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Cobra 29 High Power mod with 2sc1969 transistor

thelonegunman

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Your 2nd picture indicates using a 5.6v Zener diode, instead of the 1N4148, also, the order of Diode/Resistor is now Resistor on bottom, where the home made EKL is Diode on bottom. With the Resistor on top, it more isolates the Zener from the RF voltage being applied.

Oops, I looked again, the EKL has the 1N4148 cathode (bar end) going to the + side not the Anode (non bar side) going to ground.

Thanks. I have swap diode with 390R resistor, and the gate voltage up to 1.4v, but the output power is the same, 3.8w.
 

thelonegunman

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well,, to choose the MOSFET or 2078 - keep options open but let's see what the values you can use - to see if the MOSFET can be made to work...to replace the 2078...

Ok, the threads are bit involved, but one aspect I wanted you to pay attention to is the DIFFERENCES between the makers chose for values and what stages of the TX strip they were in...

The 1.2V is a little too small to trigger the MOSFET on.

So we need to make it greater voltage...

And there are several ways to do that - but let's use the EKL we have and make up another using different values...

Try using various values to acheive a voltage of 3.3 Volts of power that we can apply to the MOSFET from this...

Here are some examples...

First so you know the 2078 "sees" this...(Cobra/Uniden AM Types)

upload_2021-2-12_1-11-14-png.43090


For the MOSFET
upload_2021-2-12_1-5-54-png.43089


Note that R2 and R3 are now different...​

Try 3.9KΩ for the Long resistor "R2" and R3 - the short one, with the Diode? Try 470Ω Ohm

In this case - we are going UP - increasing Ohmic value - to see where our new EKL component puts the voltage at ...

Why this? Because we do not know exactly how the RF on the line will be processed using that EKL component - we don't want it to use all the power just to make a voltage and have NO RF to produce because the EKL consumed all the power in the circuit. We have to share this RF power from the driver - there's not a lot of it there...

So 3.9K and 470 ohm and try again...

Your 2nd picture indicates using a 5.6v Zener diode, instead of the 1N4148, also, the order of Diode/Resistor is now Resistor on bottom, where the home made EKL is Diode on bottom. With the Resistor on top, it more isolates the Zener from the RF voltage being applied.

Oops, I looked again, the EKL has the 1N4148 cathode (bar end) going to the + side not the Anode (non bar side) going to ground.

Hello. Swapping 1n4148 with 390R voltage up to 1.4v but power output the same, 3.8w.

If i use 3.9k and 470R the voltage down to 1.25v qnd power down to 3w.

:cry:
 
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Handy Andy

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The Zener PROTECTS the Gate from excessive P-2-P (Peak-to-Peak) voltages that can damage the Gate or worse, add extra voltages that force the gate to LATCH ON and stay on until it self -destructs - it's for mostly for the experimenter to help the user keep things in check.

The representation is schematically - referring to parts in the circuit - but Diodes themselves can add or hinder your effort when they are place or oriented in the WRONG spot.

For some chassis this isn't an issue because of a longer trace run or other localized parts are influencing the Diodes performance at that particular spot.

You can try one method then swap the Diodes location - but in most cases - the Diode does better if it's Non-Banded end is sent to ground directly and a Resistor is tied to it banded end

Research Intrinsic because the way the diode can work thru field and thermal - a good bond to Earth (Board ground) helps to offset thermal heating effects and drifting currents that exposed leads of semiconductors can act as mini-antennas in a circuit generating parasitic effects.
 
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Handy Andy

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f i use 3.9k and 470R the voltage down to 1.25v qnd power down to 3w.

Ok, if you RAISE the resistor values, power level goes down,-ok, then put 390Ω on the Diode, then KEEP the Same Resistor value (3.9KΩ) on the Long leg resistor.

One Resistor helps rectify RF into a Power (Voltage) the Gate can turn on with.

One Resistor provides the means to control this current by providing a voltage drop across the resistor - use it to shunt this power away. (Once you understand this window of operation the reason for the Zener becomes very clear)
 

Handy Andy

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These should be your current values
upload_2021-10-28_17-6-8.png

You're doing great!

As you gain experience - you're now seeing the Resistor with the Diode - provides the power...
The Resistor that has the longest leads is called R1 from now on in this set of posts
  • - It is this resistor that drops the power across it, from what voltages the other two parts generate.
  • This is the "control Resistor" it drops all the voltage present in the line and even at the Gate ACROSS ITSELF - as a means to allow a power to be generated and self contained within the circuit..
    • MOSFET Gates consume very little power
    • - so R1 is needed to keep the power this circuit generates from going too high and damaging the MOSFET
  • R2 is a working value or Window of Operation or working impedance - which allows the RF to work the MOSFET linearly.
    • You change this part to a higher or lower value to affect the SWING or Power Range the MOSFET can drive with. The Window is pretty narrow or small in dynamic range of power levels so once a value is found that allows the MOSFET to work linearly - the next higher or lower range of resistor value will affect the level of performance the MOSFET can exhibit -so the window is specific to the Chassis and the Stage it is used in.
The MOSFET Gate lead sees this circuit applying power as a voltage.

