Retrofitting a Stereo with LED lighting:
Many older stereos, radios and electronic devices were equipped with small incandescent bulbs to illuminate scales, dials and pointers. In time, these tend to burn out. Some use commonly available bulbs and are easy to replace. Others use unusual bulbs or are soldered in and difficult to access. In some cases the bulbs tend to run hot. One solution is to up the voltage of the replacement bulbs. A 12 volt bulb available at a local auto parts store will still provide enough illumination in an 8 volt circuit, running cooler and lasting longer.
We can also solve all the above issues and customize the appearance by replacing the bulbs with LED’s of a desired color. For dials that are lit from the side, this works very well as LED’s a light directional. For dials that are rear-illuminated a method of diffusing the light may be needed. The lens of an LED may be lightly sanded to help diffuse the light or you may need to use several LED’s or other creative means.
This HK has one blue LED on the left side of the scale and a red one on the pointer. Like most stereos, the dial lamps are run off a separate tap from the power supply transformer and are often not rectified. So it’s usually 6 to 12 volts AC. Be sure to confirm this with your voltmeter. That’s too high for one LED’s so we’ll want a resistor in series to limited the voltage and current.
The formula for calculating the value of the resistor is R = (V – Vf) / If
R = resistor
V = supply voltage
Vf = voltage drop across LED
If = current through LED
So if we have a 12 volt supply and a red LED needing 2 volts and 15 milliamps of current running through it then the resistor would be 680 ohms 1/2 watt. Using some jumper wires, be sure to test the circuit that voltage across the LED is within spec. Better yet, just use one of the online LED calculators listed below.
Different colors of LED have specifications for voltage and current. Hopefully that information is on the packaging when you bought the LED’s, and if not LED’s of the same color tend to have the same characteristics. Generally Red and Green ones like 2 volts while Blue and White ones like 3-3.5 volts.
LED’s can be run from AC provided the voltage and current is correct. It can be done with a single LED, when possible run a pair of LED’s in anti-parallel with one another. This will eliminate any chance of flickering and the LED’s will protect each other when the current reverses.
Now that you have the electronics part figured out you’ll need to mount the LED’s. As the leads are exposed you’ll want to take precaution that they won’t short together or to ground. Individually heat shrink tube each lead. A hot glue gun is one way to physically mount the LED’s yet will still break away if you want to change them later. Here are four excellent sources of information regarding LED’s:
— Kenn Ranous