By Jason Reilly.....

 

After years of faithful service, what happens if your good old CB starts to play up?  Do you just leave it in a corner to gather dust and go out and buy a new one or take your radio in to a repair shop?  Many CB problems are easy to fix, and can be successfully repaired by the average hobbyist at home.  Don’t worry, I’m not about to suggest that you attempt full-on retunes or major surgery, in many cases this is simply not necessary.

 

 

To begin with, you will need some tools.  Obviously, a good set of screwdrivers, both flat and phillips type, of medium and small sizes is required to undo cases, microphones etc.  A good quality soldering iron, preferably temperature controlled with a range of say 15 to 60 watts or more and some small conical tips will allow you to effect many repairs.  Note that I said ‘small’ tips - a medium or large tip just doesn’t cut it for electronic work.  A medium tip is good for soldering some large RF connectors, but that is it.  A sharp safety blade knife and a pair of small precision side cutters will also be indispensable for repair work.  A small pair of needle nose, serrated jaws pliers and a desoldering pump are also very handy.  You will also need the usual consumable items like electronic grade solder, desoldering wick, cable ties, wire, electrical tape, heatshrink, spaghetti tubing and other odds and ends.

 

 

Some basic test equipment should also be at hand, with the main item being the trusty multimeter.  A power meter and a dummy load are about the only other test items you would need. Of course, all the tools in the world are of no use if you do not have the skills to use them.  Soldering and the use of a multimeter would probably be the two main areas where your skill will determine how well you can effect a repair.  Soldering well takes three things: practice, practice and more practice.  Oh, and some good soldering gear.  Its no use trying to solder delicate electronic items using a soldering iron that is so big that you could weld two I beams together!

 

 

Correct use of a multimeter again comes with practice and some knowledge.  While I’m not about to give a lesson on how to make measurements using a multimeter, generally speaking, you measure current through a component in series, voltage is measured across a component or system, and resistance is measured across a component that has been taken out of circuit.  There are other tricks to learn, such as what readings to expect in what situations, and what readings a certain component will give when measuring it, which can all be found in general knowledge electronics books.  I learnt much of what I know in this area many moons ago from the Dick Smith Funway electronics series, which is probably a good place to start if you are unsure.

 

 

OK, so lets get stuck into a list of common symptoms, their causes and repairs.

 

 

Problem: no sound on receive:

 

Some CB's need their microphone plugged in to activate the speaker.  Make sure the microphone is plugged in.  Check the speaker for continuity and see if the external speaker jack is working.  Dirty contacts in this jack can also cause the problem.  If all this is OK, then it is likely that either the microphone has a wire broken in the mic cable, especially where it enters the microphone body or connector, or there is a problem with the audio amplifier transistor/integrated circuit inside the radio.  After years of being flexed back and forth, the wires inside a microphone cable will eventually fail.  Often the failure is not sudden and complete; you can sometimes make the fault come and go, often heard as a crackle by wriggling the cable.  You can localise the fault in the cable by small amounts of wriggling here and there, and if the fault is located very close to the microphone connector of body, you can cut this section of cable off and redo the connections.  If the audio amplifier is suspected as being faulty, it would be best to resign the radio to a repair shop at this stage: they will most likely have to order in the component required.  Another problem that occurs after time is the volume control becomes ‘scratchy’, which can often be fixed by squirting in some special electronic contact cleaner into the back of the volume pot.  In some cases, none of these items are at fault, and the repair technician will be able to find it for you.

 

 

Problem: radio dead, no lights or sound, not even a weak hiss from the speaker:

 

This is probably a power related problem.  Check the fuse - is it blown?  If so, replace it with one of the same rating.  If that fixes the problem, great.  If not, did the fuse blow again?  If it did, DO NOT replace the fuse with one of a higher rating.  This is dangerous: you can totally and utterly destroy the radio if you do this, or damage your power supply or even start a fire!  Other acts within this category are replacing a fuse with a screw, wrapping foil around the blown fuse and using no fuse at all.  The original fuse blew for a reason: find the reason.  These days, most radios are protected from having the power connected up to the radio backwards by using a hefty protection diode, which is designed to blow the fuse to protect the radio from damage.  If the radio was connected backwards, replace the fuse, and all should be OK.  If the fuse blows again, even when connected up the right way, the diode could have gone short circuit.  Open up the radio, and take a look at where the power enters the radio.  Somewhere nearby should be a larger than normal diode - this will probably be the diode in question.  Remove the diode by desoldering it and check it out with a multimeter.  If the diode is faulty, replace it, carefully observing the polarity.  While you can replace the diode with one exactly the same, you can use substitutes.  Using a high wattage rated zener diode is a good idea: this will protect the radio from both reversed power connections and power supply over voltage faults.  An 18 volt, 3 watt or greater zener diode should do the job.  By the way, this is by and far the most common problem that I see with radios.  The moral of the story is to get your power connections right and to always, always use a fuse, and use the correct rating.  Burnt looking components or PCB tracks, or even missing components with tell-tale flash marks indicate a severe over current situation, probably caused by an over rated fuse or no fuse at all, can be fixed, if there is no other damage sustained by the radio.  Often, burnt components end up being the protection diode, which can be replaced as above.  Burnt PCB tracks can be fixed by carefully scraping away the burnt section of track, exposing some fresh copper either side of the missing section and using a small section of insulated wire to join the tracks.  If the track bends, or goes very close to other tracks, it may be best to remove the entire track back to where the next component is soldered, and using insulated wire soldered to the component solder land or pad to replace the missing track.  If these tricks do not enable your radio to come back to life, it means a trip to see the radio doctor I’m afraid.

