Cobra 29, Mosfet conversion.

[Turbine Divinity]

All things being equal I was all set to try and do a 2SC1969 conversion of my old 29 ST NW radio . had the thing since 99 when it was bought new. But it has been sitting in a closet for a year after the original final had given up the magic smoke. (carelessness with the coax killed it)

So I am getting ready to do it, And Lo and behold what do I find.. More importantly what don't I find, namely the 2SC1969. Oh I can find them on Ebay but they are all fakes. And the real ones are 14 - 20 bucks, Whaaaaa?

Then I look into the IRF510 - IRF520 conversion. total parts 5 bucks for everything. I Already know from a friend how to make the secret part (the shrink wrapped one from the outlaw 621 kit)

What happens? AFTER I took it to a local shop for a tune up, I am getting a DK of 2.9 watts and a PEP swing of 35-40, Modulation is really good if I keep D-Mike down.

SO , now I have to ask is WHY would anyone try to go with the 2SC1969 build. Am I missing something by using the Mosfet build? I cannot see any reason why every CB radio does not have a conversion using these parts vs having to hunt for a non china made 2SC1969

The Cobra 29 is the ford truck of radios. I know people either love em or hate em. But for 5 bucks and a 15.00 tune up this radio loud and proud now.

would love to hear the pros and cons of the mosfet conversion ( mine was the equivalent to the 621 outlaw)

 

[DTB Radio]

I love MOSFET's in most radios. They're inexpensive (you can find IRF520's for less than a dollar each if you REALLY look hard, and don't mind waiting a bit), and at do every bit as good a job in a 29-style radio as the 1969 final did.

The specs you quoted on your radio don't look too good, though. 35-40 watts from a 2.9 key is extreme over-modulation, and the fact that you have to keep your mic gain down shows this. I recommend that you add a bit if bias to the final by running a 4.7k - 10k resistor from a TX-only 8-volt source to the gate of the MOSFET. Also, make sure you have left the AMC circuit intact, and adjusted properly. If you want extra punch, change C71 (AMC timing cap) to a 33uf unit. That will make the AMC respond much more rapidly to changing voice levels, and maintain max modulation while not overmodulating IF you've set the AMC properly.