Lowering the dead key on a radio to drive an amp

[Turbo T]

As I have found, not many people know how to do this. The last guy I paid to do this ended up snipping the modulation limiter in the radio causing it to distort.

I have a Uniden and a Cobra with a Uniden board I thought of wanting to do this on.

Is this something one can do himself at home without the requirement of any tools?

TIA

 

[J.R. Ewing]

As I have found, not many people know how to do this. The last guy I paid to do this ended up snipping the modulation limiter in the radio causing it to distort.

I have a Uniden and a Cobra with a Uniden board I thought of wanting to do this on.

Is this something one can do himself at home without the requirement of any tools?

TIA

First you need a good wattmeter, then you have to find the potmeter on the board who is for power. What kind of radio do you have?

 

[Kamikaze]

For collector modulated radios one can install a resistor in series with the modulated B+ to cut down voltage to the final. The resistor gets bypassed for audio with an electrolytic cap. This mod can actually improve audio too.
Not sure of values sorry.

 

[loosecannon]

here is a link to a sticky in this section where i explain exactly how to do this to most AM only radios:
http://www.worldwidedx.com/cb-band-cb-radios/88445-help-need-lower-deadkey-2-watts.html
LC

 

[Turbo T]

What kind of radio do you have?

Cobra 21. Old school 40 channel built on the Cobra 25 chassis.

 

[J.R. Ewing]

Cobra 21. Old school 40 channel built on the Cobra 25 chassis.

I would fix the limiter so there is no risk for overmodulation. Running an overmodulated radio over an amp will cause a lot of problems. If you will reduce dead key, you can tune it down with L8 or L10. Original it will have a dead key about 4 watt. How much do you need?

 

[loosecannon]

I would fix the limiter so there is no risk for overmodulation. Running an overmodulated radio over an amp will cause a lot of problems. If you will reduce dead key, you can tune it down with L8 or L10. Original it will have a dead key about 4 watt. How much do you need?

do not ever turn the power output down in this way. those coils are impedance matching devices and are meant to be peaked.
when they are peaked they are operating with the highest efficiency.
your modulation peaks will be significantly lower if this method is used.

the proper way to do it is to lower the voltage to the collector of the driver and the final transistors.
do this using the diodes method or the resistor/capacitor method, both will work on that radio. i prefer the diode method.

as long as the AMC remains intact, you can still tune the radio for 100% modulation if desired.
LC

 

[psycho]

As I have found, not many people know how to do this. The last guy I paid to do this ended up snipping the modulation limiter in the radio causing it to distort.

I have a Uniden and a Cobra with a Uniden board I thought of wanting to do this on.

Is this something one can do himself at home without the requirement of any tools?

TIA

Cutting the modulation limiter in a single final cobra radio won't cause them to distort, never heard one distort by just simply cutting a limiter.......... Running a power mic will. Some Export radios sound like crap with the limiters cut, not all.
Please don't bombard this forum with the same questions you do on CBRT. I remember you and slydog battling it out over the same questions over and over again.

 

[Turbo T]

I would fix the limiter so there is no risk for overmodulation. Running an overmodulated radio over an amp will cause a lot of problems. If you will reduce dead key, you can tune it down with L8 or L10. Original it will have a dead key about 4 watt. How much do you need?

Most amps only like what...2 watts total? Also the radio is stock never modified.

Cutting the modulation limiter in a single final cobra radio won't cause them to distort, never heard one distort by just simply cutting a limiter.......... Running a power mic will. Some Export radios sound like crap with the limiters cut, not all.
Please don't bombard this forum with the same questions you do on CBRT. I remember you and slydog battling it out over the same questions over and over again.

Gee what a way to welcome someone. I seldom even go to CBRT anymore. I was hoping to get some good info here but so it seems one of the know it alls migrated here. Cut the modulation limiter and no distortion huh? I think my b.s. meter just pegged into the red. Also I never have ran a power mic, so we can rule that one out.

