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307
| | Posted on Friday, November 30, 2001 - 9:29 pm: | 
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Solving squealing problems
Did you ever have a radio that would squeal really loud through the talk back circuit and you tried everything to get it to go away and failed ? Here are a few tricks I learned along the way that may help you in figuring it out . In most cases it is a simple problem but sometimes it is a problem that only a qualified CB shop can figure out . Squealing comes normally from a ground loop or some grounding problem that you may have in the vehicle . A ground loop is a resistance from one point on the ground to another point on the same ground .It should be 0 ohms or close to it but in some cases such as hooking the positive wire up on the positive on the battery to your CB radio and not hooking the ground up to the ground up on the battery because you hook it to the frame of the car . When you do this it creates a ground loop path from the negative of the battery through the chassis of the car to the wire you attached to the body and then to your radio .The ground loop is then completed when you screw in the coax because then it picks up a second ground of a different value through the antenna coax .
Since the newer radios have the chassis of the radio isolated from the main circuit board in the radio you have a differential voltage from ground to chassis when the above configuration is hooked up . When this happens the coupling capacitors that bleed the spuratic RF off of the main board to the chassis ground usually will bleed it to the weakest of the two . In either case the end result is a squeal .
There are several ways to get rid of the squeal . Never run a wire from an antenna bracket to the body . In most cases it becomes an inductor and will make the problem worse .Always run the black and the red directly to the battery and NEVER the body of the vehicle for ground . Keep the power and ground wires short as possible . Never roll up the extra , cut it off . If you ever need a new power cord , do not worry they are only about 3 dollars for a heavy duty one.
Never roll up unused coax that is to long into a coil of any kind . When you do this it becomes a live antenna and can (not always) give off RF that will be fed into the ground system of the car .
Never use longer wire than necessary on a remote speaker . Cut it off and splice it just long enough to get from the radio to the speaker .
Make sure that when and if you had a new microphone cord put on your mic that the technician used the shielded wire for the audio and no other wires come in contact with this one .So many truck stop techy's will twist the audio wire along with another one in the cord together for strength and this is the number one cause for squealing because the shielded wire has now been exposed to the outside world .
Remember never to place an added box or echo unit directly on top of the radio . (metal to metal) Always place a piece of cork board or cardboard between them to isolate them from connecting grounds .
If worst comes to worst remove the bottom cover from the radio and locate the 6 screws that hold the main board to the chassis (frame of the radio) Using a screwdriver make sure these screws are tight because that's how the radio de-couples the main board from spuratic RF is through the screws from the capacitors .
Always make sure that the coax connectors on either end of your coax are soldered . If you are unsure of this simply unscrew the back shell of the PL-259 all the way off . you will see 4 points or holes that have been filled in with solder .If you can see ANY hole that is open and if looking into the hole you see copper then you need to re-train the guy that put them on and explain to him that ALL holes must be filled with solder and flowed properly . One last thing Never run a wire from the case of the radio to the frame of the vehicle , there is a good possibility that this too could induce another ground loop . |
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