CAUTION: Don't Burn Out That Radio! One type of radio failure that is quite common can be prevented quite easily. That is reverse polarity burnout. Many radios are needlessly burned out each year when reversing the leads while hooking up 12 volts to the radio. Even a momentary touching of the leads to the power source in reverse polarity with the radio off can cause failure. CB and 10 Meter radios have protection diodes at the power source input. Many of these diodes are rated less than they should be. Even with the properly rated fuse, these diodes will permanently short out protecting the radio from further damage, causing the radio to constantly blow fuses as soon as power is applied. Many radios are marked with polarity signs on the back panel by the power connector. It’s always a good idea to check that the red wire of the power cord is on the same side of the connector as the (+) side of the power jack. In the old days, there were radios that used the same 3-pin type power cords with the opposite polarity. In addition, because these are zip cords, when the leads are pulled apart some red is left on the black wire leaving a red stripe on the black wire. If close attention is not paid, the black wire can be connected to the hot side of the power source, all that is needed is for the red wire to brush across any metal object in the vehicle that is grounded for the radio to be damaged. Some operators and some so-called techs, knowing about this diode, cut it out when they find it shorted. Sure, this will fix the short circuit, however, if the radio is ever hooked up reverse polarity again kiss it goodbye! This diode should be replaced. The diodes in the typical Cobra CB are 1 Amp units. I recommend a 3 Amp diode, and some 10 Meter radios are coming through with them now. When I replace the diode, I replace it with the same type on the circuit board as long as I can add a 3 Amp diode directly across the power jack. This is double protection is many cases, especially if the connector has filter chokes in line with the power jack. In the case of reverse polarity, the short circuit occurs before the chokes, preventing them from being damaged. If these chokes are discolored, even though they still conduct, some windings could be shorted out. Once the windings are shorted, noise from the power line can come into the radio unimpeded, as well as RF from the transmitter can exit through the power leads causing RFI in the vehicle. Power line chokes that show evidence of being slightly burned should be replaced. Over fusing a radio can and will do greater damage when the radio is wire to the power source wrong. All to often, operators replace the blown fuse with the first fuse they come across, usually a 20 Amp fuse. If the diode is blown out, the heavy fuse means that something else may have to give before the fuse blows. Usually chokes, circuit traces, the protection diode opening, or all of the above. If the radio is still wired reverse and the diode or a trace to the diode opens, it’s probably time for a new radio. Electronic components are sensitive to reverse current. The audio amp, PLL, voltage regulators, capacitors, driver transistor, final transistor, and many more components will be burned out or damaged. Tracking down these burned out parts may be relatively quick, but it becomes another story locating damaged components when a circuit isn’t acting right.
Inproper and Proper Fusing
So take care hooking your radio to the power source, never use a fuse rated higher than what the factory recommends, and never, ever cut out the protection diode. Bob F
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August 2001 Web Edition
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