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Cobra 29 High Power mod with 2sc1969 transistor

Regarding the Final & Driver - Cobra 29. I see awhile back someone asked where they are, but no answer. I believe that looking from the front at the alum tri heat sink - back left - the Final is in the middle facing the front. Is that the driver on the left corner facing the back ? They don't even show them on all of the prints of the board. By the way, this one is China W/ 5 coils & the board is # KEPC 1106-K.
I've always been confused as to that as well, I believe it is pnp, the one above l17..it is a 2314 I believe (Chinese 29 as well) the only spot I would see a 2166 going in is tr23 voltage regulator..where d880 is now..but you must turn it around because d880 is ecb, not the 2166 bce..ive been fumbling with the mods as well, it's the .2uf monos and vr4/d11 changes I have not done yet, but I have seen a 3 watt carrier increase sofar..hope this helps...
 
Hello!

I have been reading the post and it is very interesting, but in my case it is a bit complicated, because I want to do it on another station that I have, with similar endings, but not the same. It is a "Dragon CB-240N", which shares the scheme with the "memory 8012", I attach the scheme.

I'm not sure if I can make the change following the instructions in this post, but there is some element, such as VR4 or C42, that I can't find in my schematic. Can you help me? I hope I don't give too much trouble. Thanks!

edit: I have forgotten to say that I am about to make this change because the final transistor of my transmiter is already deteriorated, it gives little power, it is the equipment of my childhood. Also, I don't know if I could use the 2SC2166 and 2SC2312 with the same modifications, because I have a couple at home.
 

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Thank you!!!!
I just saw it right now! I did not receive a response notification !! I write it down, and I'm going to try it !! Thank you!!!

So I keep the Driver and use an IRF520 as final, right?

You have made my day!!
 
Yes, use the IRF520 or FQP13N10 - small differences between the two but both will get you working again.

For Further emphasis:

L13 is a "Tank circuit" - the MOSFET replacement - if FQP13N10 or the IRF520 - should be ok, as they don't have "impedance changes" as dynamic as their Bipolars they replace.

You do not need the "core" for L13 - but if you wish to experiment - the output cap C77 - it's values need to be changed to offset this loss to idealize the tuning to maximize the power transferred...

The MOSFET having the insulated gate, depends more upon the inductive to capacitive reactance interaction through a very narrow "linear" threshold - so the output is really dependent on the "Resonance" the output tank is.

So the Bipolar needed the "inductive boost" (Read as Reactance) to peak out the current to offset the lag effect current follows (as a loss) in a coil - so the inductance offered the voltage transformation thru that tank circuit - this transformation along with the output coupling cap - if both were tuned to resonance of the 27MHz then the best transfer of power occurs because the relatively low impedance to extremely low impedance dynamics the Bi-polar exhibits (thus requiring the tuning inductance as the easier method to offset the impedance change as a reactive component that can be tuned versus fixed value capacitor) when a direct (DC) power is rapidly switched on and off thru the device.

The Bipolar has two things for it, but two things are also against it.

One being - because of a physical junction voltage drops as current rises - so the "sags" in the region of the changing states, requires a wider range of reactance to offset this narrow region of impedance changes...The Base can only supply so much voltage - but current can be allowed to enter to offset - remember though - current is the amount of electrons in flow, voltage is a pressure of those same electrons (a state above a threshold)

(think of this as a plot on a Smith chart)
  • - as the Base is forced into conduction
  • - the Collector to Emitter "squeeze" (off condition) is changed to a valve opening and closing
  • - this type of dynamic affects voltage and current.
    • This generates a separate (bifurcated) condition of a tuned tank with a circulating power or current thru it - but not in exact step with the leading voltage wave. So, in one way it appears as a resistance to a power in Siemens
      • - but is a reaction to a pulse of voltage but lacks current (Water hammer effect).
    • IN the output of the transistor - a cap is used (Miller effect) to capture this energy and hold the transition region for a given rate of change to charge ratio - like a delay - but to offer the voltage energy present a place to rest - or to modify the current following it as it exits into the output tank.
MOSFET's don't need it - the switch itself and the Gate to Substrate barrier - lets the Gate and it's field effects alter the junction below without the Gate getting pulled into the mess going on below it. The Gate looks like a large ohmic structure in mega ohms Ω - versus the lower ohmic Ω values the PN junction the Base of a Bipolar - appears as, because it is as a direct connection to pull power into the circuit, from.

