Resistors used in vintage radios evolved from the early "dog bone" designs to tubular types. The way manufacturers color-coded them, such as the layout and patterns, also changed, although manufacturers retained the same set of colors and numbers. To read the ohm value of an old radio resistor, first determine its general type: a dog bone resistor is a cylinder with pronounced end caps, and the color code is painted onto the body; tubular resistors lack the end caps and have a different banded color layout than the bands used on modern parts. Decode the value based on the resistor type and its color markings.
Dog Bone Resistor
Note the resistor's body color. This is the color that covers most of the component's surface. Match the color to the standard color/number scheme (see Tips) and use it as the first digit of the part's value. For example, if the body is green, the first digit is a five.
Read the end paint color and match it to the standard scheme. This is the second digit of the resistance value. Continuing the example, a brown end indicates a one, so the value has the digits five and one.
Note the color of the dot on the resistor body and find the standard number associated with the color. This number is the "power of ten" multiplier of the resistor value. Continuing the example, if the resistor has an orange dot, it indicates a three, so the multiplier is ten to the third power, or 1,000. The complete value is therefore 51,000 ohms.
Read the body color band on the resistor. This color covers most of the part's surface. Find the color in the standard value scheme (see Tips). The number is the first digit of the resistor's value.
Read the tip color band found at the end of the resistor and look up the value in the color scheme. This number is the second digit of the resistance value.
Note the color of the band near the middle of the resistor and find the value in the color chart. This is the "times ten" multiplier for the resistor's ohm value.
Tips & Warnings
- The standard color code/number scheme reads as follows:
- Black = 0
- Brown = 1
- Red = 2
- Orange = 3
- Yellow = 4
- Green = 5
- Blue = 6
- Violet = 7
- Gray = 8
- White = 9
- Early resistors lack the fourth band, used for the tolerance percentage number, found on modern resistors.
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