06-13-2009, 06:29 PM
I have a $2 solution and it looks great. I was in Radio Shack this afternoon and started snooping through their component drawers and found a 10mm Ultra High Brightness Red LED. The clear plastic shell is 9mm and is somewhat ovoid, which matches the shape of the molding on the radio. Here's the specifications: Intensity = 400 mcd, Wavelength = 660nm, Viewing angle 16 degrees, Current = 20mA, Supply voltage = 2.4 V max. In other words, you get a very red light that's reasonably bright and the voltage out of the existing light socket is ample to light it.
I used a breadboard to find the best load resistor value. Since the voltage source is AC I connected another diode across the LED to conduct when the polarity changes, anode to cathode and cathode to anode. One end of the diode pair go to ground and the other end is connected to a resistor that draws current from the light socket in the radio. I selected the resistor to give less than 20mA of current. This measurement was done by connecting my Fluke multimeter in line. I had a 220 ohm one watt resistor and and a 330 ohm 1/2 watt resistor to choose from. The 220 ohm resistor resulted in 21.5mA while the 330 ohm resulted in 15.5mA. I decided to use the 330 ohm resistor since the sacrifice in brightness was marginal and should allow the LED to work for thousands of hours with no heating problems.
Before assembling the circuit I had to use a small metal file to cut off the flange on the LED. Once that was done I got the best fit by filing a little more to reduce the diameter of the LED so the tip was not sticking out far beyond the molding and the light would fit snuggly with a press fit by hand.
I used six inches of multi-strand wire to extend the LED leads. The diode and resistor were soldered to the extension and an another six inches of wire was used to connect to the chassis and the radio light bulb socket. All exposed metal wiring was covered with heat shrink.
As I mentioned the light look great and for some reason the viewing angle is good all angle except for a small range around 30 or 45 degrees for some reason. I know this isn't a historical restoration but the result is outstanding considering the cost and difficulty involved. I also got a set of TV tuning screw drivers that were on clearance for $2. That's about the best $4 I've ever spent at Radio Shack.
I used a breadboard to find the best load resistor value. Since the voltage source is AC I connected another diode across the LED to conduct when the polarity changes, anode to cathode and cathode to anode. One end of the diode pair go to ground and the other end is connected to a resistor that draws current from the light socket in the radio. I selected the resistor to give less than 20mA of current. This measurement was done by connecting my Fluke multimeter in line. I had a 220 ohm one watt resistor and and a 330 ohm 1/2 watt resistor to choose from. The 220 ohm resistor resulted in 21.5mA while the 330 ohm resulted in 15.5mA. I decided to use the 330 ohm resistor since the sacrifice in brightness was marginal and should allow the LED to work for thousands of hours with no heating problems.
Before assembling the circuit I had to use a small metal file to cut off the flange on the LED. Once that was done I got the best fit by filing a little more to reduce the diameter of the LED so the tip was not sticking out far beyond the molding and the light would fit snuggly with a press fit by hand.
I used six inches of multi-strand wire to extend the LED leads. The diode and resistor were soldered to the extension and an another six inches of wire was used to connect to the chassis and the radio light bulb socket. All exposed metal wiring was covered with heat shrink.
As I mentioned the light look great and for some reason the viewing angle is good all angle except for a small range around 30 or 45 degrees for some reason. I know this isn't a historical restoration but the result is outstanding considering the cost and difficulty involved. I also got a set of TV tuning screw drivers that were on clearance for $2. That's about the best $4 I've ever spent at Radio Shack.
