I bought the above RC unit on Ebay. I always wanted a Kraft unit. Back in 1972 I built a old style Pulse Proportional unit from information in a book. I was to build the electronics and my buddy was to build the plane. Well he did nothing but go drinking in his free time. So I built a simple ACE glider to use with my electronics.. After a few test flights and some crashes I lost my job. I ended up working at Radio Shack. A kid that came in everyday drove me crazy. I gave my RC stuff to him. He never did anything with it!!!
Now I am retired and I have lots of free time. During the better days I do long walks. During the colder winter days I work on some kind of project. This one is interesting to me and maybe a few other people out there!!!
The unit I bought is on the 75 MHZ band that is no longer legal to use. Some Kraft units can be reto fitted with a transmitter module. Some transmitters can be made narrow band. My idea was to rewire the Transmitter and Receiver so it would work on the 27 MHZ band.
This might sound a little crazy! After I won the bid on Ebay I went looking for some info. It took two days and BINGO! Knowing that Kraft provided some Heathkits I went looking for a Heathkit unit that was similar. In an old style files directory on the internet I found the service manuals for Kraft Series 71 and Sport. As I looked through the manual I saw it!!! They show how to make the Transmitter and Receiver work on three different bands. They list the values and locatiom of the parts that have to be changed. You need to change the crystal and coils and a few other parts. They list the diameter of the wire needed, the amount of turns and the ID size of the coil.
Now let me tell you some of the pitfalls. The guy who owned the unit must have smoked 24/7 !!! I had to take the transmitter all apart and clean it with ammonia. The guy took the pot on the stickshift apart and screwed that all up. The battery for the transmitter is in a plastic box. This has to be redone with new NICADs. If I use Eneloop cells than the charger has to be changed. I want this unit to be RETRO! The only real headache to this project is they put the parts in the board and bent the leads over on the back. Then they soldered the parts. This makes removing the parts a little more tricky.
Transmitter Before 75 MHZ
The 75 MHZ board has three sections. The Oscillator and then an amp that takes out the second harmonic and amplifies that. Then the Final output.
Transmitter After 27 MHZ
When the board is running at 27 MHZ there is only two sections. The oscillator and the output amp. The middle section is jumpered out.
I changed the main power capacitor. You can see it in the second picture. It is shaped like a bullet and it went off like a bullet when I turned on the transmitter. After replacing the cap with a normal kind of capacitor the transmitter was turned on again. My receiver, a Shortwave radio picked up nothing on 27.195! It did find a signal in the 28 mhz area. My scope probe picked up the signal just being nearby without being connected. I turned in the oscillator cap all the way. I turned on the unit again. Presto I had a signal on 27.195 I was able to adjust the carrier but I had no modulatin. I took a short break! After the break I went looking at the RF choke where the modulation comes into the final. It was wired to +9 volts. When the unit is running at 75 MHZ the modulation comes into the second transistor. When running at 27 MHZ the third transistor gets modulated. The second transistor and other parts get removed. The unused section gets jumpered out. I only had to cut and move a jumper on the RF Choke. Bingo I had modulation. Now onto the receiver.
If you start collecting Old Kraft units Watch OUT! Look at the above picture. Notice the black battery box. The plastic battery box keeps the cells from shedding acid all over the insides. Some models have just a group of cells held together with everything from tape to shrink tubing. The batteries leak all over. The acid stains the aluminum. Even a scrubbing pad can't remove the stain. During shipping dried white NICAD powder goes all throughout the unit. The Kraft Sport used a large 9 volt dry battery or a cheap cluster of NICADS. Avoid the Sport model unless the auction shows the insides!!!
Kraft Sport Front
Two different kinds of Kraft Sport Series.
You can see the clip for the Dry Battery on the right unit. The left unit had NICAD cells. Even after cleaning you can still see staining. The back cover is a real mess. The Sport Series is EL Cheapo!
Two Kraft Receivers open. The top one has it's coils removed. They will be rewound with more turns for 27 MHZ. The top receiver has descrete components in it's decoder board. The bottom board has IC chips on it's decoder board.
Email: John a quick question
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