[rescue] LCD monitor diagnosis

Don Y dgy at DakotaCom.Net
Sun Apr 16 15:49:28 CDT 2006

Phil Stracchino wrote:
> OK, folks, here's the scoop.
> I have  ViewSonic VA800 pivoting LCD monitor.  I've obtained a new power
> supply for it, and determined that all the logic is perfectly operable,
> the screen is good, but the backlight is completely dead.
> The backlight consists of four 364mm CCFL tubes, two at the top and two
> at the bottom of an acrylic scatter plate, all driven off the same
> inverter board.  None if the four tubes shows any visible indication
> whatsoever when the monitor is powered on.  I believe the two tubes in
> each set are wired in parallel.
> Is it reasonable and realistic to assume that all four CCFL tubes would
> have failed?  My guess here is actually that I'm looking at a bad
> inverter.  Given that my DMM only goes to 750V on AC and the inverter
> should probably be putting out more like 10kV, can anyone suggest any
> way I can actually test to determine whether the tubes or the inverter
> are at fault?
> (At this point, I have a pretty strong hunch it's the inverter.  But I'm
> not 100% certain.)

Shine a light *into* the display (from the front) with a signal source
connected.  You should be able to see an image if the panel itself
is operating.

In most cases (that I have seen) the power supply and/or inverter
for the CCFL's dies.  Most (?) inverters seem to run off 12V
so check the power supply for a 12V feed to the inverters
(often 2 inverters -- though could be on the same PCB).

Also, check for bad electrolytics.  Surprising how often
you'll find "bulging" caps in the power supply.  Replace
any you find (careful with polarity... I toasted an LCD
that I *had* rescued by screwing up one cap  :<  ).
And, unless there are a LOT of them, consider replacing
ALL of them while you are at it.


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