Cobra 25 unhacking project

[338_MtRushmore]

Back to the 25 now, I got a 10kw potentiometer today for the tip120 variable mod. The diagram says 5 or 10k, but the parts list says 1k or 5k. Will the 10k work?

Also, what would be the most professional way to install the cap and transistor.

http://www.cbtricks.com/radios/cobra/mods_&_info/graphics/Cobra_Uniden_Var_Pwr.pdf

 

[Robb]

Back to the 25 now, I got a 10kw potentiometer today for the tip120 variable mod. The diagram says 5 or 10k, but the parts list says 1k or 5k. Will the 10k work?

Also, what would be the most professional way to install the cap and transistor.

http://www.cbtricks.com/radios/cobra/mods_&_info/graphics/Cobra_Uniden_Var_Pwr.pdf

Yeah, it will work
But-
You will have less gradient. IOW - too much magnitude of change in a small movement of the pot. It will only have a narrower window of control; then no change at all after that.

More is not better in this case.

 

[338_MtRushmore]

According to the parts list for the 25, the rf gain pot is 1k, so I guess I will use that. I just mounded the cap to the transistor, and I'll install that through hole and wire everything from there.

 

[LeapFrog]

According to the parts list for the 25, the rf gain pot is 1k, so I guess I will use that. I just mounded the cap to the transistor, and I'll install that through hole and wire everything from there.
View attachment 20167

Yeah the 1k ohm pot works fine, I try to find a "double pot" (like echo boards use) that way the radio can still have an RF gain adjustment, the trick is finding the right value double pot. (Rarely do you see a "2x" 1k Ohm pot, but they can be ordered online)

If you don't care about the RF gain, simply solder the two wires from the pot together (use a little heat-shrink) and this will leave the RF gain at max for the receiver.

 

[338_MtRushmore]

Yeah the 1k ohm pot works fine, I try to find a "double pot" (like echo boards use) that way the radio can still have an RF gain adjustment, the trick is finding the right value double pot. (Rarely do you see a "2x" 1k Ohm pot, but they can be ordered online)

If you don't care about the RF gain, simply solder the two wires from the pot together (use a little heat-shrink) and this will leave the RF gain at max for the receiver.

I haven't decided what I will do yet. I almost never us rf gain, but I would prefer to have the variable power either on the back, or a locking pot. I went ahead and ran it to the rf gain for now, and put the talkback on the channels 9 switch.

What is the 1800 toll free keep alive mod? I have searched a bit and never found anything.

 

[LeapFrog]

I haven't decided what I will do yet. I almost never us rf gain, but I would prefer to have the variable power either on the back, or a locking pot. I went ahead and ran it to the rf gain for now, and put the talkback on the channels 9 switch.

What is the 1800 toll free keep alive mod? I have searched a bit and never found anything.

It's a modern version of the 3 diode ultra modulation modification that dates back to the tube days, only it can be applied to the newer transformer modulated radios.
[Only it doesn't work the same way, or produce the same results.]

This mod is not to help the radio become any "louder", but to help keep the carrier from pinching on an already "loud radio" (use imagination for what that is, 105-110% modulation) and hopefully clean it up some. (That's the intent anyway)
Basically the idea is to keep a voltage on one of the rails so the rig cannot crash on the neg. peaks, it works so-so.
It's not an "npc mod" by any stretch but it can help if your trying to go beyond 100% modulation, the negs won't "pinch the carrier" (most times) but the waveform can still come out distorted/sloppy.
Here is the link.
(I had better results on a 29, also this mod works fine with a TIP120 style variable RF output mod)

{You can easily lower the carrier on a TIP120 variable mod too far and watch the Neg peaks crash, this helps to stop that from happening as easily, whether the radio is actually cleaner on a spectrum analyzer I cannot tell you}
Between the slug at the final and the AMC adjustment, you can almost get the TIP120 mod to be clean at the lower carrier settings without getting a bunch of swing like a poorly tuned "comp radio" will produce. I think the resistor to ground leaves some room for the radio to "swing" even when the knob is turned all they way up, this is a bad thing in my mind. IMHO the TIP120 variable works, but it's tricky to not create a "comp tune" scenario. (this is a bad thing trust me)

Try the 33uF @ C91 compare your results to the factory value, or go for a 47uF if you are not going to do the Goldfinger mod. This one cap affects the AMC more than I thought it would.

