What Are the Adjustable Screws Inside My CB For?

Tools You'll Need

• To make any adjustable screw changes inside your CB, you will need a nylon or plastic CB radio/TV adjustment tool kit. These include different sizes of non-metal stick-like tools that have either a plastic or metal blade at one end. They are used to adjust coils and "trimmer" potentiometers (variable resistors). Traditional screwdrivers have a metal shaft; if used inside a CB radio, they can alter the parameters of certain circuits, especially those containing coils, and make proper adjustments impossible.

Modulation Adjustment

• Inside your CB radio you may see rectangular "cans" with a screw slot in the top. These are factory set and should not be touched except by technicians with proper test and alignment equipment. However, CB radios out of the box may not have their modulation peaked for best performance. Adjustable screw-type "trimmer" variable resistors can be set to realize maximum potential. You will need a schematic diagram of your radio to identify which variable resistor affects modulation. Ideally, you'll want 100 percent modulation; any less reduces your range and too much causes distortion. This variable resistor will have a slot similar to a flat-head screw into which you can insert your adjustment tool. Phone a friend who has a CB radio and talk into your radio's microphone while adjusting the modulation resistor. Keep him on the phone at the same time so he can tell you when it sounds loudest without distorting.

SWR Adjustment

• One of the most important adjustments you can make is on the CB antenna rather than inside the radio. SWR (standing wave ratio) is a measure of how much transmitting energy is being reflected back from the antenna. Ideally, all the energy from the CB transmitter should be radiated from the antenna, but slight mismatches between the radio, the transmission line and the antenna will cause some loss. Connect a SWR meter between your CB radio and the coax line feeding the antenna. Tune your CB to channel 19, which is about the center frequency of the CB's range. Push the switch marked "Adjust" or "Set" on the SWR meter to the "Set" position and slide the meter's switch to Forward (or "FWD"). Depress the transmit button and turn the meter's adjustment knob until the meter's needle aligns with the indicated set mark on the meter's faceplate. Move the FWD switch to the REF ("Reference") position, press the transmit button and read the SWR. A reading of 2:1 is considered good. Make adjustments tweaking the antenna. Refer to the antenna's owners manual; the antenna will have an adjustable tip with a screw or an Allen screw that can be turned.

Warnings

• If the CB radio you are working on is a base station that operates on a 110-volt wall outlet, use extreme caution when you remove the cover. You will be exposed to components that can give you an electrical shock.

  Adjusting the wrong screw-type variable resistor and coil components may decrease performance if incorrectly adjusted. Have someone experienced in electronics help you if you are unsure of how to proceed.