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L14 pot
- Thread starter Klint
- Start date
Yep, it's a ferrite slug after all.
Turning it clockwise will reduce the carrier and the modulation peaks (swing) both.
If the amplifier has a high/low switch this is the cheap way to get the amplifier's carrier output down to a safe level. Just flip the switch to "Low" and see if this brings the amplifier's carrier power down low enough. Problem solved.
Cheap.
Just one problem. This also turns down the peak level.
To get the amplifier's full peak power with the carrier low enough to sound good requires one of two things.
Either a radio with a separate carrier-level control, like a Galaxy 929.
OR
Modifying a radio that lacks this feature. The Cobra 29 isn't made with this feature.
The simple mod is to remove a wire jumper that feeds power to the radio's driver transistor. A resistor is installed in its place to reduce the carrier level.
Raises two questions. How low does the carrier power have to be set?
What resistance value will get the carrier down to that level?
A capacitor gets placed in parallel with the resistor to restore the lost modulation-peak power. I recommend a 47uf or a 100uf 25-Volt part. Other folks will tell you that 220uf is the lowest value to use. Seems every expert has an opinion.
The extreme version of this mod is a module with a power transistor in it. Gets wired where the jumper wire was removed Typically the SWR Cal control on the front panel gets unhooked from the meter circuit and hijacked to control the transistor. Gets you a variable-carrier control on the front panel like a Galaxy.
Typical resistor value for this trick is 47 to 100 ohms. We recommend a 1-Watt or 2-Watt part. Some folks get by with a half-Watt resistor.
But this still leaves you with two questions. How far down the radio's carrier needs to be to make the amplifier happy. And what resistor value is needed to turn it down that far.
This is a routine truck-stop trick that has been around for decades. Finding someone who has done it a few dozen times might be more efficient than tackling it yourself. The risks are the usual things, accidentally blowing out parts from solder bridges across the gaps between foil pads on the solder side of the circuit board, turning the capacitor around the wrong way. (Hint: the negative side faces the rear of the radio).
Sure would be nice if someone posted a detailed "How-To" on YouTube.
73
Turning it clockwise will reduce the carrier and the modulation peaks (swing) both.
If the amplifier has a high/low switch this is the cheap way to get the amplifier's carrier output down to a safe level. Just flip the switch to "Low" and see if this brings the amplifier's carrier power down low enough. Problem solved.
Cheap.
Just one problem. This also turns down the peak level.
To get the amplifier's full peak power with the carrier low enough to sound good requires one of two things.
Either a radio with a separate carrier-level control, like a Galaxy 929.
OR
Modifying a radio that lacks this feature. The Cobra 29 isn't made with this feature.
The simple mod is to remove a wire jumper that feeds power to the radio's driver transistor. A resistor is installed in its place to reduce the carrier level.
Raises two questions. How low does the carrier power have to be set?
What resistance value will get the carrier down to that level?
A capacitor gets placed in parallel with the resistor to restore the lost modulation-peak power. I recommend a 47uf or a 100uf 25-Volt part. Other folks will tell you that 220uf is the lowest value to use. Seems every expert has an opinion.
The extreme version of this mod is a module with a power transistor in it. Gets wired where the jumper wire was removed Typically the SWR Cal control on the front panel gets unhooked from the meter circuit and hijacked to control the transistor. Gets you a variable-carrier control on the front panel like a Galaxy.
Typical resistor value for this trick is 47 to 100 ohms. We recommend a 1-Watt or 2-Watt part. Some folks get by with a half-Watt resistor.
But this still leaves you with two questions. How far down the radio's carrier needs to be to make the amplifier happy. And what resistor value is needed to turn it down that far.
This is a routine truck-stop trick that has been around for decades. Finding someone who has done it a few dozen times might be more efficient than tackling it yourself. The risks are the usual things, accidentally blowing out parts from solder bridges across the gaps between foil pads on the solder side of the circuit board, turning the capacitor around the wrong way. (Hint: the negative side faces the rear of the radio).
Sure would be nice if someone posted a detailed "How-To" on YouTube.
73
You can't.
You can:
1. Lower PS voltage.
2. Add a few 1n4001 in series with existing one.
3. Mod radio and add power adjustment on one transistor and potentiometer.
Mike
You can:
1. Lower PS voltage.
2. Add a few 1n4001 in series with existing one.
3. Mod radio and add power adjustment on one transistor and potentiometer.
Mike
Biggest problem we see with distortion is using a capacitor that's too large, just to make the wattmeter show a few more tick marks up the scale.
If you use an amplifier, distortion is just part of the deal. Too much carrier produces one kind of distortion. Not enough carrier gets you another kind.
Find the sweet spot for it and the radio will sound good enough.