The Gate.it doesn't need a lot of power, just the voltage to turn it on - just enough to turn on the Gate usually no more than 3.3 to as much as 4.1 VOLTS DC.

However, RF power is also there so to process the RF power IT needs some current behind it so it can "pool" in the line to allow RF to pass long the line INTO the MOSFET.

Understand you don't want too much current power floating on the line.

So you create a voltage window of operation with some resistance to change - this level of change or impedance - is the purpose of the 390Ω ohm on the Diode line is just for that - a "window" to allow change of RF swing to occur as the Voltage in DC circulates thru the line - then drains off to ground at a given rate thru the 3.9KΩ resistor

As you get the voltage up - you can then INCREASE the Power Swing the MOSFET can work with - by INCREASING the Resistance Value of R2 - from 390Ω to as high as 820Ω.

However, to get THAT high of a power swing , the preceding stage has to be able to DELIVER a wide dynamic power swing rate. There are cautions with that...
  • - so that means you'll then need to consider changing D1 from a Regular Diode, to a ZENER type
  • - between 5.6V to as high as 7.2V rating to handle the power swings and not damage the MOSFET from too much RF power going into it.
 
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thelonegunman

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These should be your current values
View attachment 48041

You're doing great!

As you gain experience - you're now seeing the Resistor with the Diode - provides the power...
The Resistor that has the longest leads is called R1 from now on in this set of posts
  • - It is this resistor that drops the power across it, from what voltages the other two parts generate.
  • This is the "control Resistor" it drops all the voltage present in the line and even at the Gate ACROSS ITSELF - as a means to allow a power to be generated and self contained within the circuit..
    • MOSFET Gates consume very little power
    • - so R1 is needed to keep the power this circuit generates from going too high and damaging the MOSFET
  • R2 is a working value or Window of Operation or working impedance - which allows the RF to work the MOSFET linearly.
    • You change this part to a higher or lower value to affect the SWING or Power Range the MOSFET can drive with. The Window is pretty narrow or small in dynamic range of power levels so once a value is found that allows the MOSFET to work linearly - the next higher or lower range of resistor value will affect the level of performance the MOSFET can exhibit -so the window is specific to the Chassis and the Stage it is used in.
The MOSFET Gate lead sees this circuit applying power as a voltage.

The Gate.it doesn't need a lot of power, just the voltage to turn it on - just enough to turn on the Gate usually no more than 3.3 to as much as 4.1 VOLTS DC.

However, RF power is also there so to process the RF power IT needs some current behind it so it can "pool" in the line to allow RF to pass long the line INTO the MOSFET.

Understand you don't want too much current power floating on the line.

So you create a voltage window of operation with some resistance to change - this level of change or impedance - is the purpose of the 390Ω ohm on the Diode line is just for that - a "window" to allow change of RF swing to occur as the Voltage in DC circulates thru the line - then drains off to ground at a given rate thru the 3.9KΩ resistor

As you get the voltage up - you can then INCREASE the Power Swing the MOSFET can work with - by INCREASING the Resistance Value of R2 - from 390Ω to as high as 820Ω.

However, to get THAT high of a power swing , the preceding stage has to be able to DELIVER a wide dynamic power swing rate. There are cautions with that...
  • - so that means you'll then need to consider changing D1 from a Regular Diode, to a ZENER type
  • - between 5.6V to as high as 7.2V rating to handle the power swings and not damage the MOSFET from too much RF power going into it.

Okay, I think I have some 5.1V zener, bigger I don't think. Could it be worth two zener in series of 3.3V? like 3.3v + 3.3v.

Yesterday I tried to vary the value of R2 both higher and lower and it only managed to decrease the value of the voltage, but I don't remember what value of R1 I left at the end. I'll try again.

Today I am the one who will have jobs "for the house" that cannot wait, but during the weekend I will try.

I have a doubt. I measured the 2sc2314 with the tester and it showed an hfE of 220. Is that correct?

Could a "damaged" 2sc2314 be to blame?

Some component after L13 could be "limiting" the maximum power? Because I have never managed to exceed 3.8W, neither with the 2sc2078 nor with the IRF520.

Thanks for your help.
 

