Cobra 19 Plus Mosfet mod?

[Robb]

Got a 19 Plus someone gave me, and I cannot find the mosfet mod for it. I know it can be done; just can't find anything on the 'net that shows how to do it.



Was wondering if anyone has a link for this mod, or can give a quick mod plan to do it.

Also, if there is a cap/resistor swing mod for it, I'd like to experiment with that too.

It also has a problem with the LED channel display (some segments won't light up, and this doesn't use the standard 25/29 channel LED); but that is another can of worms . . .

Thanks Mucho

 

[711]

Robb,
The 19's come in different flavors and much of the Mosfet conversions I've done haven't necessarily followed any instructions. The way I tackle these is this. Fill the basic needs of a mosfet , gate bias voltage, input capacitance and output capacitance. Most final setups in these series are class C, which makes it easy, source will be to ground , gate will be to a 3.5 volt source and drain , normally bypassed to ground by some cap. Use the 29/25 mosfet instructions with one caveat, the companion parts creates its own self bias using the RF from the driver. I use a voltage divider and tap a source from TX 8volts. Some have used a 4.7k resistor tapped right off the TX switch straight to the gate but that's up to you. Collector of Q203 looks like a TX 8 volt source, or anywhere along that node that provides 8 volts during TX. Cap values for input and output , you'll have to play with but it doesn't hurt to try more.

 

[711]

With these series , I also don't play with cap values for deadkey changes, I use the trusty TIP120 variable deadkey (JP36) mod. Here the placement would be R309and R308. If it was me, I'd lift the side of R309 that shares the connection to C311/R308 and ferrite . Place the negative of the cap to the hole and the positive to the side with the lifted R309 lead. It's a little more work but it is very convenient and consistent in results. Disregard the jumper numbers and refer to where I recommend . Hope this helps.

 

[LeapFrog]

With these series , I also don't play with cap values for deadkey changes, I use the trusty TIP120 variable deadkey (JP36) mod. Here the placement would be R309and R308. If it was me, I'd lift the side of R309 that shares the connection to C311/R308 and ferrite . Place the negative of the cap to the hole and the positive to the side with the lifted R309 lead. It's a little more work but it is very convenient and consistent in results. Disregard the jumper numbers and refer to where I recommend . Hope this helps.View attachment 20431

Never seen the capacitor value affect carrier power, out of curiosity what value cap do you use (100uF?) for this mod?

And have you ever tried this mod with a 1uF or a 47uF capacitor?
Thanks 711
-LeapFrog

 

[711]

The value is arbitrary, as far as the limit on the low end , I don't know. Something to keep in mind , is the reason for the cap is to pass the audio signal through. Also to note that , capacitors and inductors has an interaction with the frequency component of the signal, i.e. In series , Smaller value impedes the ability to pass lower frequencies. So the value is more of a tonal choice. Me I choose between 220-470 uf, The action of the resistor is to affect the D.C. Bias going to the driver. You already know that a short is what ever the full deadkey is at stock. Increasing the resistance , lowers the deadkey. 5k pot seems to give a good smooth range of deadkey. You could make it fixed and do a standard JP36 mod but I like options.most don't use the squelch or RF gain knobs if its available. I've been doing a lot of the new unidens that way.

 

[Robb]

Since the final in this radio is a 2sc2078; then it also should be similar/same (same pinout) to modding it as one would with the Cobra 29 mosfet mod?

 

[binrat]

I did a mosfet mod on a Cobra 18. It was petty easy to do considering.

 

[711]

Since the final in this radio is a 2sc2078; then it also should be similar/same (same pinout) to modding it as one would with the Cobra 29 mosfet mod?

That's the premise . It's one of the greatest lessons I learned from Goldfinger over on Pirates forum. Extrapolate, radios with the same layout tend to use the same mods, i.e. A 25 and a 29 or a galaxy and a ranger. Those were easy comparisons but you'll see similar layouts that mods or improvements and be shared due to the same layouts.