Hey guys, I have a cobra 29 ltd classic that has variable power. All the way down it goes to 1 watt. All the way up it shows 6 watts. When I drop the dead key to 1 1/2 or 2 watts and I modulate, it only swings to 3 rms. Peak it does 20. How can I get it to swing the whole 6 watts? What would be causing it to do that? Thanks for the help
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Cobra 29 variable power.
- Thread starter Ocean cruiser
- Start date
Cheap variable mod. The better ones will still swing forward with the carrier level reduced.
What kind of wattmeter are you using? They should all read the same on a carrier alone. But what a wattmeter will read with modulation can vary all over the place.
73
What kind of wattmeter are you using? They should all read the same on a carrier alone. But what a wattmeter will read with modulation can vary all over the place.
73
Is the Limiter still intact?
Variable Power - the mod itself, does it only control the amount of power ALL of the power Signal (audio) and BIAS (DC power)
On a Cobra 29 - you have 12 volts DC Power available to use, with audio - you still have that 12 volts - but audio is ADDED. (12 volts reference here is "battery power supply level)
So with a "variable" mod - there are two things to remember.
Controlling the power - means you adjust all the power - or you can control the CARRIER and let the Audio thru unhindered to modulate that carrier level. There's a difference between Variable power and Variable Swing mods.
The reason I ask about "limiter" is - when you install a variable mod, the power you control is already past the limiter and is unable to offer any other sort of way to "restore" audio already clipped and limited back at the Audio amp mic amp states where it works,- it's already done
Think of this like; having a limiter in a radio and using a variable mod (not a variable swing mod) - is much like - you driving a car - with the brakes on - using it to control your speed unable, never ever letting off the throttle to even allow the car to slow down. You "slow it down" by applying the brakes that much more. The engine can't offset the throttle - it's set - so all you do is modulate the Brakes - producing the heat in the brakes all while using a poorly performing car.
When you have a variable swing mod, the CARRIER (DC side of this) is controlled more than the Audio power present. So more audio can be applied to a lower-power-level (lesser) Carrier signal - creating an envelope that the others see on their meters as having a lot of swing.
Variable Power - the mod itself, does it only control the amount of power ALL of the power Signal (audio) and BIAS (DC power)
On a Cobra 29 - you have 12 volts DC Power available to use, with audio - you still have that 12 volts - but audio is ADDED. (12 volts reference here is "battery power supply level)
So with a "variable" mod - there are two things to remember.
Controlling the power - means you adjust all the power - or you can control the CARRIER and let the Audio thru unhindered to modulate that carrier level. There's a difference between Variable power and Variable Swing mods.
The reason I ask about "limiter" is - when you install a variable mod, the power you control is already past the limiter and is unable to offer any other sort of way to "restore" audio already clipped and limited back at the Audio amp mic amp states where it works,- it's already done
Think of this like; having a limiter in a radio and using a variable mod (not a variable swing mod) - is much like - you driving a car - with the brakes on - using it to control your speed unable, never ever letting off the throttle to even allow the car to slow down. You "slow it down" by applying the brakes that much more. The engine can't offset the throttle - it's set - so all you do is modulate the Brakes - producing the heat in the brakes all while using a poorly performing car.
- When you have simple variable mod, that effect is what it is like, the signals already amplified, you're choking down what is there.
- You can make the argument for restrictor plate too, but i those matters the throttle is adjusted but you still have brakes - Restrictor plate would more akin to Limiter and Variable - only not always at full throttle.
When you have a variable swing mod, the CARRIER (DC side of this) is controlled more than the Audio power present. So more audio can be applied to a lower-power-level (lesser) Carrier signal - creating an envelope that the others see on their meters as having a lot of swing.
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First I’d like to apologize for not logging on and responding back. I’ve been so busy that I haven’t even been able to play radio. Thank you for the responses. I’m finally back at the bench and will be logging on more often. Thanks again.
Nomad has these, don't know if he sells them outside of his shop, though.
https://www.worldwidedx.com/threads...e-key-for-the-cobra-29-radios-no-joke.240485/
https://www.worldwidedx.com/threads...e-key-for-the-cobra-29-radios-no-joke.240485/
Shadetree Mechanic
Delaware Base Station 808
I have used these in several radios. Quick and easy. Drop it in, set it up, good to go.
Thanks Andy, the variable I installed was the one your showing but the diode is not in the original so I’ll try it. Thanks
So you know...
There are some that "tap" in a different part of the "audio + bias" stream - the Diode keeps the BASE lead from becoming too Negative Biased, from where they tap.
Saw this mod without the diode, destroy too many of the TIP-120s' too often - I put it in there to remind me when I install them - do it this way - if they want to keep the part working for a lot longer timeframe.
Others copy this mod and try to get more out of it - I've seen wiring schemes that - well, make a wire loom look like spaghetti....
I use this variable key method on all cobra 29, 25, etc...and it works great. The only change I make, is to replace the 100 ohm resistor coming from the lowest tab of the external potentiometer to ground with a 1k ohm variable resistor.
In place of the 100 ohm resistor, solder the wiper tab of a 1k ohm variable resistor to a convenient electrical ground on the board. Then solder a wire to the lowest position tab of that same 1k ohm resistor, and the other end of this wire will go to the lowest position tab of the external potentiometer (RF Gain, squelch, SWR cal, etc...
Then you can adjust the lowest deadkey the radio will key when the variable power (key) is turned all the way down by adjusting the added variable resistor you installed above.
Turn RF power (RF Gain, squelch, SWR cal, etc...) to it's lowest position (counter clockwise).
Turn mic gain off.
Key radio.
Adjust the newly added variable resistor to desired lowest deadkey.
This does not affect the highest deadkey with variable power all the way down. It works the same as before, it just allows you to adjust your lowest deadkey to your liking, instead of always going to 0w all the way down. You have control over the lowest deadkey.
Oh, and I also use a 470uf 25v cap instead of a 100uf cap. And I stick with the stock final/driver, with no limiters or anything cut.