Handy Andy

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2314 with 220 Hfe - should be ok, I am more concerned that the radio may have had other components along the chain not working.

Why? After the experience you've had it seems the radio has some support component sucking-up all the signal. For if you we're getting less watts on the 2078 than that of a typical CB Radio would give your meter - then the radio may have needed work to get it up to snuff so you can even do the conversion right.

So I know - if you used a Cobra 19 or Uniden PC-66 what watts does your meter show for those types of typical CB Radios? Then to know this we'd have a reference.

So to blame the 2314 - the only way to know is to swap in another and recheck your efforts. I'd hate to think it was this part all along but who's (or Whom is) to know?

As I see now, the typical Driver for this type of chassis usually gives about 10 volts Peak-to-Peak to present to the Final (about 1/4 to as much as 3/4W (250 to 750mW)) - which after much of the processing it needs to turn on - has a working window of about 4~6 Volts or RF Peak-to-Peak to obtain a signal from - so the "gain" the stage will have is limited only to the Hfe or mΩ (Siemens) the Part used shows the rest of the circuit - with the lower the ohmic resistance - the higher the gain (Higher Hfe) which means higher power.

So you know 2078's are not the highest in gain - they have a higher junction resistance (intrinsic effect) so the gain is a bandwidth product which limits this to the 30MHz or less. So that means Maximum ?Useable? Frequency - sticks this part to become the standard for the CB radio as the "workhorse" and Go-To part.

The "4-watt" limit may be other issues within the output tank - but when you use the MOSFET as the drop in, the values you need in the EKL conversion part are showing that even that high of resistance of R1 at 3.9KΩ (Unless RFC4 is still in there - if so remove it and check again) that is about the upper limit you need to be to even get 4 volts rectified from the RF to turn on the MOSFET - it should be enough power produced by the Driver to do this without any sort of help.

So in light of your hard work you should be commended for the effort. This should have been a simple drop in but the parts needed to be removed were to only keep the inductance away from the gate that would otherwise add a concern for ringing and latching on - which would put you right back to square one.

If you had a scope you can see the peak-to-peak and observe any part along the chain causing the power drop or hindering your efforts in making this work.

Ok, with that said, change the C77 (150pF Original) to a 0.0047uF (472) Disc - it should have the same power out - Why? Because if the inductance is the only choke point against the output of the MOSFET then the capacitive Reactance on the output tank is more than low enough to no longer be a concern - this way - as you locate and find the right EKL values on the new 2314 part installed (if it was bad in the first place) and you have the radio working like you want it to, then you can redo C77 capacitance values to reset the output bandwidth to the right levels and keep out of other peoples stuff when you use that radio.
 

thelonegunman

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Nov 19, 2011
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2314 with 220 Hfe - should be ok, I am more concerned that the radio may have had other components along the chain not working.

Why? After the experience you've had it seems the radio has some support component sucking-up all the signal. For if you we're getting less watts on the 2078 than that of a typical CB Radio would give your meter - then the radio may have needed work to get it up to snuff so you can even do the conversion right.

So I know - if you used a Cobra 19 or Uniden PC-66 what watts does your meter show for those types of typical CB Radios? Then to know this we'd have a reference.

So to blame the 2314 - the only way to know is to swap in another and recheck your efforts. I'd hate to think it was this part all along but who's (or Whom is) to know?

As I see now, the typical Driver for this type of chassis usually gives about 10 volts Peak-to-Peak to present to the Final (about 1/4 to as much as 3/4W (250 to 750mW)) - which after much of the processing it needs to turn on - has a working window of about 4~6 Volts or RF Peak-to-Peak to obtain a signal from - so the "gain" the stage will have is limited only to the Hfe or mΩ (Siemens) the Part used shows the rest of the circuit - with the lower the ohmic resistance - the higher the gain (Higher Hfe) which means higher power.

So you know 2078's are not the highest in gain - they have a higher junction resistance (intrinsic effect) so the gain is a bandwidth product which limits this to the 30MHz or less. So that means Maximum ?Useable? Frequency - sticks this part to become the standard for the CB radio as the "workhorse" and Go-To part.

The "4-watt" limit may be other issues within the output tank - but when you use the MOSFET as the drop in, the values you need in the EKL conversion part are showing that even that high of resistance of R1 at 3.9KΩ (Unless RFC4 is still in there - if so remove it and check again) that is about the upper limit you need to be to even get 4 volts rectified from the RF to turn on the MOSFET - it should be enough power produced by the Driver to do this without any sort of help.

So in light of your hard work you should be commended for the effort. This should have been a simple drop in but the parts needed to be removed were to only keep the inductance away from the gate that would otherwise add a concern for ringing and latching on - which would put you right back to square one.

