Antenna Grounding Problem

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2006exploder
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Antenna Grounding Problem

#409700

Post by 2006exploder »

Hello everyone,

THE PROBLEM:
I recently ditched the magnet mount and installed a new antenna for my cb radio, and decided the check the SWR. It was a tad high, and I suspected that the antenna was not well grounded. The antenna is mounted on the lightbar on my roof. So I set about grounding the antenna mount better. I ran a piece of 8 gauge (all I had at the time) to a nice ground bolt in the roof. So, after grounding the antenna, I plugged my SWR meter back in and turned on the radio, and poof! smoke out the back of the radio, and instead of static, a rapid pulsing buzz came through the speaker. I quickly turned it off, and after waiting a few minutes, turned it back on, and no more smoke, no more buzz, just static. In fact, my reception seemed a lot better. I immediately started picking up skip on several channels. However, the meter light on the radio was burned out. The radio proceeded to work normally, but I could not test the transmission. So my question is thus: obviously grounding the antenna mount caused something to burn up in my radio. The antenna mount is connected to the coax shielding in my antenna mount, so by grounding the antenna mount, I grounded the coax shielding. The coax shielding is connected to the frame of the radio, so I essentially grounded the frame of the radio when I grounded the antenna mount. Upon inspection with a multimeter, the frame of the radio is not usually connected to the ground. I did recently replace the incandescent bulb that illuminated the meter on the radio with an LED after it burned out, but I had to use two resistors to make a voltage divider, as there was too much current for the LED to handle. This voltage divider was likely touching the frame of the radio, and upon taking the radio apart afterwards, sure enough one of these resistors had completely burnt up. So my questions are as follows:

I understand why grounding the antenna mount and in effect, the frame of the radio fried the light circuit I had for the meter light if it was touching the frame, but would that short out other components in the radio?

Am I correct in my analysis that when one grounds an antenna, the frame of the radio becomes grounded as well? is this supposed to be the case?

THE REPAIR:
The radio I am working on is a Realistic TRC -425 for reference. So upon taking the radio apart, I noticed a component that looked like it was totally burnt up and fried, So I pulled it, and another component next to it that appeared suspect. Both components are next to the final transistor. The component that I was sure was fried, I was unable to identify, but after looking at the photofacts service manual for the radio, I determined it to be a resistor, albeit an abnormally large resistor to me though. In the photofacts wiring diagram is is listed as R908, with the number 8.2 listed below. other resistors in the circuit diagram have numbers such as 33k below them, which I assume is 33 kila-ohms. So would the 8.2 mean 8.2 ohms? this seems like a small resistance, especially for such a physically large resistor. also, how would I go about finding one to replace it? also there is another component next to the blown resistor that I removed because it looked burnt, but I am not sure what it is. It is a bright blue bulb shaped thing that sticks up from the circuit board, and says +1.0 on the first line and 50V on the second. Anyone know that this component is? A capacitor maybe?

Apologies for the long post,
Thanks,
Dylan
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443 Arizona
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Re: Antenna Grounding Problem

#410118

Post by 443 Arizona »

i think the radio smoked because it wanted to, not because of grounding unless you screwed it into a wire,or,,, has anybody ever modified the radio? look for solder or loose debris that may have cause a short, even dust.
as for the resistor, start with an 820ohm and see what happens, the other blue one sounds like a cap, look at similar RS radio and see whats there, might get lucky and see the same circuit, unfried. are the other resistors marked with numbers instead of color-bands ? might be something other than a resistor.
you are on the right track with the photofact schematic,
grounding the radio helps to eliminate static,
what kind of truck is it? and where are you getting the 12v power from? sometimes it makes a difference.
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jessejamesdallas
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Re: Antenna Grounding Problem

#410119

Post by jessejamesdallas »

...my head hurts after trying to read all that... :roll:

I agree with 443 Arizona...think the radio smoked because either it was it's time, or something else totally unrelated to the antenna.

First problem was trying to mount the antenna to the light bar...bad idea..."looks good", but you'll be pulling your hair out trying to get a decent ground, and running ground wires is not going to cut it...

Mount to roof, or you can mount to a pedestal or riser, if the riser is well grounded to the bed of the truck, and the bed itself is well grounded to the frame...

I have my Predator 10K mounted to a home made riser I built using 1" square metal tubing...The riser is bolted to the floor of my trucks bed, And there are some added 1" ground braid straps running under the bed to the frame in several spots...My SWR is flat as a pancake...

least it's flat "most of the time"...did run into a issue about a week ago when I noticed the SWR was a little higher than normal...Found out the coax connector that screws on the bottom of my antenna mount had oxidized a little over the years, and I had to clean it up and screw it back together, and it was good to go again!...Probably first time in 3 or 4 years I have unscrewed that coax! :lol:
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