So the Gate can now offer a image for the substrate to mirror from via a field - the Bipolar has to do this directly with the base because the Collector Emitter DIRECT connection thru the Base - steals electrons and power from the Base to make the switch operational.

MOSFET's only need a Voltage (electron pressure not volume) to cause a Major flow of current (Large volume) at a given voltage (even Higher Pressure)

- so in doing / using a MOSFET​
  • - the Inductor plays a weaker role in the circulation of resonance currents present in a tank circuit
  • - because of the impedance changes in the substrate alone are generating the interaction
    • - you're not involving a Base and it's direct connections in this mess.
So it should work just fine - and you always have a method to adjust the tank resonance by adjusting or subbing in a larger or smaller value of capacitance at C77 to offset the lost inductance if you need to improve the power transfer.

C77 is shown as a 150pF ...
  • - you might want this to be a larger value
  • - since they did away with the core
    • - that extra inductance now lost is why the C77 value was so small
    • - However; in later Chinese versions, they changed the output Tank coil by winds and diameter
      • - to allow both the Bipolar and MOSFET parts to work in the same output tank system without too much interactions and losses that others whom had previously designed the outputs to meet a given emission standard
    • In doing so;
      • - allowed the Output PI trap to then fix those spurries and idealized the output network for 50 ohms to match more systems but doesn't solve the SWR mis-match failures
    • The Major Drawback being...
      • - Reflections within the antennas' system are not properly terminated at the radio the PI filter networks are simple two - way filtering systems with little to no ability to absorb or alter reactive elements that damage power devices...
      • Refer back to older designs you will see that output networks were not designed as symmetrical filters, they were purposely altered by wind or capacitance to affect harmonics as well as provide a method or means of dissipation of other reactive power reflections arriving back to the system.
      • Since the newer designs are symmetrical in nature they simply re-admit reflection power back to the device that now has to dissipate it, they are still too high due to this change. (Subject to failure)
  • So when you change C77 - don't swap, just "stack" - solder another cap or two onto the foil side as a parallel design can let resonance of one given set of board support parts work with your MOSFET better; as if a larger cap was straddled across C77's existing one - you have to sub in the right values (yes even more than one can be done to offset effects to improve the power transfer)
To help...

I see a means here to help others that follow these threads about MOSFET's -

The above explanation - although generalized, may help you understand how the MOSFET and it's own interactions are causing many changes in the realm of Bipolar.

So as the transition continues, Bipolar being more analog than MOSFET types, are still a subject of study because as we work with small signal versus small short wavelength (read higher frequency synthesis) small signal class A amplifiers the Bipolar still predominates because of the ability of it's biasing to follow and track audio to LF-VLF regions of spectrum faithfully without the FET interactive effects of noise induced distortions caused by the slower field region effects skewing slewing waveforms that need to be kept as true to source as possible.

The below term will come up a lot in the study of MOSFET and Power designs...
upload_2021-10-22_7-39-58.png
 
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Yes, use the IRF520 or FQP13N10 - small differences between the two but both will get you working again.


  • So when you change C77 - don't swap, just "stack" - solder another cap or two onto the foil side as a parallel design can let resonance of one given set of board support parts work with your MOSFET better; as if a larger cap was straddled across C77's existing one - you have to sub in the right values (yes even more than one can be done to offset effects to improve the power transfer)
To help...

I see a means here to help others that follow these threads about MOSFET's -

The above explanation - although generalized, may help you understand how the MOSFET and it's own interactions are causing may changes in the realm of Bipolar.