73
If you use an amplifier, distortion is just part of the deal. Too much carrier produces one kind of distortion. Not enough carrier gets you another kind.
Find the sweet spot for it and the radio will sound good enough.
73
depends on what amp you are going to be using. that is why you need to match the radio to the amp to get the best sound out of both. just as Nomad was stating you have to watch what cap you use and what dead key the amp wants to run at.
I will use from 47uf up to 220uf to see which one sounds the best. as Nomad mentioned some guy's will go up to 1000uf on the cap mod and it can make the radio sound bad and the amp in line even amplifies the terrible sound.
on my last 29 I matched to one of my Texas Star amps a 100uf sounded the best for the setup.
I will use from 47uf up to 220uf to see which one sounds the best. as Nomad mentioned some guy's will go up to 1000uf on the cap mod and it can make the radio sound bad and the amp in line even amplifies the terrible sound.
on my last 29 I matched to one of my Texas Star amps a 100uf sounded the best for the setup.
Klint, Mike raised some good points.
You can operate the radio itself on 12 Volts flat and that'll reduce the power with most all CB sets.
If the 47 Ohm resistor doesn't lower the dead key enough, and you don't feel like removing it, maybe remove D8 instead and use (2X) 1N4001 rectifier diodes in series, that'll lower the voltage going to the final section by 0.7 Volts..
You could lift one end of the diode, clear the pcb hole (use a tooth-pick while the solder on the pad is still molten) and simply add an additional rectifier type diode to the existing one, "banded ends" facing the same direction.
I suggest you "tack the resistor in" on the foil side of the the PCB instead of completely installing it, at least until you know you have a value close enough.
Tack a resistor to the bottom side and test the dead key (using a dummy load), record the measurement and what value resistor you used, remove the resistor carefully, try another value, record the results etc.
Be careful you don't want to destroy the delicate foil traces on the circuit board.
Nobody wants to try and stay 4:1 when they run a box... I don't get it, they all say "audio & headroom" but I say It can sound rotten like that. Why not use some proper attenuation or a "box" that'll happily take "Stock" power levels.
A clean signal is more pleasing to listen to.
Well most CB linears are better suited operating with FM, but that's another topic for another thread.
Carrier control is what the 29 needs in my mind, the TIP120 or TIP31C mod is not the "proper way to get swing", but it is better than a simple resistor, I used a 33uf capacitor one time,
it worked out okay; but I don't look for ridiculous swing on my meter, so I omit the capacitor if i'm doing this mod...I haven't done this mod in some time, since I've moved on to other "controlled carrier" schemas.
The resistor/capacitor trick goes back to the tube days, far be it for me to say but I think it worked better in that type of radio..
Oh well, I guess no one really cares about the adjacent channels anyway..
Okay i'm done rambling..
---<{(This post is for educational purposes only, always follow local regulations.)}>----
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The advice I give a customer about this has to do with how the radio gets used.
If you use it with only one amplifier, The fixed resistor+capacitor trick works fine. You just need a few resistors with values between 47 and 120 ohms. Choose a resistor that gets the desired OUTPUT carrier from the AMPLIFIER. We're not so concerned with the exact carrier power of the radio, only the correct carrier power coming out of the amplifier.
Linears are not all created equal. For a typical small two-transistor linear, a carrier power between 20 and 40 Watts is typical. If you have a meter that reads peak power, set the carrier for about one-fourth to one-fifth of the peak reading.
Once you have the amplifier's carrier set where you want it, install the capacitor. It's easier to leave this step until last when the first resistor you try shows a carrier power either higher or lower than the target. Once you have the resistor in place that provides the desired result, install the capacitor. The negative side is usually the one that's marked on it. This side should face the rear of a typical Cobra 25 or 29 type radio.
And if you may use more than one amplifier, a variable knob allows you to match the amplifier you're using at the time.
73
If you use it with only one amplifier, The fixed resistor+capacitor trick works fine. You just need a few resistors with values between 47 and 120 ohms. Choose a resistor that gets the desired OUTPUT carrier from the AMPLIFIER. We're not so concerned with the exact carrier power of the radio, only the correct carrier power coming out of the amplifier.
Linears are not all created equal. For a typical small two-transistor linear, a carrier power between 20 and 40 Watts is typical. If you have a meter that reads peak power, set the carrier for about one-fourth to one-fifth of the peak reading.
Once you have the amplifier's carrier set where you want it, install the capacitor. It's easier to leave this step until last when the first resistor you try shows a carrier power either higher or lower than the target. Once you have the resistor in place that provides the desired result, install the capacitor. The negative side is usually the one that's marked on it. This side should face the rear of a typical Cobra 25 or 29 type radio.
And if you may use more than one amplifier, a variable knob allows you to match the amplifier you're using at the time.
73