"You fried the final," he said. Antenna the culprit?

Keep in mind that hardly any short circuit will clear itself. Some are even built in to save a user prone to "oops-dang"-kinda good-ups. An all too often such goof occurs when odd color codes are used in a CB mounting position above or below the windshield in a big rig's cab. Sometimes a pair of wires, like say green and white, are used. Simple guesswork oughta give a 50% chance of having them right, right? Well, for that *other* 50% of times, most CB manufacturers install a 4-cent diode "backwards" across the power leads just inside the radio's housing. When power in hooked up correctly, the diode is "reverse biased" so it doesn't conduct current. At this point, all is fine & dandy (that's tech talk thar....)
For the 50% of times when the color code is guessed at incorrectly, that 4-cent diode shorts out, and stays shorted out. Thereafter, as long as the right size fuse for the CB is used, that fuse will keep shorting until some tech opens the radio up and replaces the 4-cent diode.
It *is* possible, however, to "out-dumb" the 4-cent protection diode. As I all too often find, a radio user will keep using larger and larger fuses until a value that will still allow current to flow BACKWARDS through the expensive components in the radio, as soon as the 4-cent diode finally works like a fuse and burns out *open*: audio chip, driver and final(s) transistor(s), and the like.

If, once the proper positive and negative leads are identified and tied to a CB with a typical 3-amp fuse that blows as soon as it's hooked up, that reverse polarity diode is doing its job and is showing a dead short. (An ohmmeter will show the same result without any power being applied to the radio, saving on the cost of fuses). At that point, unless you're capable of recognizing a diode that's installed on the circuit board right next to the power input connector, it's time to get help to identify the part and get shown how to replace it.

If the user keeps using bigger and bigger fuses until the fuse can handle the backwards current through the radio, then that's when big, expensive parts, and even the traces on the circuit board, will burn open. That's about the time to think of that particular CB as ballast for small bag of empty beer cans, to keep them on the bottom of the river/lake/pond/ocean/cesspool.

If the DC wiring to the radio is OK, then troubleshoot by changing only one variable at a time, which is the essence of the scientific method. If a run of new coax from the radio out to where the new antenna will be mounted shows full power with a good dummy load on the output side of the wattmeter at the end of the coax, then the coax is good.

Most of the "dead" radios I come across either have the DC power leads reversed, or the hot element side of the antenna (the part that's connected to the center of the coax) is shorted to the frame. A $10 DVM, is plenty to show DC polarity and continuity for the coax and the power leads, and a $20-40 SWR meter is enough for an average CBer to install and check his own install(s). Pay attention to where any insulators on the antenna mount are supposed to be installed, and the average user can do his own install correctly in an hour or two if it's his/her/its first install, and less than an hour for any subsequent jobs.

Hope that helps.
73

 

Bravo,
Very good post,

CMFofNC

 

Folks, I reread the OP's post 3 times and something really bothers me. Melted components indicate to me that there is excessive current draw. Are the correct size fuses being used?
.
.A blown final [final amplifier section] would indicate excessive signal strength is being reflected back to the radio. That could be caused by the following faulty items: antenna, coax or ground plane.
.
.To the OP, when you install a radio, the very first thing is to check you Standing Wave Ratio [SWR] with an external SWR meter not the one that comes with the radio if it even has a SWR meter in the radio. Then change one component and retest. As a previous poster listed, use the scientific method. If you are not sure of what you are doing, come back to this forum for advice before you start using your radio. That will save you some money instead of a costly mistake.

 

My first suspicion is in the "tweak" job on the radio.

When components have their rating exceeded (which can easily happen on a tuneup), they do funny things. Most of the time, they fail "open", and the radio simply quits working. (Usually receives OK; no transmit power.) Occasionally, though, they can fail "short". A resistor that limited the current to the final now lets a LOT more current through. Or, the value of some component has changed enough to let the final amplifier circuit go into oscillation. In either case, the result is a fried final output transistor.

Recommendation:
1. Replace the radio. No sense throwing good money after bad.

2. Find a *good* shop. There are lots of recommendations on this board. (I've talked to Dave at DTB Radio (dtbradio.com) on the air; his stuff sounds good. PA Turnpike at I-81; he does mail-in service.) The shop may take the old radio as (partial) payment for a tune-up; they can always use the parts.

You may want to reconsider getting your radio reworked in the first place. Most of the people on the air are running a stock radio. Increasing your power doesn't help your reception one bit.

 

By the radio at rest do you mean turned off? If radio is on 1/2 scale meter on receive while powered up that wouldn't be unusual during the day or if your trucks electronics send out harmonic signals. If your radio powered up shows full signal while in receive regardless of time of day and regardless if vehicle is running or shut down that would seem to indicate faulty meter adjustment at the very least. Watched a online video of a fellow tweaking his own radio, he just went in and cranked every variable resistor wide open. Since some of the resistors he screwed with were for meter adjustment no way it could have still been correct. He didn't include a sound check. Possibly something similar happened to yours.

 

If your in texas, go over to Kilgores CB Shop off I-20 in Fort Worth. He's a older gentleman that's been working on radios seems like forever, does real good work...May have to leave it with him is he's busy, but he can get ya fixed back up...I-20 west towards Fort Worth, take the Mansfield 287 business exit, and he's about a couple blocks north on the left.

open 8:00AM till 1M Mon-Fri...SAT. 8 till noon.
5136 Mansfield Hwy,
Forest Hill, TX 76119

(817) 478-2111

 

Didn't know they were still around, I bought a Clear Channel AR 3500 from them about 30 years ago and he seemed old then .... Thanks for the info Kerry.