Resistor Coding

A few visitors have asked for information about the resistor colour code. This is set out in the table below, but some explanation is also required. This picture (taken from the CPC catalogue) shows a 4-band resistor, although many of those used in vintage radios will have only three bands. The first band (yellow in this picture) will always be the one closest to one end of the component.

The first two bands indicate the first two digits of the value. In the table below we have called these "x" and "y". In our example in the picture, the first band is yellow which means (by referring to the table) that the first digit (x) is "4". The second band is violet which means the second digit (y) is "7".

The third band indicates the number of zeros that have to be added after the first two digits to produce the value on Ohms. This is somewhat confusing, so I have converted these into Ohms (R), Kilohms (K) and Megaohms (M). In our example the third band is red. Referring to the table, red is "x.y K". If we replace "x" and "y" with their digits for the first two bands, we get a value of "4.7 K".

The fourth band indicates the tolerance of the resistor. In other words, it indicates how many percent the actual value may vary from the marked value. This is because it is difficult to manufacture resistors of exactly the correct value, and in most situations some variation from the nominal value is no problem. In our example, the fourth band is brown, which by referring to the table we can see indicates a tolerance of + or - 1%. So if we were to measure the resistance of the resistor with a test meter, the value could be anything between 4.653 K and 4.747 K.

Most resistors encountered in vintage radios will be either 10% (silver fourth band) or 20% (no fourth band). So if a resistor has only three bands, it has a tolerance of 20%. This may seem quite a wide tolerance, but in practice it is not a problem because the circuits were designed to accommodate this sort of variation.

If you are checking the resistors in a set with a test meter, you should bear in mind the tolerance before assuming a component is faulty. A 10 K resistor with a 20% tolerance could actually be anything between 8 K and 12 K. In practice we can accept a 20% resistor if it is a bit outside this. I usually regard a 20% resistor as acceptable if it is within 25% of its nominal value. You may find it helpful to have a calculator with a percent function in the workshop!

Since this can be rather confusing initially, here are a couple more examples:

First band green, second band blue, third band yellow, fourth band silver. From the table, the first band (x) is 5, the second band (y) is 6, and the third band means "xy0 K". Replacing the "x" with the 5 and the "y" with the 6 gives us a value of "560 K". The silver fourth band indicates that the tolerance is 10%.

First band brown, second band black, third band green, no fourth band. The first band (x) is 1, the second band (y) is 0, and the third band means "x.y M", so the value is 1.0 M. The lack of a fourth band indicates that the tolerance is 20%.

Resistor Values in Printed Materials

On this website I am using the letter "R" to indicate Ohms. This is a convention that has come about relatively recently because most computer fonts do not have the proper omega () character.

Another modern convention, which you will probably find in some places on this website, is to replace the decimal point with the units character. So 4.7K could be written as 4K7. This is because it is quite easy for a small decimal point character to be lost when a diagram is photocopied or otherwise reproduced.

Older Resistor Coding

Some earlier sets use resistors that use a different colour coding system, as shown in these photos. The colours are read in the sequence Body, Tip, Spot. The colours themselves represent the same values as in the modern code above. The left resistor here has a red body, a violet tip (not really visible in the photo) and a green dot, so the value is 2.7 M. Some, like the other photo here, use a centre stripe in place of the spot. This one has a yellow body, violet stripe at one end (tip) and a brown stripe in the middle (spot), so the value is 470R.

Replacement Resistors

As well as a resistance value and a tolerance, resistors also have a power rating. This is the maximum power they can dissipate as heat, specified in Watts. The power rating is not generally shown on the component itself, but can be established from the service sheet for the set. The general rule is that all resistors in a set will be 0.25W or 0.5W unless it states otherwise. Higher power resistors are physically larger so it is easy to see those that have a higher power rating.

Vintage resistors do not generally fail as seriously as vintage capacitors. Resistors just tend to go high in value, and it is not uncommon to find a resistor that has risen by 50% of its original value. Surprisingly, this sort of variation will generally not significantly affect the operation or performance of a set! There is no point in replacing resistors that are high in value, unless they are clearly causing problems. Indeed, I generally do not even bother to check the values of resistors in a set unless they look discoloured or otherwise sorry for themselves.

Nigel Hughes made the following comments:

Although resistors can be very reliable, you should not assume this. High value resistors in particular, can go very high in value with age. For example, I have found an anode feed resistor in a local oscillator circuit which had increased from 47K ohms to half a megohm. This was more than enough to put the oscillator out of action.

If you do need to replace a resistor, you will have no problems finding a modern replacement. Like other components, modern resistors are much smaller than their vintage counterparts for the same power rating. Most repairers use 1W modern resistors to replace 0.25W and 0.5W vintage resistors, because they are about the same size as the originals. The modern resistors will probably have a tolerance of 5% or better, which is better than the originals. With higher power resistors simply choose a modern resistor that is of a similar physical size to the original, and you can be sure it will me more than adequately rated.

Another trick with resistors that are high in value is to connect a smaller modern resistor in parallel to bring the value back down. The modern resistor can be hidden behind the original, giving a more original appearance to the underside of the chassis. The only downside to this is that the original resistor will probably continue to rise in value over the years, but it is likely to be tens of years before it has risen sufficiently to stop the set working again.

To calculate the resistance of two resistors in parallel, the formula is:

Where R1 and R2 are the values of the two resistors in parallel, and R is the resulting resistance.

To calculate the value of resistor to add in parallel with a high resistor, to bring it back to the right value, we need to rearrange this formula. The result is:

Where R2 is the current value of the resistor, R is the value it should be, and R1 is the resistor you need to add in parallel to achieve this.

A formula is fine for those who understand these things. For the rest of us, we need to know who to work it out with a basic calculator and a piece of paper. The units of resistance (R, K or M) you use are irrelevant; as long as you use the same units for all three values, the result will be correct. The procedure is:

• "1" divided by the current value of the resistor, equals. Write down the result.
• Clear.
• "1" divided by the value it should be, equals, minus the number you wrote down, equals. Write down this number.
• Clear.
• "1" divided by the number you just wrote down, equals.

The result is the value that you need to connect in parallel with the existing resistor. Just use the closest readily available value.

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