CB Radio Modulation Modification
A Citizen Band or CB radio signal without modulation is a carrier wave. When you speak into the microphone the AM carrier wave is increased or modulated relative to the level of your voice. The modulation can be increased by speaking more loudly or using a power microphone. The more the wave amplitude is modulated, the virtual output power will seem higher, thus achieving a noticeable power boost in your transmission. Beyond what can be achieved at the microphone it is possible to further increase modulation by internal modification of an adjustable resistor known as a potentiometer, and possibly by removal of a modulation limiting resistor.
Most CB radios have a speaker on the bottom. It is the bottom case cover that is typically removed to gain access to the internal components. When removing the bottom case lid take care not to pull the cover away from the radio such that the speaker wires are jerked off of the speaker or board.
Look for the AMC Adjustment point (modulation potentiometer) or modulation adjustment pot. Use a reference for your brand and model of radio to identify the correct potentiometer. It should have a label on the board itself. The label will be something like "VR5" but not necessarily that. Again, you need a reliable reference for your model, or a schematic diagram so that you can identify the correct potentiometer.
You will typically turn the adjustment to increase the modulation. You can start by turning the adjustment counter-clockwise until it stops to achieve full modulation.
Turn clockwise or counter-clockwise? It depends on the radio. The way it has the furthest to turn is probably the wrong way. The logic follows that there should be less room to turn it up as opposed to down as modulation is typically factory set well above 50% of the potentiometer adjustment value. In fact, it will typically be turned over three-quarters of the way at about 85% relative modulation. Make note of the position prior to making an adjustment. It will not turn more than 360 degrees (it is not multi-turn) and you will likely not have far to turn it before achieving maximum modulation.
Increased modulation is typically desirable, however, maximum modulation is often counterproductive. Turn it up all the way and then back it off just slightly.
At this point you will want to speak into the microphone and if the reading constantly peeks you are probably over modulated. Back off the modulation until speech is no longer distorted. If over-modulated, you will sound distorted and your radio can cause interference to adjacent channels or on other frequency harmonics.
Use the built in display to determine what your modulation swing is before and after the modification.
Some models the adjustment is going to be clockwise to increase as opposed to counter clockwise.
There is also a modulation limiter resistor on some CB radios. A modification known as "clipping the modulation limiter" is the removal or bypass of this resistor to achieve even higher modulation. In some radios the limiter may be a diode. There are CB radios that do not have an adjustable potentiometer for modulation, and simply use a fixed resistor. The resistor can be replaced with a jumper made of wire or solder, or a resistor of a lower value.
Another modification that can easily be made by adjustment of a potentiometer is to increase transmit power. This potentiometer is typically located close to the amplification transistor often referred to as the final stage amplification transistor or the final.
If you overdrive the final amplification transistor you may damage it.
Adjustments should be made with a non-conducting ceramic screwdriver.
- CB radio people tend to refer to the potentiometer or pot as a "can" and they will talk about tuning the cans.
- The potentiometer may be referred to as a "trimmer" or "trim pot." The potentiometer is simply an adjustable or variable resistor. Trimmers can actually be variable resistors (potentiometers), variable capacitors, or trimmable inductors.
- CB radio people tend to refer to the final stage transistor as a "pill" and will talk about burning out the pills.
Make sure you identify the correct potentiometer prior to making an adjustment. Other potentiometers perform functions related to alignment and the operation of the receiver. You can create problems with your CB radio which can seriously inhibit the functionality of the unit or cause damage to other components inside the radio.
Take note of a potentiometer position prior to making an adjustment so that you may return the position to where it was before you modified it. You want to have the ability to put everything back to where it was before you began in the event you are not able to achieve the results you were after or you find you have diminished the functionality of the unit.
Make only one adjustment at a time. Make the adjustment, then test to discover the result. Don't adjust multiple potentiometers prior to performing a test of the results. Tune the modulation to the desired result, then after completion of testing and verification you may attempt to adjust the power output if that is your desire. Don't try to do them both at once prior to testing.
If you increase the modulation too much you will sound distorted to listeners. You can only get "louder" to a point before clarity is sacrificed and your efforts become counterproductive.
Do not max out the power output as you can damage the final stage transistor and also distort your audio.
Some of the "export model" CB radios have more than one final stage transistor. Modern export models have two MOSFET type final transistors. These models are not legal for use in the United States. Export models with a high and low power setting will also have two potentiometers that may be modified for audio modulation and two potentiometers to modify for power output, making a total of four potentiometers that may be adjusted.