[sdiy] Alesis D4 backlight update

[harry]

in our last episode the Alesis D4 backlight had gone splotchy
and then died.

An e-mail to Alesis got the reply "The D4 is obsolete"

Thanks loads, guys that was less information than I needed....   ;^P

Nonplussed... the plucky diy-tech (me) opened up the display...
its an LED backlight, not the EL panel you might expect. It uses
24 LED in two rows of twelve. Its a 2 x 12 series parallel arrangement
with one dropping resistor... on a single sided board. The LED chips
were
bonded direct to the copper, then potted with some silicone or epoxy
clear coating.  Runs on 5VDC.

Obviously, one or more died and screwed the whole pooch. So I'm going
to use 24 SMT LEDs (I'll report later).

I built a constant current source to 'match' the LEDs so that current
sharing will
be good.  I tested a few LED just for fun.

Red LEDs at 2mA all came in at 1.7V... even different manufacturers !!!

I have some Bipolar Red-Red LEDs that I use as symmetric clippers...
they
ALL read 1.64V in one direction... 1.68 in the other. Hmmm they don't
make
very symmetrical clippers by design do they. I'd be better off with two
LEDs
if I cared about the difference (I don't....)

Just wanted to let folks know that it looks like the D4 backlight could
be repaired
easily...   I was tempted to do a wierd color, or rainbow effect... but
someday I
might want to sell it (fat chance...).

But why did it fail ?  It should have run for 100,000 hours ???   I'm
thinking maybe
the coating was not perfect and water vapor or some other mechanism got
in and
degraded the unit. Electormigration perhaps ???

H^) harry

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