LCD brightness gone, can I fix this?

[Peter de Vroomen]

Hi,

I have an old Magellan Meridian XL GPS (1996). Good GPS, bit old, but still
works. Except that the LCD has gradually become dimmer and dimmer up to the
point that it is very hard to read.

If I switch the GPS on, the LCD is already hard to read (brightness/contrast
is already at maximum), but it even gets a little dimmer over the next
couple of minutes.

I can RMA the GPS at Magellan, but I think it's not really worth it any
more. Today's GPSses are much smaller, much faster and much more accurate,
and I think I'd rather invest my money in a newer one .

But... I'd still like to have a shot at repairing the LCD. It must have
something to do with power to the LCD. The only thing I can imagine that
degrades over time is an electrolytic capacitor. Anyone have any other
hints? What would make an LCD dim, even at the highest contrast setting?

It's this one: http://www.euronet.nl/users/nautika/magxl.htm

PeterV

 

[Peter de Vroomen]

The display's contrast is set programmatically (i.e. through a menu), not a
potmeter. Could it be that there's a 'digital potmeter' in there? I have
also seen a method using PWM, but that is pretty fool-proof, i.e. as long as
the pulse generator works, the rest of the circuit should also work.

I have been looking for more info on LCD's on the internet. I gather that
the LCD's contrast is determined by some BIAS voltage. This voltage is
adjustable between 0V and VCC in simple designs. But better designs use a
negative voltage. How big a negative voltage can this BIAS voltage be before
I damage something? The display is a mono display, probably STN.

Are digital potmeters known to degrade over a time-span of, say, 7 years?

I know: a fool can ask more questions than a wise man can answer . But I
do know something about electronics, just nothing about driving LCD's.

One last question: will an LCD display become dim because it has been in the
sun for an extended period of time?

PeterV