Hey guys i got a cobra 29 classic pushing a grey 150 bi linear the radio has been tuned and peaked and was told it was set to push amp fine however when i key up i get a humming noise what needs adjusting thanx

I will answer the question. Figuring that 10 amps is continuous not peak. 10 amp multiplied by 12.5 it should be good for 125 watts. But you are on the ragged edge. I do agree you should lower dreaded key but you did not ask that question. But a swing kit will solve that. That is amusing SSB and the 80 watt cobra 29 comments.

some one tuned their radio to the wrong side of peak and now you have downward modulation .Try re-tuning it back to to its original settings and if you get it back don't mess with it unless you know what the shell you are doing.

Not if the power output from radio to amp is set correct and the jumper for coax is correct length and there is absolute perfect match between them and the antenna used. Afterthought: also the amp better be connected to something big enough to power it.

You are actually tuning the antenna to the vehicle it is mounted on here are some very good resources. http://www.rightchannelradios.com/pages/cb-antenna-tuning-guide-pv-c0-61.htmlhttp://www.wilsonantenna.com/swr.php

Be aware that if your radio has been modified it may produce a dirt signal that an swr meter may read as reflected power. In this case it may be more accurate set with a unmodified radio also if your radio is setup for a linear it may have a deadkey to low for the swr meter to calibrate correctly.
Hope this was of help.

Only a qualified technical person is allowed to do so per FCC, (Federal Communications Commission). That means what it says. If you are not one, then you cannot do this procedure....hope this helps....

did you make sure the caps stripe is facing the back of the radio? If not, its the wrong way.

Also, don't use a 1/4 watt resistor because it will get too hot for it to last. use 1/8th.

changing the cap size will not change output, however it will change the tone. Bigger the cap, the deeper the tone of audio.

to be honest with you, high output mods are worthless in these radios. they are fine with out them. mosfets are unstable and are very dirty transmit finals. As well if you are planning to use a linear you may hit it too hard unless its a high drive, in which case you will need more then 35 watts swing to drive it fully. A stock cobra 29 that has been peaked and tuned, then the swing mod put in, 220uf e cap and 100 ohm resistor, which should give you about 2 watts, should swing to about 27 watts, and kick an amp in the pants.

I run a cobra 29 in my blazer with a dave made 5 pill, and I key on 6 pills and I near keyed on a 2x8. very close. All that I have done to the radio is peak and tune, (of course I have a few tricks with that up my sleave which I don't usually share with just any one.) and variable power, wich will control the dead key, which is basically a varible swing kit. I run it just under 2 watts, just enoug to get a 125 watt dead key out of the amp. The amps swings over 920 watts.

any more swing from the radio will beat up the pills in the amp.

A swing kit is nothing more then a Cap and a resistor. There really isn't a different way to do a swing kit, unless you do variable power, which you don't need unless you are going to run a linear, and even then you can just set the dead key with a swing kit.

you need a watt meter, preferably one that reads peak and average watts. If not, average will be find.

Open the bottom cover of the radio. Hook the radio up to the watt meter, use a dummy load or have it hooked to the antenna. Never key the radio with an antenna or a dummy load.

Find VR4. Its toward the back right of the radio. Turn it all the way to the left. That increases your modulation.

Next find L14. it is at the back in front of the final resistor. While making a steady sound into the radio, just make an ahhhhhhh noise. Back the slug out of the tank until the needle hits peak. Don't move it out any more or you will over bias the final.

Now you will need a 220 uf electrolytic cap, and a 100 ohm 1/2 watt resistor, both of which you can get from radio shack. (this is your swing kit.

To the right of the tank slug is a jumper maked JP36. Us soldering wick to remove the solder and pull out the jumper. Now put the cap in it with the stripe facing the back of the radio.(that is very important. the other way you will blow the radio.) Solder it in place and cut the extra leads off. Now on the back of the board were you soldered in the cap, solder the 100 ohm resistor in between the 2 points.

The amount of resistance determines you dead key. you will be at about a 2 watt dead key which is good for swing and if you want to run a linear. The higher the resistor the lower the dead key, and the lower the cap, the higher the dead key.

That is your basic peak and tune with a swing kit.

If you're using the RFX-75 amp section. Then you'll need to make sure your transmiter section is tuned for full output. The last last unit I installed in a Cobra 29 LTD, the radio modulated right at 100 watts output.
Hope this helps,
J.R.

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.

nearly all problems are caused by poorly tuned or untuned antennas. a good antenna system with a cheap stock radio will out perform a great tuned radio with a bad antenna system any day of the week. if you dont believe me, check the firestik antenna website's tech notes. or wilson's tech notes.