View Single Post
  #2   Report Post  
Old July 25th 03, 12:24 AM
Posts: n/a
Default hopeless question about lamp in Alpine CDA-7861

its you wrote in message
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

with markings on the casing in this configuration

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

Or just get the lamp, from PCP.