Cobra 25 LTD Classic

[Big Ben]

Where are the adjustments for the deadkey and modulation on a Cobra 25 LTD Classic? I've seen videos on YouTube of other Cobras (the Cobra 29 LTD BT), but I don't know if you use the same adjustment points or not. I tried finding this info on Google, but the only results I've gotten have been from less than reputable sites saying "change this" and "remove this". I'm not looking to clip the modulation limiter or anything here, I just want to get to the modulation and deadkey adjustments. Is there even an adjustment for the deadkey on this radio, or do you have to swap something out? I also have seen guides to other radios on more trusted sites (won't list specifics here because I'm not sure which sites we're allowed to link to), but again I'm not sure which radios, if any, would have everything in the same place.

I have also seen the "Super Modulation" mod before, but I'm not interested because I don't want to have an over modulated signal.

Also, I was thinking about adding in some variable power to this radio in the future. How would I go about doing this? I might add a another knob in, or I may just want to wire it to the instant channel 9 switch, as I never use it. What are my options for this? Thanks.

[Montana4413]

bump. also how do you make talkback louder on the 25LTD other that jumping with a 100 ohm resisitor?

[TheCBDoctor]

Hi Big Ben,

Welcome to the forum. The Cobra 25 LTD has a fixed audio limiter. To disable the audio limiter cut C-70. To adjust the carrier, or dead-key, adjust the slug in front of the final.

There is no such thing as a swing kit or or super mod. It is just a way to separate an operator from his money. The most important part of the radio system is the antenna. To make the talk -back louder simply lower the value of the resistor that is across the speaker or mic plug, pins 1 and 4.

Respectfully and good luck,

[freightliner06]

Hi Big Ben,

Welcome to the forum. The Cobra 25 LTD has a fixed audio limiter. To disable the audio limiter cut C-70. To adjust the carrier, or dead-key, adjust the slug in front of the final.

There is no such thing as a swing kit or or super mod. It is just a way to separate an operator from his money. The most important part of the radio system is the antenna. To make the talk -back louder simply lower the value of the resistor that is across the speaker or mic plug, pins 1 and 4.

Respectfully and good luck,

The Cobra 25 has a modulation adjustment internally, it's VR5. Put the radio on a service monitor and adjust VR5. There is no need to remove C70, there is plenty of modulation available via VR5. If you look at an over-modulated radio on an o-scope while injecting a steady tone into the mic jack, you will see brief periods where the radio's output signal "flat-lines". These flat spots are periods where the radio is putting out 0 power. If the AMC is intact, there will be no flat lines, only a tiny, brief instant where the output drops to barely above 0. The result, with the AMC intact, is a higher average output from the radio. The reason you see extra "swing" on a power meter when you clip a radio is that power meters do NOT distinguish between signals, they only add the total power seen and give a single reading. That extra swing is NOT more power from your radio on the channel you are on, it is the meter adding the power from the garbage signals generated by over-modulation to the power of the real, usable signal. A spectrum analyzer should be able to see this quite quickly even with the mic gain cut back. Just a thought!

[TheCBDoctor]

Hi Big Ben

My mistake, freightliner06 is right the 25 GTL does have an adjustment, it is the Uniden 68XL that does not have an adjustment for the AMC or a 10.240 MHz crystal alignment capacitor, yet they are 99% the same radio.

I have added a link that will show what Freightliner06 was talking about when over-modulating.

http://thecbdoctor.com/_mgxro ... 10912.html

Respectfully,

[Big Ben]

Thank you for the bump, Montana4413, and all the replies from everyone. Sorry I didn't see them for a while. I haven't been able to be on CBRT as much as I would like lately and I didn't think I'd had any activity in any of the threads I'd created because I didn't get an e-mail notification. Must've gotten eaten by my spam filter.

TheCBDoctor: Excuse my ignorance, but what do you mean by a slug? The only thing I've heard referred to as slugs in this hobby are the things that go in bird meters. Thank you for the link to the scope pics.

Everyone: I don't have a scope. Since I'm getting into this hobby more, I may buy one eventually, but if I'm not mistaken, even lower end ones are pretty expensive. Instead of using a scope, could I just use a PEP meter to set up the radio with a 1:4 dead key to peak ratio? I know this is a terrible way of doing this, and it might make you cringe, but i ought to be able to get pretty close, right? Close enough where I am getting more out of my radio without bleeding channels or talking over people's home theater systems, right? To make sure I don't over modulate, I could be a little conservative and set the top number a little lower than four times the deadkey to account for meter inaccuracies. Also, since Cobra has many similarly named radios, I want to make sure I accurately conveyed which radio I'm talking about. It's a Cobra 25 LTD Classic, recently purchased, with the mic connector on the front. I say that because I think I've seen an older Cobra with the mic connector on the side that has 25 and either "LTD" or "classic" in the name.

Thanks!

[pokeyman]

you could cut d9 but i do not suggest it it will defeat vr5 i usually tune vr5 and like doc told me when i was learning you can also put the sc1969 final in and really get the power up there good luck

[Big Ben]

you could cut d9 but i do not suggest it it will defeat vr5 i usually tune vr5 and like doc told me when i was learning you can also put the sc1969 final in and really get the power up there good luck

Yeah I know cutting the limiter can result in overmodulation, so I'm not planning on doing that. Thank you for mentioning swapping out the finals. For some reason, I hadn't even thought of doing that. I'll have to look into that.

[pokeyman]

Install 2SC1969 power transistor. Exchange D10 with schottky diode.
Exchange R43 (at driver) with a 1.5-2.2 ohm 1/4 watt resistor. (Stock unit is a 8.2-10 ohm)
Exchange R40 (at final) with a 8.2-10 ohm 1/4 watt resistor. (Stock unit is a 22 ohm)
Optimize output for each radio by bridging a 33, 47, or 68pf at C46 and C42

[pokeyman]

i would not mess with d10

[pokeyman]

sorry about all the post but i just noticed you wanted variable power well heres how i do it
get a tip31 and 47uf 35 volt electrolitic cap solder the two together and remove jp6 and stick the cap transistor
combo in right leg toward the final cuth the rf gain wires and solder them on the legs of the combo be sure
to solder the rf gain wires back together if i left anything out please post it also take the bottom tab on the
rf gain nob and solder to ground

[pokeyman]

theres some more stuff you gotta do but i lost the second sheet of directions

[Big Ben]

Thanks for all the info. There's more to do for what? The final swap? And where are you getting these instructions from?

[pokeyman]

i have like 50 different tech manuals i got from a old ham operator before he died because he found i was interested in the hobby also got a 8 tube and 10 tube maverik base amps but as for the other stuff to do you have to do like 2 more things on the variable power mod im sure someone will feel in were i left off

[Big Ben]

How does it have the right board numbers if it's that old of a book? I have a fairly new 25 LTD classic. Are the numbers the same for all the cobras because they all use the same board?

What does changing those resistors do? Let more power get to the driver and final? And what about bridging with that capacitor? What does that do? And what does d10 do?

Thanks again.

[pokeyman]

d10 is the spiking diode cobra 25 uniden 66 and 68 all practically the same the ltd classic is a classic because of the classic board it uses your right on the resistors thatas all they do these manuals are up to may 2008 so they are not old lol just the guy that gave them to me