So as the transition continues, Bipolar being more analog than MOSFET types, are still a subject of study because as we work with small signal versus small short wavelength (read higher frequency synthesis) small signal class A amplifiers the Bipolar still predominates because of the ability of it's biasing to follow and track audio to LF-VLF regions of spectrum faithfully without the FET interactive effects of noise induced distortions caused by the slower field region effects skewing slewing waveforms that need to be kept as true to source as possible.

The below term will come up a lot in the study of MOSFET and Power designs...

Thank you! how much information! I will read it a couple of times to understand it better.

What values of C77 do you think I can add in parallel? Or rather, what maximum total capacitor do you recommend? I don't know if adding another 150pF cap in parallel will be too little or too much.
 
I'm trying to avoid this getting bogged down with theory - but when you look at the MOSFET - it's like you have a big capacitor input to it...

From: MicroType Engineering
Understanding MOSFET Gate Capacitance

Understanding that the gate of a MOSFET acts as a capacitor is crucial in understanding how to design MOSFET circuits. As with any capacitor, an applied voltage is seen as a short circuit initially. Once the capacitor is charged, there is essentially zero current that flows, with the circuit viewing the capacitor as an open circuit. It takes a certain amount of time to fully charge a capacitor. The same is true of the capacitance of a MOSFET gate. The most useful specification on a MOSFET datasheet for this is the “Total Gate Charge.” “Injecting” this amount of charge into the gate of a MOSFET fully turns it on.

This gate charge is one thing governing the speed in which you can switch a MOSFET. Before any charge is injected into the gate, it is fully turned off, with no current able to flow from the drain to the source. As charge is injected into the gate, more and more current is able to flow from drain to source, until the gate capacitance is fully charged. All charge must then be removed from the gate to turn the MOSFET off. Determining the time it takes to charge and discharge the gate helps to determine the maximum switching speed of a MOSFET circuit.​

So you know, in ohms law of parallel - Capacitors in parallel have LESS capacitance...they are DIVIDED by the number of capacitors and their values used
  • It is opposite of Parallel Resistance
So two caps of equal values in parallel - are 1/2 the value of each - meaning

upload_2021-10-22_9-41-42.png

This gets interesting when you experiment - because I don't know your exact situation - so I'm letting you take to this - but as you ask, look back on the DATASHEETS of these MOSFET's you're thinking about using...

There's a critical key to why I mention and discuss what I do...
upload_2021-10-22_8-14-18.png

See the RED ARROW ?
This is more for the "appearance" of how the MOSFET
look as (like) - in a circuit.

So you'll need to offset these values - so the input can be considered parallel and the output can also be considered as parallel - so these capacitance values are HALVED - or lessened - when you have input from the Driver (C72 originally 220pF) will have to be nearly TWICE the value of the gate to get enough power to transfer across the MOSFET.

The output side is also parallel and to see 150pF (C77) means it needed a 4 to 5-turn output tank coil to offset this LOW capacitance - this design kept 2nd harmonics trapped to the C75/C74 section (Miller effect) and REDUCED the reactive "window" of reflected power (Bad SWR) from being reflected back into the Bipolar (2078) because the reactive effects are reciprocal when it comes to opposing power flow being transformed back to a low-impedance state. They did their best in a trade off of power flowing out into the network and holding back power flowing from the network back into this circuit and the Transistor.

The output too, but not from the MOSFET - but GOING INTO the Pi-Network filter - you'll need to use a higher value of between 560pF (561) to as much as 1200pF (122) - since the network is the most reactive part - remove C75 if not done so already - and use this parts value AS A PART of the other suggested value.
Why?

  • - you're retuning this tank circuit to offset a broadbanded part
  • - a part that can't use all the parts the Bipolar needed
  • - so use these parts IN PARALLEL with a larger value part and observe the results to obtain the right choice of capacitive values to offset the lost coil core
- that is now up to you to decide the needed values you lost when you no longer are able to tune the coil...
Since we are working on this
we will need a template...
upload_2021-10-22_9-4-49.png
 
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It has been brought to my attention...

This is incorrect, and I did this as a TRANSPOSITION...

