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Kraco kcb4007 (what I learned before I let the smoke out)

Uncle Ronnie 336

Well-Known Member
Apr 28, 2020
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This radio gets a lot of bad rap for being a junker and it's justified. the driver is weak and not attached to the heat sink because the pinout is backwards..... And it gets wicked hot but if you want more juice out of that, remove r81 and put a 100 ohm resistor in place of r79 to put it in class c bias. (The tuning coils on both the driver and output need to be changed out with a tower coil with a slug)

For the irf-520 Mos I would use en 2030 or the appropriate resistors/diode as I did. I was only able to achieve 18 Watts out of it with the stock driver in class c getting really hot. Remove r83 and c86 as well as c87.

So back to that driver, I got a wild hair and decided to put the old final in its place since it's just reverse pinout and I would be able to bolt it to the chassis. Ehh probably could have worked better. Then I pulled that out and put in an irf510. Once I figured out my bias voltage this actually sounded pretty good but again I wasn't getting the peak readings I wanted to see. So for irf510 driver I used the same (homemade) bias part as the final but I also ran from the 8-volt TX line a 15k resistor to give the driver enough bias to wake up. It actually sounded real good.

Vr3 is the AMC pot.

As far as the output tank parameters I was not able to maximize that before I threw smoke in multiple spots and made the audio transformer hot
 

I have friends who are Jeepers and like these small size radios so if anyone has one to donate or some information on modding from some experiments, I would like to see it. With some mods I believe we can make this thing talk. I really liked how the audio sounded when it was irf510 driving a irf-520
 
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DTB had instructions on his website at one time on adding Mosfet to the plastic bodied Cobra 19s. I had him built me one a while back. I ran it in a Subaru Forester. I sold it with the vehicle, I had put a ball mount to run a 102 whip on the Forester. The Forester looked like a remote control car. The Cobra 19 fit well with the interior of the vehicle.
 
Update: back up and running, still got some work to do keying around 3watts and swinging up to around 17. Has never sounded better, but we need to pump these numbers up
 
Been trying a bunch of different cap combinations but the thing doesn't seem too want to push past 17 so tomorrow after work I'm thinking I am going to slap new electrolytics in to see if it will bring the peak numbers up a little bit. Also grabbing a four-pin connector off of a parts pig radio because while my stock radio shack microphone does sound really good, I feel like I need a Little more gain. Going to be poking around the modulator tomorrow
 
Just remember you must double your power to gain 1/2 of an S unit on the other stations receive. So if you have a 20 watt radio doing a mod that gives you 5-10 more watts does nothing for you really. Making sure your radio is at 100% modulation and running the best antenna you can will give you the best results.
 
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Okay bro. Thanks for the reply BECAUSE I TOTALLY NEVER SEEN ANYONE DO THIS BIT ON ANY OTHER POSTS ABOUT MOSFETS LOL

Let's keep the eyes on the project at hand here. This radio is generally seen as a throwaway and because I'm a glutton for punishment I like to make sleeper radios sometimes for the fun of it...... So I literally don't care of how many s units a mosfet will show versus a bjt because that is obviously not the purpose of this post.
 
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R381c87dc76b5b4295a577d2e7a82ce63


upload_2021-3-1_16-46-13.png

What problems are you having?
 
Just got myself a wild hair my friend lol. It was a freebie and literally everyone I know who has had one of these replaced it with something else. It's a pretty piss-poor design stock and I'm basically cramming a big block into a 87 Honda Civic. Have you ever looked inside one of these?
 
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I've been messing with different cap values in the output and driver stage. The stock driver transistor which is a 2314 is not heat sinked at all stock. It's in the whole backwards but they made it real convenient to drop an irf510 in there and it actually sounds pretty good driving a 520. I'm doing a recap later on tonight of all the electrolytics, which is honestly not that many, and I will see where I am at from there. I suspect I'm going to poke around the audio section and Mic preamp and maybe change out the AMC limiter release cap to something faster
 
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Also on my list on this radio as the last order of business, the case covers will be painted a hammered gold. I have not found any documentation anywhere on it except the secret CB entry and let's just say that's not a very flattering review.
 
No, only thing I ever owned from Kraco was a crappy Mayday - useless...
upload_2021-3-1_17-27-49.png
Didn't even have LED display - just a little RED LED to show you're TX-ing...​

From what I do know they were built in the mid-80's to compete with GE's versions - like the 5809

upload_2021-3-1_17-29-54.png

Not much there either...
What I kick myself over was letting these things go, good parts radios - cheap HELP case but you could rebuild another radio from the spare parts these were...

Mayday used the 7130/31 PLL chip - the rotary knob was the BCD HAND-WIRED in - it was that cheap...wow...

The GE though, had the better multiplexer LED Display brother using the C5121 you just had the Up-Down toggle switch on the front.

So to me they made similar chassis parts - but in getting more power out of it - the problems were with the lack to big stuff that could handle the amp draw. Assembly was "bolt" or push to the back Mayday used a shield "square" the Driver and Final pressed against- well aware of the Driver mounted backwards in both - in the GE version they did use the bolt thru "tab-like" heat sink but most of the work to TX was done with the Final.

So sorry, don't have a working radio to help work alongside you but wanted to let you know they did use discrete's that are no longer available under OEM - like the Varactor and the MIXER SIPP chips - the C5121 PLL was an all in one we could use these days to help with getting rid of the need for the BCD or Rotary knob encoder - but that's a pipe dream for the moment.
 
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