[sdiy] Bizarre LCD Display Behavior & Is backlight AC or DC Voltage

[Tom Wiltshire]

This definitely sounds like a problem with a backlight inverter.

The question is how to test whether it is the backlight or the  
inverter that is at fault. If you can disconnect the inverter from  
the display, perhaps you can test the inverter output on the scope.  
Looking through some catalogues, you can expect 10s of volts at 100s  
of Hz. One example here provides 82V at 410Hz.

I've always been very wary of putting high AC voltages in a circuit  
with logic levels, so I buy displays with LED backlighting. Then you  
can run it off a +5V rail. I'd have a regulator for this alone, since  
the current for an LED backlight is often 100s of mA.

T.


On 24 Sep 2008, at 05:47, Ken Elhardt wrote:

> I'm getting private posts and forum posts and it's getting confusing.
>
> To Bob Weigel, I had spent a lot of time going over the ribbon cable,
> resoldering the connectors, pushing, pulling, bending PCB's, and none
> of that would cause the display to go out, and none of it fixed the
> problem either.  It was only after removing the backlight power that
> the problem seemed to be solved, except of course I now have no
> backlight.
>
> To Scott Nordlund, Some of symptoms you mention like digital glitches
> and a such also seemed to happen on rare occasions.  For instance
> today when the display went out, it changed the patch number.
> However, I'm not sure if getting a new LCD display will solve the
> problem yet, because of the nonsense voltage or signal I'm getting
> from the power supply board.  It's not currently DC which worries me.
> There's AC there, but I can't measure because my scope will not
> display a signal when there is a continuous spark on the probe tip.  I
> noticed that when I moved the probe closer to the LCD PCB where the
> power comes in, I started seeing some kind of AC signal on the scope,
> even at a couple of inches away.  I suspect the boost converter or
> whatever that is on the power supply board.  There is a high pitched
> sound and noise that I can physically hear coming from there.  As per
> my previous post, I temporarily disconnected the backlight and that
> seemed to fix the problem, but since I wanted to put this keyboard up
> for sale, having no backlight isn't preferable.  I'm just trying
> confirm if it's a power supply problem so I don't waste time and money
> buying a new LCD unit and not solve the problem.
>
> Thanks,
> -Elhardt
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