is a sign that your CCFLs are dying.

Let's assume that you have a LCD with 4 CCFLs (numbered 1 to 4) and a good spare. We are trying to narrow down which CCFL is bad by substituting in a good one.

Pay ATTENTION as there is a VERY HIGH VOLTAGE (600V to 1000V)! As a safety precaution, it might help to have someone around when you are doing this if something screws up.

a) TURN POWER OFF and unplug the monitor and wait 1 minute
b) disconnect CCFL #1 and plug in spare CCFL into spot #1
c) plug in monitor and turn monitor on - note if "2 seconds to black" occurs
d) TURN POWER OFF and unplug the monitor and wait 1 minute
e) reconnect CCFL #1 and disconnect CCFL #2 and plug in spare CCFL into spot #2
f) plug in monitor and turn monitor on - note if "2 seconds to black" occurs
g) TURN POWER OFF and unplug the monitor and wait 1 minute
h) reconnect CCFL #2 and disconnect CCFL #3 and plug in spare CCFL into spot #3
i) plug in monitor and turn monitor on - note if "2 seconds to black" occurs
j) TURN POWER OFF and unplug the monitor and wait 1 minute
k) reconnect CCFL #3 and disconnect CCFL #4 and plug in spare CCFL into spot #4
l) plug in monitor and turn monitor on - note if "2 seconds to black" occurs

If you have a bad CCFL, one of the tests above should show you which one. If you still have "2 seconds to black", then we can assume it is not due to a bad CCFL.
I would like to point out that this test will always fail with four "2 seconds to black" outcomes. This is because the unit will operate only with all of 4 lamps in good condition; if any of them goes bad then the feedback circuitry will either detect excess load (less likely) or no load at all (cracked lamp or wires) and will shut down the inverter to protect it.
This test, however, can isolate which, and how many CCFLs have gone bad as with the working lamp plugged in the picture would still be visible (for a short period) but quite dim. However, when the bad lamp is plugged in there would be no picture at all in that 2 (or less) seconds.

There's another interesting thing described in this thread, where a digital MM or even a capacitor connected at right place acts as confusing element for feedback protection circuitry and the picture will stay on even with 3 working lamps, hence no black picture symptom. This can sometimes be useful as the repair can be uneconomical. And after all, replacing the lamps is a tricky as one can score a perfect "hat trick" only to completely mess on the 4th attempt (usually on friend's panel ).
My point is, running LCD with 3 or even 2 lamps can still sometimes be acceptable until all of them die, but this requires finding a way to confuse protection mechanism by attaching something that the feedback sees as a good lamp.

Question is: what is the best and safest way to do it?

BTW: the "remains" of the monitor can then be used to build pretty good standalone CCFL tester.


Cheers.
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