Originally Posted by modeltronics
I can open up the transmitter and take some pics later (I'm at work now). I will try to write this up as follows:
You will need to open up the back of the transmitter. Pull out the module. You will see two rows of female pins in the transmitter that the module plugged into. Ignore the top small row. On the bottom row as you are looking at the back of the transmitter the right pin will be the + power. The center pin will be the modulation PPM line. The left pin will be the ground. I got a pin connector that would plug into the female connector in the Kraft transmitter to wire everything to.
On the FRsky (or Corona) hack module there will be 3 wires. + power, ground, and the PPM modulation input. DO NOT CONNECT THE PPM MODULATION INPUT TO THE CENTER PIN ABOVE FOR THE MODULATION OUTPUT OF THE TRANSMITTER!!!!
The PPM modulation output of a Kraft is inverted with respect to all the other popular transmitters on the market. Also the voltage swing on the Kraft is larger than the FRsky (or Corona) module wants to see. So the modification I needed to make is to invert the modulation PPM pulses and lower the voltage. I did this with a simple NPN transistor inverter and I used a 5 volt regulator on the collector. (If you need I can detail this.) So I built a little PCB with one transistor, a regulator, and a few resistors. This PCB must be connected to the power pins (per above) in the transmitter and the PPM modulation pin. The output of this PCB goes to the FRsky PPM modulation input line. The FRsky also gets it's power from the pins in the transmitter. (I have thought about a different way to do this would be to use a single op-amp as a inverter with negative gain. Any thoughts out there on this?)
The little PCB that inverts the signal I built up as small as I could. Seeing I was using parts that I had on hand, I used a larger regulator than I needed but it works. I enclosed the PCB with some heat shrink tubing with 4 wires coming out of it, +,-, PPM modulation input and PPM modulation (inverted) output. I put this little PCB under the PCB that the original Kraft module plugs into with double stick tape. I put the FRsky module with double sticky tape where the original Kraft module would be located. Once everything is wired and mounted there is one last step, mounting the bind pushbutton and LED PCB. FRsky unlike Corona does not give you a drilling template. I very carefully measured the centers between the LED and bind button to be .312 inch. I located this on the top of the transmitter between the antenna and the slider for channel 5. I drilled the two holes with a .130 inch drill on the top. Next I used two 2-56 screws and nuts to mount this assembly inside the transmitter.
Antenna....remove the Kraft assembly. The hole in the transmitter is too large for the FRsky antenna mount. Inside the transmitter I used a quarter inch washer and on the outside I used another quarter inch washer. The washer I used on the outside is thin copper and matches the gold look of the transmitter very nice. I got them at Lowes for pocket change. I put the original antenna ground lug into this assembly then fed through it the FRsky RF connector to be used as the antenna mount.
The original Kraft antenna wire (white) with a solder lug is still in the transmitter. I put a piece of heat shrink over the solder lug to insulate it and tucked it under the module mount PCB.
One of my goals in planning this out was to be able to put the transmitter back to 53.2 mHz if I wanted. The only thing I can not undo was drilling the holes for the bind button and LED. I don't think I will ever restore the transmitter back now. I am going to leave it on 2.4 gHz. I do have one more of these Kraft Spectrum 6 transmitters to think about doing a modification to.
I hope this helps. Let me know if you need more information. This can be done in a weekend once you have all the parts for a low cost. I hope I didn't get too detailed but I don't know your background so more info is better than not enough. I've also done a Kraft series 74 for a good friend and it involved similar work.
There is a thread in RCG that is over 100 pages long on the FRsky hack DIY module. Lots of good info there.
I hope you can help me.
Definately not electronics person per se.
I have Kraft Series 79 7C want to install new FRSky DHT.
Have definately found the +/_ and yellow wire for the ppm.
It binds the RX but does not operate the controls.
Suspect I have problem you are describing here.
Can you draw me a picture of what I need to construct showing hookup please?