Cobra 142 GTL Modulation Adjust

[BigMike357]

I want to add a little bit more modulation to this Stock Cobra 142 without
clipping things. I have heard there is a resistor you can replace or add
a pot to it. Any info is appreciated.
Thanks,
Crankshaft 35 Delta 357

 

[loosecannon]

Remove R105 which connects to the base of TR33.

replace it with a 1N4148 diode and a 5K trimmer pot in series.
unbanded end of the diode goes to the base of TR33, and banded end connects to the trimmer pot. other side of trimmer pot goes to ground.

I like to wire the pot like a rheostat which means you solder the center or "wiper" lead to one of the ends so it only has two leads on it.
one side goes to the banded end of the diode and the other end goes to ground.
LC

 

[nomadradio]

Those two added parts are what make the AM limit in other models adjustable. Just left them out of this model.

73

 

[brandon7861]

I'm getting about 92% on my washington (same board). I may have to consider doing this mod as well. It would be nice getting it closer to 100%.

 

[Ghetto]

Agreed, I am going to do this mod to my Navaho TRC490 (pc385) which has no mike gain.

 

[brandon7861]

I took a slightly different approach. TLDR~tacked a 6.8k resistor across R105.

6.8k wasn't a complete guess, it was my first try. This is how I figured it out (simple version). I measured the driver collector voltage minimum (the audio lows) at about .5v. The divider R124 and R125 drop that by 31.25%, and since .5v*.3125=.156v, I knew the base of TR33 had to also drop by .156v. I then measured the base voltage at 1.038v. To drop that by .156v, R105 had to drop by 290ohm. And to do that, we need 6.8k in parallel.

Edit: Of course, YMMV. You likely need to measure/calculate it.

Edit2: both this waveform and the one in my previous post were whistling as loud as my toothless @$$ can into a cobra 25 mic. Having that limiter back is nice. Last owner had it in backwards and I finally fixed it a month ago.