If you had a scope you can see the peak-to-peak and observe any part along the chain causing the power drop or hindering your efforts in making this work.

Ok, with that said, change the C77 (150pF Original) to a 0.0047uF (472) Disc - it should have the same power out - Why? Because if the inductance is the only choke point against the output of the MOSFET then the capacitive Reactance on the output tank is more than low enough to no longer be a concern - this way - as you locate and find the right EKL values on the new 2314 part installed (if it was bad in the first place) and you have the radio working like you want it to, then you can redo C77 capacitance values to reset the output bandwidth to the right levels and keep out of other peoples stuff when you use that radio.

I think there is something wrong with the transmitter.

RFC4 is out. I have used 3.9k in R1, and in R2 I have placed 710R together with a 1k pot in parallel, to be able to adjust that resistance. With values of R2 that are close to 300R is when I get the maximum voltage, 1.75v, and maximum power of 3.8w.

I think I will wait to receive a new 2sc2314, although it seems to me that it is not him the problem, but I am not sure.
 

Handy Andy

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I'll work on something I found over here that seems more closer to your radio...

upload_2021-10-29_20-34-55.png
This is for a Cobra 19 XS​

To me this is about as close to the design you're working with as I can locate - but what I wanted to point out are the chosen values they used.

Especially from the DRIVER to FINAL - there is no RFC4 on the RF line to the FINAL - and note the changes to the Resistors used at the Input Collector Coils at both Driver (R222 1KΩ) as RFC205 and Final (R228 1.2KΩ) RFC206 - L205 is tunable - but what I find interesting is how the took care of an admittance issue to the Final from the Driver (2314) The Coupling Cap is 82pF (C245) (which would vary in year to year thru production changes) but is vastly smaller in value - against the "Miller" Cap (C247) of 220pF - but similar in construction and layout.

L206 is shown as a 5.5T slug - but you've already idealized your circuit so I'm only showing this for comparisons.
 
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thelonegunman

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Agree. So do you think that with this scheme you will be able to propose another combination?

I give you the time you need, maybe better wait for the 2SC2314 I bought to arrive (I don't know if they also make fakes of these), and then apply more changes?

That 3.8W barrier is very strange.
 

Handy Andy

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That 3.8W barrier is very strange.

The radio has FM - so this sems to be more of a "contstraint" issue of carrier power - so there may be trick or two they applied to this chassis - because FM doesn't use "swing" in envelope power, the swing is in deviation.

So there may be other issues with that...but one thing I do want to point out...
upload_2021-10-30_11-18-16.png
It should not be this hard to make the parts work.
Your radio ...
upload_2021-10-30_11-23-45.png

We are not doing something right
I'm trying to figure out what that is...
But in the meantime _ I see tremendous amounts of inductance​
 
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thelonegunman

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The radio has FM - so this sems to be more of a "contstraint" issue of carrier power - so there may be trick or two they applied to this chassis - because FM doesn't use "swing" in envelope power, the swing is in deviation.

So there may be other issues with that...but one thing I do want to point out...
View attachment 48064
It should not be this hard to make the parts work.
Your radio ...
View attachment 48065

We are not doing something right
I'm trying to figure out what that is...
But in the meantime _ I see tremendous amounts of inductance​
I must be the most unlucky person right now. today I received the 2sc2314. hfE of 334, good point. I install it on the radio without changing anything else since the last time and 0W and a voltage of 0.6v.
 

Handy Andy

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Man you are not having good luck with that at all...

Ok, reset the Driver back to what you had.

Remove that darn coil

upload_2021-11-2_16-58-52.png
Locate C72, Remove that 220pF cap - Install ACROSS C74's 56pF

Going back to C72, locate a 68pF to 82pF disc cap - install it at C72's location.

C74 has to have something there...

 
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thelonegunman

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Okay I'll give it a try, but remember when I removed that coil, first try, I got 0W power. Only when I put it back on did I get some power.
 

NZ8N

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I looked at the schematics of the Memory 8012 (clear) and what I could find on your Dragon CB240N (fuzzy from Poland), the Memory drives the Final and Driver directly from the audio IC chip TDA2003 through D39, no transformer. Does your Dragon have the transformer or is without?

- Memory 8012 TX Audio.jpg

- Memory 8012 component side.jpg

- dragon_240_schematics.jpg
If without transformer, then the voltage being applied to the Collectors of the Final and Driver will be about 6v in AM (or about 1/2 of VCC).
If it has the transformer, then the voltage being applied will be 13v.

Do you have a better scan of the schematic for your CB240N ?

The reason why I ask is at 1/2 VCC, this means the output RF would be 1/4 of what it would be if full voltage through transformer.
 
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