 

 

Problem: other people tell you your transmit audio breaks up:

 

This is most likely the microphone cable again.  As above, locate the faulty section of cable, and if close to one end, cut out the faulty section and re-terminate, or get a new microphone cable.  This is obviously not the only fault: if the radio is used in a mobile situation, all the vibration of motion in your vehicle over the years can cause solder joints to fail.  Look for these by removing the case of the radio and looking for any obvious solder joint faults, if necessary by using a strong light and magnifying glass.  If none are apparent, you can connect the radio up to power with the cover still off, and using the insulated handle of a screwdriver, gently tap the PCB in all locations to try to provoke the fault.  Generally, you will be able to localise the fault this way.  Another thing to check is that the antenna, coax and connectors are in good shape with no intermittent contact problems.  An interesting fault that occurred to a radio in my car emulated this fault, but the cause wasn’t a faulty solder joint.  For all the world it sounded as if a solder joint was at fault since only when the car went over a bump did the transmit audio break up.  After much searching, I found some corrosion on one corner of the case of the radio, exactly where an internal spring tab made contact with the case to ensure good earthing to the chassis of the radio.  With the corrosion there, a good earth was difficult to obtain, and whenever the car went over a bump, an earth was momentarily obtained, causing the transmitter to exhibit an effect called microphony, where the transmitter is affected by vibration which can change the capacitive relationship between surrounding components and the transmitter.

 

 

Problem: no transmit:

 

Modern CB radios are, electronically speaking, quite reliable.  If you can not find a ‘mechanical’ problem, such as a dry joint, damaged antenna connector or connector tail, then an electronic component fault should be suspected.  Sometimes this does happen for seemingly no reason at all, and this is bad luck, but more often than not, a situation has provoked the radio to fail in this way.  When the radio failed, was it in continual use for an extended period of time?  When any radio transmits, heat is generated by the transmitter, which must be dissipated, or the heat build up can damage the transmitter.  Feel the back of your radio now: if you have been talking for, say the last five minutes, and the back has warmed up so that it is warm to hot, then it is possible that you will be at risk of this heat damaging your radio. Adding a heatsink to help dissipate the heat would be a very good idea.  The point here is that prevention is better than cure.  Unfortunately, repair of a non-transmitting radio is best left to a proper repair establishment.

 

While we’re on the subject, lets talk about another common reason for transmitter failure.....

 

 

Screwdrivers.  Why, why, why do people always ‘wind up’ the transmitter power (and everything else for that matter) and expect to get away with it with no consequences?  Certainly, the power level of many CBs can be adjusted upwards, but why bother?  For starters, it is illegal.  But that is the least of your worries; the transmitter in your radio is designed to do about 4 and 12 Watts for 27 MHz radios, and up to about 5 Watts for UHF CBs.  If you get it to do double the power it is designed for, failure of the transmitter stage is inevitable.  It may take a few seconds or it could take a few months, but it will happen.  Lets take an example: an AM only 27 MHz CB is happily working away at 4 Watts.  Mr I-want-more-power comes along and winds up the wick to, lets say 10 Watts, 150% more than the radio is designed for.  The heat now dissipated in the transmitter amplifier is now more than doubled.  The more heat causes the amplifier to become more inefficient, making it work even harder to try to keep up with the new power level requested of it, which in turn causes more heat and so on.  The voltage regulator is also straining to keep up with the increased current demand, and your power supply may also be baulking at the new demand being placed on it.  Now, lets also imagine that the antenna is not as good as it could be.  Some of this 10 Watts of power is being reflected back into the transmitter, causing even more heat.  Mr I-want-more-power is also disappointed that, even though he has more than doubled his output power, signal strength reports haven’t increased all that much, and neither has his overall communication range.  According to the inverse-square law, which all radio waves obey, doubling the output power does not double your communication range.  It only increases it by 50%, and that assumes that everyone else has done the same to their radios so that they can talk back to you, too.  Not only this, but the new 10 Watts is pretty ugly; not all of it is being produced on frequency, and the modulation has suddenly become unclean.  People are finding it hard to understand you now, and after a while, they cease to hear you at all.  One of three things has happened.  One: the power amplifier has ruptured, and has ceased to work.  It may also have short circuited, causing a large inrush of current to overload your power supply.  But don’t worry, the fuse will protect your power supply from any damage.  Oh, you didn’t use a fuse?  Oh dear…  Two: your power supply has finally decided that it can’t supply any more current, and its internal pass transistor has gone short circuit, causing up to 30 volts unregulated DC to hit your radio.  The radio is now very dead…  Three: any other number of component failures relating to the transmitter or power supply regulation part of your radio have put you off air…

 

 

That pretty much describes the most common problems and how you can go about repairing them.

 

 

For the more tricky problems, I really do recommend that you take the radio to an established repair shop.  Full on retunes after having a screwdriver mad person attack a radio is not something that can successfully be tackled at home, as are the more tricky transmitter or receiver problems. This sort of repair requires more test equipment than you might imagine; frequency counters, good quality power meters, modulation measuring meters, oscilloscopes, audio and RF signal generators, spectrum analysers, noise and distortion meters, and so on.  It also takes more skill and knowledge than the average back-yarder will have, and a factory service manual will most likely be required.

 

If you are a keen hobbyist, and have the tools, knowledge, skill, test equipment and time to do your own repairs of this magnitude, that is great, you can save a lot of money doing your own repairs, all while learning more and more about radio principles, but if not, please, if you are out of your depth with a repair, take the radio to someone who really does know what is going on.