So you know, in ohms law of parallel - Capacitors in parallel have LESS capacitance...they are DIVIDED by the number of capacitors and their values used
  • It is opposite of Parallel Resistance
So two caps of equal values in parallel - are 1/2 the value of each - meaning

upload_2021-10-22_9-41-42-png.47890


The issue surrounds a set of capacitors in SERIES - they are treated as if they are PARALLEL resistors - their combined capacitances are LESS than using a single capacitor. AS if they are Parallel resistors..

The Formula is correct for capacitors IN SERIES - use Parallel Resistance Formula...but the wording is bad...my bad...

It should have read...

upload_2021-10-23_18-19-7.png
So sorry for the confusion - I can't correct it now because it past an edit time window ...
 
Okay, I did not go into that detail, but I have the concept, I know that two capacitors in parallel add their capacity. Agree.

Do I continue with your recommendation to add capacitors IN PARALLEL to C77?
 
Yes - straddle them - two legs one hole - two legs other hole -

upload_2021-10-24_11-19-51.png

As stated in the Datasheet, use output capacitance 2X stated value - to handle power capacity - use at least TWO capacitors - equal value or otherwise, to RAISE the output capacitance as needed (or use just one but it has to have a strong wattage rating capacity which makes the part more expensive in the process) to overcome the L13 inductance - as as you do, remember to keep any eye/concern for the wattage the part produces will generate heat too. So - too much wattage, you're going to have poor results.

The larger overall main work here is to experiment for this chassis is unique to itself but has similar roots - it's different component values used in the same lattice or matrix of parts to produce work - but layout and locations as well as where the part is positioned in the chain - produces different results amongst the different radios that use this type of construction and design for their outputs.

So concern here is - Quality of signal - so that means listen to the audio this produces thru another radio to MONITOR this. Audio that sounds pinched means you have a drive problem, Heat is also produced when the MOSFET is driven too hard. So Listen for the quality of audio and also keep an eye on the heat generated for too much wattage may indicate a 2nd harmonic is being produced and will kill the upgrade.

How to figure out if the wattage is too much input or output?

Retune the Drivers input coil - to reduce the RF drive - does the Audio sound better? Then that means C390 is needed or tweak the C72 - make it smaller to transfer less power (Keep C73 in there else you will have Driver issues too) - and re-peak Drivers Coil (L10) to compensate for this and improve the audio level to RF drive level mixing happening at the MOSFET from the driver.

If the Final seems weak, place, piggyback a larger cap across C77 to see if this helps the power transfer - and check for heat - a lot of heat signals either the input is still not right or the output tank is not resonate - rework the output cap C77 BACK to stock and then slowly inch up in value...C75 - change this cap to a higher value and observe the results to see if the output Miller Effect is helping or hindering the RF output.

The above may seem like a lot of work but as explained earlier -each chassis is slightly different than their other builds - so you have to be ready to make some changes to improve the transfer.

None of this was ever cast in stone...for MOSFET - only Bipolar...
 
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Yes - straddle them - two legs one home - tow legs other hole -


As stated in the Datasheet, use output capacitance 2X stated value - to handle power capacity - use at least TWO capacitors - equal value or otherwise, to RAISE the output capacitance as needed (or use just one but it has to have a strong wattage rating capacity which makes the part more expensive in the process) to overcome the L13 inductance - as as you do, remember to keep any eye/concern for the wattage the part produces will generate heat too. So - too much wattage, you're going to have poor results.

The larger overall main work here is to experiment for this chassis is unique to itself but has similar roots - it's different component values used in the same lattice or matrix of parts to produce work - but layout and locations as well as where the part is positioned in the chain - produces different results amongst the different radios that use this type of construction and design for their outputs.

So concern here is - Quality of signal - so that means listen to the audio this produces thru another radio to MONITOR this. Audio that sounds pinched means you have a drive problem, Heat is also produced when the MOSFET is driven too hard. So Listen for the quality of audio and also keep an eye on the heat generated for too much wattage may indicate a 2nd harmonic is being produced and will kill the upgrade.

How to figure out if the wattage is too much input or output?

