[sdiy] Alesis D4 backlight success

[harry]

I got my SMT LEDs today and fixed my backlight.

First I built a constant current source with an LM334 and
adjusted for about 2mA.   In retrospect I'd shoot for 5mA.

Then I bought 100 512-QTLP650C3 LED from Fairchild (Mouser number)

Binning the parts I found the forward drop varied from 1.810 to 1.820
volts
with the peak values around 1.815.  I chose 24 pcs. between 1.84 and
1.86.

Better matching might have helped the display be more evenly
illuminated.

I soldered these all to the board that originally held the LEDs... watch
the polarity.
I got mine wrong and used two jumper wires to fix it ;^)

Because the white paint on the board was damaged, and the "jelly coat"
of
silicone was gone... the light is slighly brighter just below the lines
of text... almost
perfect.

It took about an hour and a half to do.  I'm an old man (mind you)
almost as old as
Larry Hendry   ;^P...   so that SMT soldering was really hard on the
eyes. I used
long tweezers to hold the parts, tacked one end... then soldered all the
other ends.

You can see that the light is not as even as the factory stock unit...
but its better to
light 24 LEDs than curse the darkness

H^) harry

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