Also I believe slydog was banned like what...3 years ago? LOL I forgot all about that guy until now. You sure do like to bring up old stuff.

Thanks to everyone else who helped.

 

[Kamikaze]

The diodes are a little better than resistors as it's a little easier to predict power changes.

They should still be bypassed by an electrolytic unless one wants to throw modulator power out the window.

 

[W5LZ]

Something to think about.

The relationship between 'dead key' and 'modulation' deals with the percentage of modulation. Varying one of them without varying the other changes that percentage, not good if too much or too little.
Amplifiers don't just 'look' at 'dead key', they wouldn't know a 'dead key' from a strawberry malt. Amplifier's deal with, or look at, total drive power being fed to them. If you need to lower the input power to that amplifier, lower the whole mess, not just 'part' of it.
You can 'over-do' anything if you don't know what that 'something' consists of, and why.
- 'Doc

 

[Kamikaze]

Something to think about.

The relationship between 'dead key' and 'modulation' deals with the percentage of modulation. Varying one of them without varying the other changes that percentage, not good if too much or too little.
Amplifiers don't just 'look' at 'dead key', they wouldn't know a 'dead key' from a strawberry malt. Amplifier's deal with, or look at, total drive power being fed to them. If you need to lower the input power to that amplifier, lower the whole mess, not just 'part' of it.
You can 'over-do' anything if you don't know what that 'something' consists of, and why.
- 'Doc

Doc, the configuration several here are assuming is a collector modulated AM final. It must be class C to work properly. As such the final will modulate at the same exact percentage at all power levels out when the B+ is cut down with a device which effects audio and DC power similarly. That device would be a power resistor.

If the resistor is bypassed with a DC blocking capacitor the audio will not be cut down. This will require lowering audio gain and re-adjusting compression but will add dynamic headroom to the modulator stage.

Drive power should not be of much concern here as the final will simply work a little harder into Class C.

One sticking point here is the driver is usually modulated along with the final in a lot of CB radios.

Now the amplifier following this whole thing is another subject.....

 

[eagle1911]

HOLY FREAKING CRAP!!!!!!!!!!!


Kamikaze posted some valid technical information!

OMG OMG!!!!

Unbelievable.. This deserves some sort of commendation!

I'm almost sorry I've been hounding the crap out of you for trolling constantly Kamikaze.. maybe you do know a thing or two. See, I can admit when I might be wrong about someone.. and to think you didn't believe me lol.

 

[Turbo T]

Something to think about.

The relationship between 'dead key' and 'modulation' deals with the percentage of modulation. Varying one of them without varying the other changes that percentage, not good if too much or too little.
Amplifiers don't just 'look' at 'dead key', they wouldn't know a 'dead key' from a strawberry malt. Amplifier's deal with, or look at, total drive power being fed to them. If you need to lower the input power to that amplifier, lower the whole mess, not just 'part' of it.
You can 'over-do' anything if you don't know what that 'something' consists of, and why.
- 'Doc

Doc, I was under the impression to set up a radio to drive an amp efficiently without overdriving it, was just to drop the dead key, so what you're saying is I also need to lower the modulation? Please clarify.

I would fix the limiter so there is no risk for overmodulation. Running an overmodulated radio over an amp will cause a lot of problems. If you will reduce dead key, you can tune it down with L8 or L10. Original it will have a dead key about 4 watt. How much do you need?

Well right now I'm looking at buying a new Palomar 250 I found. Not sure how many watts should be driving it. I'm assuming the pills used dictate how much should be fed into them? Is there a chart or something out there that will show me how many watts the radio should be putting out to a certain amp to not overdrive it?

 

[Robb]

I used eight 1N4003 diodes strung in series at D8 to get 1 1/4 watt dead key from a Uniden PC78XL for a local AM operator today. He bought it new and is going to run it with a TS Mod V and a TS Sweet Sixteen. MUCH better radio than any new Cobra 29 IMO; but I really don't do AM any more myself.