The backlight for the LCD display in my Alpine 7861 (car cd changer)
one day was dimmer, but not gone. I was expecting to find one bulb
burned out of two, but instead there was a single dual filament bulb,
one had burned. This set has optional illumination capabilities via a
single wire, so that when you turn on your headlights the faceplate
dims, but I haven't hooked it up. Regardless of this, the lamp
configuration was as follows, with pins 1, 2, and 3

2 1
3

with markings on the casing in this configuration
OA
Z

Because one filament was blown, and the bulb was frosted, it took a
little circuit tracing to find which pins were filaments. Pin 1 is
definantely ground, 2 was the good filament, and 3 was the blown
filament. The cold DC resistance across the good coil was 59 Ohms.
When the lamp was hooked up and on with one filament, I measured
13.2VDC at pin 3, and 5.6VDC at pin 2 across the filament. At the
time, this made sense. I then removed the lamp, and I decided to
check out the voltages to hopefully find some simple readings to get
two replacement lamps and just make do. Pin 3 jumped to 26.4VDC, and
pin 2 stayed around 6V.

Confused, I started trying replacement lamps and LED's. With a LED
from pins 1 to 3, and pins 1 to 2, the voltage across pins 1 to 3
dropped to 1.64VDC adn the voltage from pins 1 to 2 dropped to about
1.7VDC. So, close in voltage, but the LED's brightness was different.
The same occured when I hooked up incandecent bulbs, but the voltages
were both around 2VDC and the brightness difference was vastly
different. Therefore, I started measuring current. Pin 2 supplied a
varying amount (by varying I mean it changed with load) of current
anywhere up to approximately 185mADC. Pin 3, however, supplied a
steady 22mADC regardless of load.

So after all these readings, What I've come up with is that pin 2 is
basically an unregulated output, no load voltage of 5-6VDC, loaded
voltage of 1.7-2VDC, seeing as the voltage and current change with
load, and is probably for the main filament. Also, pin 3 has an
unloaded voltage of 26.4VDC, loaded voltage of around 1.64-2VDC, and a
constant current of 22mADC. In addition, all these readings are
proportinal to eachother, i.e. if you hook a lamp from pins 1 to 2,
the unloaded pin 3 drops in voltage, and vice versa. So, for now, I
just h ooked up 2 LED's, but they're not bright enough. So, until I
can get smaller incandecent lamps (the only ones I have now are 12VDC,
way too much voltage, they barely glow) I'll just leave it and forget
this ever happened, I'm too confused now, because I have to keep in
mind this is a car CD changer, running off 12VDC, so for the voltage
on pin 3 to go to 26VDC there has to be some more complex circuitry
than just a resistor, Bah, I wish it had never burned out, and that
the bulb was a simple replacement. Any thoughs on this subject? Does
any of this make sense to anyone? What's the professional opinion on
the matter. Thanks, Steve