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Cobra 25LTD HiFi Base Build

A place to ask questions about base setups.

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KP68
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Cobra 25LTD HiFi Base Build

Post by KP68 » Monday, 20 July 2015, 7:55 AM

Well, I finally put it together. For quite a few years I talked about building an all in one HiFi CB base station but I found myself dragging my feet on the idea and motivation. The saying “talk is cheap” is very true, so keeping that in mind, it gave me the push I needed to see this project through. Although not %100 done, it is fully functional with all controls performing their intended functions.
When I find more time I will post some pics of the inside, but for now here she be. The front panel is temporary until I get the cut outs for everything in the aluminum face plate. I’m still not sure of what control knobs I want to use so that’s why you can see a mix of various types. The radio is based around the Cobra 25 LTD and as for that choice and many other things, many decisions were made upon space restrictions. So I don’t bore everybody with details I’ll give a basic rundown of everything.
The base is built around the Cobra 25LTD which was completely stripped down to the chassis, and the case is a three space aluminum rack mount. The watt meter is a Workman HP201s also completely stripped down, modified, and recalibrated. As for the HiFi, I built the MMM *Censored* board from scratch and included the dual polarity power supply (pos13.8, neg13.8 volts) the 24 volt supply and the digital voltmeters as well. As for RF power, I built a two pill class AB amplifier which sits upon a 120mm fan at the rear right corner of the unit. The power supply for the amplifier is a regulated 13 volt 30 amp, and in order to keep everything running cool and clean, I installed an air intake fan and air exhaust fan on the rear panel. The thing that I found was the most time consuming was the channel selector. If you know, the original radio has the channel readout mounted above the selector knob in a one piece unit and in mine the selector knob is installed beside the readout. The one piece unit had to be dismantled and extension wires run from the LED board to the channel selector, and the original ribbon cable from the LED board to the radio PCB had to be removed and longer ribbon cable installed. I could have kept them together, but it would have been in the way of the right speaker and also I wanted to keep it custom.
Like I said, I don’t want to bore everyone with the blah blah blah stuff so here are the control knob functions. In the upper left corner are the master power switch and the amplifier power switch. The five knobs at the bottom of the unit going left to right are audio, asymmetry, tube emulation (which is just a high gain mic preamp), SWR set, and range selector/ SWR DIR,REF. The knob just below and centered between watt/SWR meter and S-meter is the carrier control. The three silver knobs to the left of the digital volt meters and going from top to bottom are radio volume, RF gain, and squelch. The knob at the top right corner is the channel selector and if you can follow what I just typed you’re a genius!
I still have to do some fine tuning and all in all, working nicely. At first I was concerned that I might have to deal with electrical interference from the fans and AC but through careful planning, it is throwing a silent dead key and quiet receive.
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High_Speed
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Re: Cobra 25LTD HiFi Base Build

Post by High_Speed » Monday, 20 July 2015, 11:00 AM

Wow. This is really cool. Very creative. I'd like to hear it!
HighSpeed556 in the Bluegrass
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KP68
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Re: Cobra 25LTD HiFi Base Build

Post by KP68 » Monday, 20 July 2015, 16:17 PM

Hopefully conditions will permit. I'm a ch28 junky and I try to get on as much as I can. I finally got a decent video cam and I plan on doing some videos of my station as well as DX gates.

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Rcr619
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Re: Cobra 25LTD HiFi Base Build

Post by Rcr619 » Tuesday, 29 December 2015, 10:41 AM

That is awesome!

I have an old dak mark X radio that I've been contemplating doing something similar with. I just haven't come up with enough courage to start stripping it all down yet.

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KP68
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Re: Cobra 25LTD HiFi Base Build

Post by KP68 » Wednesday, 30 December 2015, 10:04 AM

Ya, courage, determination, and patience. So far, I couldn't ask for better performance. I already have parts coming in for my next HiFi base station. The latest one will dwarf my current build. This next one will be the Cobra 29LTD base station.


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MDYoungblood
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Re: Cobra 25LTD HiFi Base Build

Post by MDYoungblood » Wednesday, 30 December 2015, 15:06 PM

It is definitly interesting, can't wait to see the next and a suggestion would be SSB.

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