Question about Cobra 29 LTD CB Radio
Essentially what is done is replace the RF output transistor with one that can handle the 30 Watts output. Since you are not going to achieve it with higher B voltage, you have to modulate more current, and the bias voltages adjusted to accomodate the different characteristics of the new of the new final. Then the radio is checked to make sure the tank circuit is still in tune.
This is the basic outline of how to do it in a manner where you actually learn hands on, and apply what you should already know.
The alternate method is to install a linear amplifier. A 10 meter linear can usually work with only slight modifications.
BTW. the common search engines are worthless for finding radio mods. By design- they are oriented to a controlled marketplace.
Posted on Feb 25, 2012
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Posted on Jan 02, 2017
Wow. That's not at all a solution to what this guy is asking. And why all the caps??
The one point jm77437 has that is correct is that you do not set the radio, you set the antenna. The rest is blah blah.
Here is how you tune your antenna for you radio. First off it depends on what kind of antenna you have. most fiberglass antennas have a tunable tip to adjust the height of the antenna. Metal whips either have a collar at the base that the whip will either slide up or down in to adjust the height. The other type of metal whip's you have to physically clip to shorten the antenna, you must be careful with this type because once its clipped, that's it. You cannot add what you cut off.
I am going to give this example using a metal whip with an adjustable collar.
First after your radio is hooked up in the vehicle, drive to an open lot. Buildings and other structures can cause extra reflection. turn to channel 1. Flip the meter to CAL. Key the mic without talking. Now turn the CAL knob up until the needle reaches the little upside down triangle all the way to the right marked CAL. Now you have just celebrated your radio for your setup to measure you SWR. Now while still holding the key, flip the meter switch to SWR. The needle will move, and where it stops is your SWR reading. The lower the better. Anything under 1.5 is considered really good. But still, the lower the better.
Make note of what your SWR was on ch 1.
Now flip to channel 40. Repeat the SWR process. You must recalabrate each time you take a reading.
Now like jm77437 said. If your SWR reading is higher on 40 then 1, then your antenna is too long, and you must make it shorter. Loosen the collar and lower the antenna in the collar 1/4 inch at a time. check the swr each time you adjust the antenna.
If your SWR reading is higher on channel 1, then your antenna is too short. You must make your antenna longer. Do this by loossening the collar and pulling the antenna upward in the collar 1/4 inch at a time, again checking the swr each time you adjust the antenna.
What you want to achieve is having the same SWR reading on channel 1 and 40. then you have the best match you are going to get with that antenna.
If your swr is under 1.5 on all channels, you don't need to fuss unless you are a perfectionist. then by all means go ahead.
If your reading is higher then 3, or your ant light comes on, something is really wrong. Make sure your antenna is connected all the way, and the coax is not damaged.
Its important to have an appropriate SWR because if you do not, you are reflecting too much signal back into the radio, and eventually you will burn out your final transistor, and your radio will be toast.
Some tips: make sure you check the swr with the doors and windows shut. Make sure you are in an open lot. Remember to calabrate ecah time you take a reading. If you have had your radio peaked by a radio shop, and they have lowered your dead key, or have done a swing modification, then the meter in your radio will not work when taking an SWR reading and you will have to use an external meter.
Posted on Apr 24, 2009
SOURCE: not enough watts
All he is asking for is this...
Find VR4. Its near the big transformer. Turn it all the way to the left. That is your AMC. that will turn your modulation all the way up.
If you have a power meter you can raise your dead key, or carrier signal as raising the modulation only effects your swing.
If you don't have power meter, then don't mess with it. Increasing the modulation will be a big improvement.
If you do have a power meter then here is what you do. Find L12 near the final, is near the heat sink in the little space near the transistor. that is your final. Turn this out, to the left, with the radio keyed until the needle stops moving up. don't turn it out any more.
***** NOTE IMPORTANT *******
Do not do this with no antenna hooked up, or without a dummy load. You will blow up your final. You can never key the radio with out a 50 ohm load on the radio.
You can make a simple dummy load to do this inside instead of out in your vehicle. Take a CB coax and 5 150 ohm resistors. Hook the 3 ends of the resistors together and solder them, and do the same with the other side. then solder on end to the center piece of a coax, and then to the out side piece. that will give you a 50 ohm load when you connect it to the radio. If the Ant light goes on, stop right away you did something wrong.
You will also need a power supply. If you have an old computer, you can use the power supply from it to power the radio. Yellow wire on one of the power supply ends to the red wire, and the black can go to either of the black wires.
If you are not sure, just increase the modulation with VR4. that will make the radio swing to about 20 - 25 watts.
Posted on Feb 19, 2009
You should not calibrate the antenna before you install it on the car!! the antenna needs to be calibrated on the vehicle that it is going to be on. Type of vehicle, different mounts, and many other variables can change the amount of reflection you will get.
j0hn9999 did a much better job but left some info out.
Take your SWR readings on channel 1, then on channel 40.... NOW, if your SWR is higher on ch 40 then ch 1, then your antenna is too long and it needs to be shortened. Refer to the antenna instructions on how to shorten the antenna.
If your SWR is higher on ch 1 then on ch 40 then your antenna is too short. You must lengthen the antenna. Again refer to your antenna instructions on how to lengthen your antenna.
You must calibrate each time you take a reading.
Make sure your doors and windows are shut when taking a reading
Make sure you are in an open lot when taking a reading as buildings, trees and other structures can cause reflection and give you an inaccurate reading.
If you adjust the length of your antenna on another vehicle and then put it on yours, the SWR will be off. It will need to be tuned again.
An external meter is better then the one in the radio, but will need to be hooked up in the vehicle you have the antenna on.
If you are not running any kind of power, or do not have a swing kit installed, and the radio is stock, the meter in the radio is more then enough to get a correct SWR reading.
Posted on Apr 24, 2009
i would highly suggest you take it to a cb shop to get a cb tuned and peaked. before you do that though, make sure you have a good antenna system. a good antenna will have a much greater effect on your cb than tuning and peaking. as far as output, who knows? it all depends on the skill of the person doing the tuning. be warned that any output over 4 watts is illegal and you could recieve hefty fines if caught with an illegal radio.
Posted on Sep 24, 2009
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