CB Radio Fix?
ActualDragon wrote 01/13/2018 at 22:48 • 2 pointsI have a CB radio my dad gave me. He ran freight for 20 years, all over the country, so the radio has some sentimental value. Not enough, however, to take it to a actual shop. XD It only broadcasts a few miles, when hooked up to a 2 or 3 foot whip antenna. Pretty sure that means SWR needs adjusting, but the dials broke. The radio's here: https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=2ahUKEwi25q3bgdbYAhVr4YMKHcUaD_wQjRx6BAgAEAY&url=https%3A%2F%2Fwww.bobscb.com%2Fshop%2Fproduct%2F119%2FCobra-29-LTD---Classic-Professional-Instant-Channel-9-SWR-Calibration-Antenna-Warning-Indicator--AMPA-40-Channel-Mobile-CB-Radios%2F&psig=AOvVaw0Y7v0HHrJkrNQtR99u21Cs&ust=1515969792279246
The dial on the left is the SWR, and the know second to the right is the dial. He said it's never worked (as far as he can remember). It's never really a problem until now, because it's range is shitty. I found this: https://www.ebay.com/itm/Cobra-29-CB-Radio-aftermarket-Replacement-Signal-Meter-Roosters-Radio-Connectio/222651068386?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20131017132637%26meid%3Db6649814c8fd4095a68b542515458c05%26pid%3D100033%26rk%3D4%26rkt%3D8%26sd%3D112740807594&_trksid=p2045573.c100033.m2042
I think it will work, but i'm not sure. Anyone know? And i know I'm probably putting to much effort into this (the radio is 25 years old and the same one is $50 on ebay), but is there anything else I can try? I really want to get it working for him.
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I don't think it's the antenna, but for argument's sake, i'll add this- The antenna is a 3 foot whip antenna: http://www.antenna.com.tw/l2whip.html. It was mounted on the top left of his toyota corolla, but he switched it to our 4Runner, same spot. I don't really understand much, but you adjust the SWR on it by moving it slightly up and down via a hex screw in the side.
Are you sure? yes | no
If you get or make an antenna specifically made for CB band, you will have the best luck at getting better range. With radio antennas, height and length are 2 predictors of range (without going into directional antennas). If you are getting 2 or 3 miles, with a random antenna, the radio is probably OK.
There are lots of books on making antennas available for hams. With a little interpolation, they can be adapted to the frequencies used on CB. In the US, it is 11 meters or 27MHz. No idea what other countries use. An external SWR meter is probably the best bet if you are going to mess with making an antenna. RF wizards can probably simulate/predict lengths accurately enough, I wound up cutting to tune my home made ham antenna.
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I have a cb antenna. Like I said, my dad was a trucker for twenty years, so we already had a 5' fiberglass antenna. Needless to say, though that looks fine on a tractor trailer, it looks a bit stupid on anything else. That's why we got a 2 or 3 foot whip one. One of the magnetic kinds.
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Do you have the antenna mounted on a vehicle now? The vehicle forms part of the antenna (the ground plane), so it needs to be on a large piece of metal. If it is on a trunk lid, you might make sure that the lid is grounded to the rest of the vehicle with a ground strap. Other than that, basic SWR adjustment should be a one-time thing.
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Well, the meter movement itself is just a DC ammeter movement. You can check if that works easily enough before buying a replacement. The SWR measurement circuit behind the meter may also be at fault. A common SWR measurement circuit uses a directional coupler and two detectors (probably just diodes in your rig) to measure forward and reflected power. Any of those components could be bad, too.
You can check out the schematic for that very radio here.
http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltd-st_29ltd_sch.pdf
In block C6 of the diagram, you can see the coupler and two detector diodes. The meter movement is just below.
The main page for that radio:http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/
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how would i check that?
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Disconnect one of the meter leads and apply a small current (try 100uA to start) to the meter to see if the meter deflects. Neither the schematic nor the service manual show the full-scale current for the meter, so you have to guess.
Try a 47k resistor in series with 5V to get around 100uA , and see if that moves the meter. If it moves, it's probably OK. If it just sits there, you could try 1mA (4.7k resistor), but it probably means the meter is dead.
I looked at the ebay meter you found. It seems to be the proper replacement part if the meter movement is really dead.
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You can also buy an external SWR meter that goes in-line between the radio and the antenna, and use that to tune the antenna. They're relatively inexpensive. I haven't used one of the dedicated CB ones, so I can't give you a specific recommendation.
Or, you may have luck borrowing an SWR meter from a local ham.
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I know, it would just be really great to get the dial working again, it's bulky enough as it is, and that's cheap, and it's just a matter of soldering some wires
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