Cobra 25 WX NW ST mods

[Cutlass327]

I have found some different things listed for these, one is that you just replace the final with a 1969. According to the schematic from CBtricks I saved, it looks like the driver, TR8, is a 1957. I tested the 1969 before installing, and it showed a Hfe of 143. My output went from a 2-5W to a 1-2.5W output. Nothing in the directions I find say to change anything else, but is the driver sufficient?

Also, I found this:

Cobra 25 GTL/LTD/CLASSIC
SAMS PHOTOS: 207
AM LIMITER: D-9
POWER TWEAKS:
1. Change R76 (3.3k) to a 1k (located near mike socket)
2. Change R43 (10 Ohm) to a 2.7 Ohm.
3. Change R108 (1 Ohm) to a .47 Ohm.
4. Replace stock final with a 2SC1969 transistor.
5. Re-tune coils L10,L9,L8 for maximum forward power.

L8 and L9 are just coils, look like the traps, so I didn't touch them. L10 is the external wound with plastic housing style.

I also found this on Coppertalk, listed for the WX NW ST specifically:

AM POWER: ADJUST ~ L18, L17, L14
AM MODULATION: ADJUST ~ VR5
SQUELCH ADJUSTMENT: ADJUST ~ VR4
"S" METER: ADJUST ~ VR1
RF METER: ADJUST ~ VR7

L18 is a double can, L17 and L14 are single metal squares. Turning them, the DK maxed at around 1W.

Did I screw up and wasting time and radio? Do I need to change the driver to a 2314 like in the thread https://www.worldwidedx.com/threads/cobra-29-high-power-mod-with-2sc1969-transistor.31449/ mentions?

 

[Handy Andy]

Hey Cultass.

SIGH...It's best to start out knowing what you're up against...

In simpler terms...

​

This might be a long rant so brace for Impact...

From a Cobra 142GTL
...Thru CB Tricks...​

Now, just so you know...
A Cobra 25 Final Output looks like this...Schematically...too...

Cobra 25NWST - Thru CBTricks...
Minus all the extraneous...​

Look at C139 versus C44 - LOCATED AFTER the Final - also look at the C46 (25) and C150 (142) Before the Final...

There is a reason to show you the above...

It's in how inadequate the Cobra 25 Output Network is before it even gets to the Matching Network and 52MHz trap filters further towards the Antenna Jack.

It is why you are having these problems...

Take a look at some other radios that use the 2078 in a different setup but they still produce as much power if not more - than your Cobra 25 as far as CONVERSION efficiency...

TRC- 423
​

Note C316, it's value
Also C311 - it's value as well...

Even from a General Electric...
​

Why so much smaller - well the main reason being to keep spurious emissions down...but another is to allow the Transistor to load into the output tank more favorably. It impedes the effort in a different way.

You want 27MHz to pass thru this Cap and Coil combo.

If you use too much of one, it affects the other and requires that you retune it.

This is why those Slugs are used - but you may need to "trim" the capacitive effects into this Tank circuit by reducing the capacitance and allowing for another effect to help produce power...

A simple screen grab...

You can research this at your own leisure...
​

That's where cap like C149 (Cobra 25) - although not always used but it is present to help with input matching or input admittance - but also note the use of C45 - BEHIND it - this is where the main role of the Miller effect comes in to play - especially with biasing and the limitations of the CLASS of operation you are in...

So when you're in SSB modes, you need to have the most-est, best-est allowance for power to arrive to the Base to become amplified in the best possible way - by making the Class of Operation as well as Ability to transfer that power you're trying to get - into the circuit without any sort of limitation...

I've got to stop and take a moment with my Wife to enjoy a Sunday with her.

After all the events of the week - it's important I share some time with her.

For you, to try this, gather up small value caps in Pico-Farad values - like 100pF on up to 470pF for your attempt and simply swap out C44 with these smaller values and retune the Tank coil for best results and observe those results...also try the same for the Driver out - to Final In - portion of this circuit too C46 and C149...
.

 

[Cutlass327]

Ok, thank you. Long way to say "No, they didn't give you full instructions for the modification"

I appreciate it. I have 2 of these, and I have kinda decided to make this one my guinea pig. I did the talkback resistor on it, using the PA/CB switch to turn it on/off, but somehow turning the talkback off either with the switch or desoldering it, I lose receive volume.... grr.

Probably going to end ups saying 'screw it', put the original Q back in place, and leave it alone.

 

[Handy Andy]

In shorter words, the 1969 Final - being what it is, would make any radio work - well, but it has needs.

It needs to be tuned to REACT properly.

Too Much signal input on the Base - you get poor performance...
Too Little - same result..

In basic sense, larger caps allow for more input - or greater strength of signal to be transferred to the BASE from the output of the previous stage.

Same applies to smaller levels of capacitance...

The best results are achieved when the Input Admittance is as close to being as equal to the required needs of that Transistor to accept signals and also to reciprocate and develop the best transfer of power into the output...

So no, it's not an exact science and no - the bigger finals (as well as even Small Signal like the 2SC2999) require the right conditions to be met before they accept, process and deliver power as an amplifier in any RF circuit - it has to be tuned for the part that is to do the work.

You don't hire a Taxicab to haul 10 people, you also don't hail a Greyhound bus either - you call for the right vehicle to obtain the best ability to transport the people to their destination...

​

A Bicycle is not the tool you need when you are drag-racing - you'll need the right tool...

It's ok to experiment - I just wanted you to see how I learned was by looking back and seeing how the different Transmitter Strips were designed and researched them.

That tool? Is you...

 

[Cutlass327]

Just so you know, I have copy/pasted this to a Word Doc for future study. Finals week is coming up (all of my classes are online) so I can dig into it later.

All of this advice is not for naught!

 

[Handy Andy]

Hey... good luck on those Exams...

*SIGH* - am I really that old...?

No - not too old to learn...

 

[Cutlass327]

Well, this is my 2nd time thru college - first was my Associates in Automotive, graduated in 99. This is a career change for me - Health Information Management. Been working on cars and semi trucks the past 20+ years, time for something easier on the body... I'm a couple weeks shy of 44....

 

[Handy Andy]

Hate to say this but...

Healthcare and "Time for Something easier on the body" - are not exactly synonymous with each other

But I do understand.

Comparing the "Lesser of the two evils" - especially when it comes to MSDS and how long toxins stay in the body...definitely a better move... just don't stress or burn out - which Health Aides and Caregivers tend to do...

 

[Cutlass327]

Hate to say this but...

Healthcare and "Time for Something easier on the body" - are not exactly synonymous with each other

But I do understand.

Comparing the "Lesser of the two evils" - especially when it comes to MSDS and how long toxins stay in the body...definitely a better move... just don't stress or burn out - which Health Aides and Caregivers tend to do...

I'm going to start with coding, as it is a job I can work from home and usually with flex time so that we can work around my wife's schedule and avoid daycare/sitters with our kids.

 

[9C1Driver]

You have to consider also you have the ST version of the Cobra 25. The soundtracker Cobra 25,29, and 148's were not the best made. Our local shop called them soundcrappers.
Your best bet would be to have one of them given a mild tune and alignment and then just run it. If you need more talk power get a small amplifier. And the other you can mess around with based on what Andy suggests for mods.

 

[Cutlass327]

Thats what I'm thinking. Just trying to find a place for the tx250 in the CJ is hard. It gets wet inside once in a while.