Cobra 29 ltd with high and low power

[805centralcal]

Anybody know of a good way to install high and low power on a cobra 29 ltd via switch like ch.9 switch or cb/pa switch. I was thinking running jp36 in series. Any ideas? Thanks

 

[sonoma]

that is the way it has been done for years to change power on a 29 etc radio

 

[543_Dallas]

Install more diodes in series with d8 to get the desired low power. Then mount a relay back there to jump across as many diodes as necessary to get the high power you want.

 

[805centralcal]

Install more diodes in series with d8 to get the desired low power. Then mount a relay back there to jump across as many diodes as necessary to get the high power you want.

I though d8 was the protection diode for the transformer. Did you meen the limiter diode, I believe it's d11. I'm confused...

 

[sonoma]

the easiest way to do this is put a resistor capacitor at jp36 position , be sure to take the jumper out. get the cap resister set where you want the low power set. next just put a spst switch across the location you put the ,cap in and this way it will bypass the cap resister and give you the high power mode.. switch open will give you low power then closed you will have high power.

 

[543_Dallas]

I though d8 was the protection diode for the transformer. Did you meen the limiter diode, I believe it's d11. I'm confused...

I meant d8. Each diode will lower voltage to the final and driver by 0.6v. Audio will still pass through the diodes and it will have full swang. I've always used the jp36 method but some of the guys here like the diode method. I finally tried it on a cobra 29 a few weeks ago and I liked the results.

The cap and resistor across jp36 with a switch will work, I wouldn't do it that way but it will work. If you're going the jp36 route google "cobra 29 tip 120" use that mod. You might as well have variable power. You could use that mod with a switch if you wish. Use fixed or variable resistors to dial in your high and low power where you want it.

 

[sonoma]

here is the variable for that radio.

http://www.cbtricks.com/radios/cobra/mods_&_info/graphics/Cobra_Uniden_Var_Pwr.pdf

 

[805centralcal]

here is the variable for that radio.

http://www.cbtricks.com/radios/cobra/mods_&_info/graphics/Cobra_Uniden_Var_Pwr.pdf

I have built numerous variable power on cobra 29s. I prefer tip31c but the tip120 and tip29 will also work. I am interested in the way people do the high and low power. So your saying do a cap resistor on the solder side of jp39? Thanks

 

[805centralcal]

the easiest way to do this is put a resistor capacitor at jp36 position , be sure to take the jumper out. get the cap resister set where you want the low power set. next just put a spst switch across the location you put the ,cap in and this way it will bypass the cap resister and give you the high power mode.. switch open will give you low power then closed you will have high power.

So your saying install a switch to bypass jp36 and then you can run the wires from the switch to the ch.9 switch. Thanks

 

[nomadradio]

A resistor in place of JP36 reduces carrier power. The bigger the resistor, the lower the carrier power. A range of 50 to 120 ohms is typical. Just one problem This reduces the modulated "swing" power of the radio along with the carrier. A capacitor in parallel with the resistor will allow full audio to reach the driver transistor while holding down the steady carrier power.

An electrolytic capacitor is placed in parallel with the resistor. The positive side of the cap goes to the end of JP36 towards the front of the radio. Negative side to the rear-most hole.

The size of this cap that sounds best is a matter of conflicting opinions. We have used 47 uf, but the guys who say to use 1000uf are just meter watchers. A listen to the radio's transmit audio will reveal that bigger is not always better. A watt meter is not an audio meter.

And a switch that simply shorts across the resistor/capacitor combo puts the radio back to its stock carrier level, as if those extra parts were never added.

73

 

[805centralcal]

A resistor in place of JP36 reduces carrier power. The bigger the resistor, the lower the carrier power. A range of 50 to 120 ohms is typical. Just one problem This reduces the modulated "swing" power of the radio along with the carrier. A capacitor in parallel with the resistor will allow full audio to reach the driver transistor while holding down the steady carrier power.

An electrolytic capacitor is placed in parallel with the resistor. The positive side of the cap goes to the end of JP36 towards the front of the radio. Negative side to the rear-most hole.

The size of this cap that sounds best is a matter of conflicting opinions. We have used 47 uf, but the guys who say to use 1000uf are just meter watchers. A listen to the radio's transmit audio will reveal that bigger is not always better. A watt meter is not an audio meter.

And a switch that simply shorts across the resistor/capacitor combo puts the radio back to its stock carrier level, as if those extra parts were never added.

73

Thanks! I finally got it all figured out now. I didn't know the cap affected audio. Good stuff, thanks again.

 

[Mk121]

If I wanted to use the ch. 9 switch on the front for a hi/low, what do you do with the wires currently attached to it? Can they just be removed? Thanks

 

[Shadetree Mechanic]

If I wanted to use the ch. 9 switch on the front for a hi/low, what do you do with the wires currently attached to it? Can they just be removed? Thanks

If the switch connects two wires in the normal position, then they would need to be removed from the switch and soldered together and taped. If two wires are not connected by the switch in the normal position, then remove the wires from the switch and tape them each separately.

 

[500kpot]

Is it okay to run your DC + modulated audio for the final through such a small switch?
I mean is the switch hefty enough to take all that?