 

[338_MtRushmore]

I am planning to recap this 25 and send it off for an alignment. I have an old China 25 and 29 that I may try the mods on before I go crazy on this one. Other than the recap, I just need to figure out a way to mount the echo board. Not much room in that little chassis

 

[LeapFrog]

You can use two (or one) of the "L" brackets used to mount the echo boards in the exports, I just checked and you will have to drill a hole or two.

Here is an echo board Mount I did on a cobra 25 just now, I used only one clip but I'd rather put the echo board on the other side of the radio away from the PLL. (just a preference)

It looks better and it's more solid with two L brackets, use loctite when finalizing the installation.

[photo="medium"]4435[/photo] .
Never mind that it's slightly crooked, I don't have any Dremel cut off wheels and I didn't account for the little slit that the L bracket slides into to stop it from turning.
And I just did this literally to kind of give you an idea. (it's not a real install, just a dummy install to show one way you can mount the board, not every board has a hole at the end where you can put it long ways, in that case you'll have to drill 2 holes and use two brackets)

You could just cut off the little tab that sticks out on the L bracket and you won't have to worry about cutting a slit, just keep in mind though the brackets can move about if they don't have the Locking slit/groove, loctite works good for most installs.

The ideal setup for an older side mic Cobra 25 LTD, with Echo, Variable RF output:
Combine Volume/Squelch into a "double pot" that uses the same values as the original ones.
Use a dual 1k Ohm "double pot" for RF gain/RF power.
install the Echo "double pot" in the squelch's old spot.
Leave "DynaMike" alone in it's spot, you can order a custom faceplate decal so the labels match the new controls across the front of the radio.

This way you won't have to drill any serious holes in the chassis and the radio could be returned to near mint condition by a collector down the road. (Minus the two little holes drilled for the echo mount's "L brackets", but you wouldn't see these once the lids are on)

This part goes without saying, but I'll add it for any newbies who come across this thread; The moment anyone drills a hole in the chassis to mount a potentiometer or switch, you can expect the value of that radio to decrease (to collectors anyway), regardless of the quality of work performed.

 

[338_MtRushmore]

Can anyone tell me why my soldering iron tip eats away, and the best ways to combat it? I've been kinda busy lately, but I am hoping to get that echo board mounted in the next few days. I will have to post a video of my work on youtube for you all to critique.
I appreciate all of your help, and this project has really pumped me up. I'm not much on electronics, but I have learned a lot just from this radio. Yesterday I ripe the dash of my wifes 2005 Yukon apart to replace bulbs with led's. It was the smallest through hole soldering I've done yet. Not a problem though, except for one board that was wired 2 series, that threw me for a loop because it was a multi layer board. I'm feeling cocky now...

 

[LeapFrog]

All that cap is doing is allowing peak watts to get through. However, if you don't want it to swing, all you would need to do is not install that cap.

I tried without a cap, just the TIP120, I saw some kind of "ghost image" (hard to describe) on the scope when I put the power any lower than max. (A sample within a sample if that makes sense, when I lowered the power I saw a the sample change in a weird way)

With a 1uF 450 Volt cap this ghost image was not on my scope, no matter the carrier level. Without the cap I saw something on the 'scope that looked wrong.. Kinda hard to explain.

Yeah 338_MtRushMoore for a variable RF output without swing non-sense try this: leave the AMC alone, and use a 1uF cap. I was able to adjust the carrier while seeing hardly any distortion, at all carrier levels.

-LeapFrog