Retune the Drivers input coil - to reduce the RF drive - does the Audio sound better? Then that means C390 is needed or tweak the C72 - make it smaller to transfer less power (Keep C73 in there else you will have Driver issues too) - and re-peak Drivers Coil (L10) to compensate for this and improve the audio level to RF drive level mixing happening at the MOSFET from the driver.

If the Final seems weak, place, piggyback a larger cap across C77 to see if this helps the power transfer - and check for heat - a lot of heat signals either the input is still not right or the output tank is not resonate - rework the output cap C77 BACK to stock and then slowly inch up in value...C75 - change this cap to a higher value and observe the results to see if the output Miller Effect is helping or hindering the RF output.

The above may seem like a lot of work but as explained earlier -each chassis is slightly different than their other builds - so you have to be ready to make some changes to improve the transfer.

None of this was ever cast in stone...for MOSFET - only Bipolar...

Thanks for answering.

Before reading all this last post, I was doing the conversion. Before conversion, the transmiter delivered a power of approximately 3 watts.

After conversion, zero watt. I have tried different IRF520s, I have tried different values of C72 and C77, I removed C75 from the beginning, as you indicated in a post, I have done the "piggy parts" twice. With the last "piggy parts" it seems that it wants to give 0.1W, but it is negligible. I have checked all the capacitors, all the bridges, that I would not have mistaken an element ... Nothing, it does not give power.

I have seen that L10 does not have a core, they should have removed it. I have seen it before the conversion, but if I added a core, if I introduced it, the power would decrease. I have added it also after the conversion and it has not increased anything either.,

Perhaps it is too complex for my skills to adjust this circuit.

Wouldn't it be easier to try the 2SC2166 with the 2SC1969? I don't know if the stock parts would work. Certainly not.
 
Ok, this is what I was afraid of, you can still try this last mod as a method - but the predominate problem this is from the LACK of power the driver will produce - the part (your EKL converted part) can't generate enough "juice" to turn the MOSFET on directly.

So if you want to continue with this - I'll leave this here to be expanded on later once someone comes along and wants/needs extra help - I cannot blame you for the effort for this is not meant to be done by a novice nor anyone with little first time experience with RF circuits.

Else it ok, to restore the board back to OEM and use a 1969 but 2166 cannot be used very well - look at the schematically changes supplied here and you'll see that the 2166 is not the right part for either UNLESS you only use it on the FINAL. Else keep the 2314 for best results using a 2078 or 1969 as the Final.

Also, I need to know if you removed all the parts I mentioned?

R81 and L11 - R81 needs to be a Jumper - Reinstall L11 and try again...
If power Returns - you're ok...just finish up and move on...
  • - did you also remove RFC4? (Hope you SAVED this part...)
    • This part "shorts out" DC voltage present past R81 back to the Driver - it's for RF only
R81 if it's left in there - will hinder the turn on voltage and the power trying to get to the MOSFET to turn on.

Don't worry, you have several options...we have not done everything we can to make this work...we've only done the EASIER mods to make it work - let's get back in there to finish...

Ok, read this thru because as you have already found out - the radio not set up for this conversion - so what works for Cobra and Uniden does not work on the Midland/CTE designs...

1st method...

Reinstall L11 - try again

2nd method -
Requires a Pull Up Resistor to help supply power to turn on MOSFET.

upload_2021-10-24_15-25-58.png
So that means it will need a trickle voltage from the TX line to "raise" the DC voltage and this is why I am not a big fan of MOSFET's - I just do the work ... and for some radios it's a LOT of work...

If you have RFC4 - this is a power choke you can use to obtain a trickle power from the TX line to help boost the turn on voltage to the IRF520N put in series with the 10K to keep RF out of places it doesn't need to be.

So think it over - there is always back to square one also...

I also have to leave this note here...
Google auto corrects and uses your browser as a means to use this extension. So I find that when I'm typing and holding for a moment - I'm forgetting to proofread back behind - at where I last left off. It tends to want to inject what it things (<--Here) THINKS you are trying to write as a "look ahead" and this can make for a disaster when it comes to interpretations. So I'm expending a lot of time and effort wording this right since I got caught last time.
 
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