1 Jan
2006
1 Jan
'06
2:48 p.m.
I repaired two units of 'higher-end' audio equipment for a friend a while back
in which all the incandescents backlighting the LCD displays had burnt out and
the displays could no longer be read.
I used white LEDs, brighness was sufficient but not quite as bright as original.
Dispersion was a bit of a problem. In this instance, the trick I found was to
aim the LEDs towards the display at oblique angles (such that their beams
crossed over), e.g. with two LEDs an inch or two behind the display, aim the
one on the left towards the right side of the display, the one on the right
towards the left side, so you get two overlapping ovals at the display instead
of two independant circles.
(Edge-lit displays or those with some other form of internal diffuser may be
another issue.)
The overall result was not quite perfect but certainly adequate to keep the
equipment usable. More LEDs would have helped, but for white LEDs in small
quantity, the cost was adding up quickly. I got by with two or three LEDs for
each display of roughly (IIRC) 2-to-3 inches by 3/4 inch. (general note: at
12V once can string more than two in series.)
If the incandescent lamps were fed with AC it may be that one should add some
rectifiers. Even though LEDs are diodes I don't believe they are rated for, or
tolerate, application of reverse voltage of any significant magnitude. (I know
I checked into/experimented with this for another reason a long time ago. I
forget what the exact outcome was but IIRC, AC was bad.)
Chuck Guzis wrote:
While not directly germane to vintage computers, I can see that this might
have some real interest for those restoring old displays.
I've got an old (1990) 120w-per-channel receiver in which I've just
replaced the hybrid power amp module--I think I can expect another 15 years
or so of life out of it. What bothers me is that some of the incandescent
12vdc back-of-panel lamps are starting to go.
They're soldered onto small PCBs and are the small 25 ma variety. They
illuminate the back of small LCD displays that have an orange filter on
them, so the color is basically that of an NE2 neon.
I was thinking of replacing the lamps with high brightness LEDs and current
limiting resistors--since the bulbs are paired up, I'd put the 2 LEDs in
series with a single resistor.
Now for the questions:
1. Has anyone tried to do this?
2. Will LED's have sufficient brightness and dispersion to substitute for
incandescents in this application?
3. What would the best LED to use?