Cobra 29 LX lower deadkey with 100 ohm resistor and no cap?

[Pappas and Beer]

I just bought a brand new Cobra 29 LX and would like to lower the deadkey to around 1-2 watts. Could I just solder the 100 ohm 1/2 resistor on the solder side of JP36? Or do I need to install the capacitor on the other side also? I also turned up AMC V4. That is all the mods and adjustments I want to do on that radio.

 

[loosecannon]

you definitely need to remove JP36, or the resistor will have no effect.

the purpose of adding the capacitor is so that the modulation will not be attenuated by the resistor being there.

there is a DC voltage coming from the modulation transformer, through a rectifier diode, and going to the collectors of the driver and final transistors.

this DC voltage sets the deadkey level, and there is an AC component "riding on top of" this DC voltage, and that is your modulation.

when you add the resistor, you lower the DC voltage to the driver which in turn lowers your deadkey. however, that AC voltage (your modulation) has to also go through the resistor, which lowers it's level.

so, the capacitor gets added because a capacitor will pass AC but will block DC.
so now, the deadkey is lowered, but the modulation still passes without being lowered.

you can also lower your deadkey without affecting modulation by adding more rectifier diodes in series with the one coming out of the transformer.

hope this helps.
LC

 

[Pappas and Beer]

Very helpful . It seems like adding the rectifiers in series is a better or at least easier way?

 

[JoeDirt]

you definitely need to remove JP36, or the resistor will have no effect.

the purpose of adding the capacitor is so that the modulation will not be attenuated by the resistor being there.

there is a DC voltage coming from the modulation transformer, through a rectifier diode, and going to the collectors of the driver and final transistors.

this DC voltage sets the deadkey level, and there is an AC component "riding on top of" this DC voltage, and that is your modulation.

when you add the resistor, you lower the DC voltage to the driver which in turn lowers your deadkey. however, that AC voltage (your modulation) has to also go through the resistor, which lowers it's level.

so, the capacitor gets added because a capacitor will pass AC but will block DC.
so now, the deadkey is lowered, but the modulation still passes without being lowered.

you can also lower your deadkey without affecting modulation by adding more rectifier diodes in series with the one coming out of the transformer.

hope this helps.
LC

You would have to add a lot of diodes to lower your deadkey a considerable amount.

 

[JoeDirt]

You can add a resistor and that will lower your deadkey but also your pep modulation like Loosecannon said. Depending on how much you want to lower it you would have to add a lot of diodes to lower it a few watts.

What are you trying to accomplish?

 

[Pappas and Beer]

I am trying to get a 2 watt dead key to run a TS 350. By turning up v4 I hope to have enough pep. That's all I want simpler the better.

 

[loosecannon]

JoeDirt is right, it will take a few or more diodes in series to cut the deadkey in half.
each added diode will drop the voltage by .7 volts.

i have had to use four or five in series before. i actually like to do it this way, and if you orient the diodes the right way, you can make a very compact little doo-dad, and just heat shrink it.

both ways are cool, and both have been used for decades.
LC

 

[Pappas and Beer]

Thanks I will choose one way and post the results.

 

[JoeDirt]

I am trying to get a 2 watt dead key to run a TS 350. By turning up v4 I hope to have enough pep. That's all I want simpler the better.

You can run the TS350 variable power lower (if equipped) or run it in SSB mode. The variable power or SSB acts as a attenuation. The same as adding a resistor in JP36.

You would lose pep power either way, as soon as you add that cap you add a lot of distortion to your signal. If you add the cap I would leave your mod limiter stock.

 

[543_Dallas]

I haven't tried the string of diodes yet but would you really lose that much pep? Seems like most of the audio would still pass through the diodes.

 

[wavrider]

I only use the diodes to lower dead key, a little of the PEP output is lost but not that anyone on the receive end would notice, Looks great on the scope, usually about 2 or 3 diodes added will bring the dead key down around 2 or 3 watts, PEP around 12 to 15 watts, that is my results, yours may vary.

 

[Pappas and Beer]

I added the cap 470 uf and 100 ohm 1/2 watt resistor and than added the RF Gain wires to either end of the resistor. It works great. But I am deadkeying 2.9 watts with RF gain full down, and 3.6 with RF gain full up so the "variable power mod" works a little. I am swinging to 16 regardless of the RF gain setting.

I need to bring the deadkey down to 1 watt. Could I add another resistor on top of the existing one? What are my options?

 

[Pappas and Beer]

Where would I add the diodes, and what kind of diodes?

 

[543_Dallas]

Where would I add the diodes, and what kind of diodes?

Location is D8 on a cobra 29. Any 3 amp or better rectifier diode will work.

If you are going to keep the variable power get rid of the wires going to the RF gain pot and check this out.

https://www.google.com/url?sa=t&sou...TEWhETZP3da_6brRw&sig2=keWXC-Pyl4cTVBKewHQmsw

That isn't exactly how I do it but it will be much better. The way you have it now will eventually burn out the rf gain pot.

 

[Robb]

Try a 220 ohm/1 watt resistor . ..