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This thread is privately moderated by Scott Todd, who may elect to delete unwanted replies.
Aug 05, 2018, 09:23 PM
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So you want to convert an old radio...

Many of the older 40+ radios like Kraft, EK, Orbit, Futaba are corroded from old batteries. The Futaba ones are not as bad as the US ones but quite often the caps and transistors go bad too. Even if they work now, its questionable how long they will continue. Manufacturers all used different centering pulses to so that can cause issues if switching radios. Also, some need a signal inverter to get the PPM stream correct. Although many of the new modules like the FrSky ones can handle both.

But there is just something cool about those old metal cases. Many like the Krafts and Futaba's really changed our Hobby. They are still out there. They are all over eBay and in peoples collections. Should be just toss them? NO! Even if you are not an electronics technician, your average modeler can still restore/save one and be the cool kid at the field flying a 40-50 year old vintage radio. We are going to stick to conversions more than repairs. By that, we are going to usually rip out the old electronics and replace them with a modern RF module and an Arduino acting as the encoder. We'll go over cleaning too but most people can figure that part out.

The Arduino replaces the old encoder. Its one little simple board that is modern and reliable. The Sketch (program) can be easily changed to customize it for whatever features you want that were not available on the original.

So by gutting a 40 year old Kraft, or whatever, you can thoroughly clean it and make it look nearly like new. They were built pretty well. Adding a $5 Arduino and $25 module can bring an old classic up to speed to operate safely and with things like reversing, rates, end points, throttle cut, mixers and whatever else you can dream up.

Phil's singlechanneluk site has some good stuff. Everything you need is there. I'm going to just summarize a little and add some thoughts about my learning process.

If you can't solder, learn or stop here. Get some help. I think you should buy the crimps and crimp tool to make your own servo plugs. I tried it several times thru the last 40 years and always struggled. Well I finally gave in and watched some Youtube videos. Using a magnifier and practicing making a few dozen plugs, I got it! OMG! Everyone needs to be able to do this. Its really pretty easy and opens up many options.

These are what I bought and they work good. I also bought some servo wire but you should have most of what you need when you gut the old radio and you can use old servo extensions.

I usually start at the switch or pot and solder those wires on if you don't re-use the old ones. Then route the wire to the Arduino and cut it to the proper length. A minute or two of crimping and you have a perfect length, custom cable.

Of course if you don't have the crimper, you need to use servo extension leads and work the other way. Amazon has 30 packs for roughly $10. Then you would plug the lead into the Arduino wherever you have mounted it. Then route the wire to the pot or switch. Cut it and solder on. Plan ahead a little and run all the wires together. Some small zip ties or Dental floss to tie the bundles together makes for a really neat job in the end.
Last edited by Scott Todd; Dec 11, 2019 at 12:37 PM.
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Aug 05, 2018, 09:52 PM
Thread OP

Radio Basics

I think we should talk about whats going on inside these magic boxes? When proportional first came around, there was a standard that most manufacturers followed. The signal built inside the transmitter was a modified square wave also refered to as PPM or pulse Position Modulation. Many people discuss and argue what this really stands for but lets keep it simple. A little vague is fine. There was a voltage pulse for each channel. This pulse changed length to tell the servo how much throw you wanted. All the channels added together made a Frame. This was usually done at about 20 ms or it updated 50 times a second. Each channel was 'centered' at a certain pulse width. This has changed and manufacturers used different values. But in more modern times, the industry as generally migrated to 1.5 ms or 1500 micro seconds. The entire channel is generally considered from 1000 to 2000 and centered at 1500. In modern computer radios, there is often a place to see these exact numbers.

In old radios, this was done with transistors as switches, and resistor/capacitor pairs to get the timing correct. Later there were entire IC chips designed to do this but thats another story. In our Arduino, we just need to build the PPM stream to match what we want it to do. Arduino lets us tell a pin to go High, which puts the supply voltage on it. We can also tell the same pin to go Low, which puts zero or no voltage on it.

I'm sure I'll hack this but lets look at the picture. Its Ron's PPM tester from the discussion board linked from singlechanneluk. The beginning space plus the width of the channel sends the signal to the transmitter module that then sends it to the receiver. The receiver sees the Sync pulse being significantly different from the rest and knows its time to start over. This is done every 17956 micro seconds in Ron's picture, or about 55 times a second. I think some of the DIY modules like 18 ms so Phil used that as a standard instead of 20. Maybe he will chime in and correct me This easily fits 7 channels in so that's what he picked.

In Ron's example, channel one is at 1536. This is roughly centered for the way we read the stick pots. Again, different manufacturers used different values so we'll just assume this is near center for whatever radio Ron is looking at. We don't know if its actually an Arduino or other legacy encoder. Channel two is again near center at 1542, and channel three is apparently throttle at a low of 1037. There is no telling what is going on with 4 but early radios often had extra channels, especially for lower channel count radios. It was cheaper to make standard boards.

Here is Phil's code that 'builds our PPM stream.

//send ppm frame, last channel holds sync value
for (int ch=0; ch<8; ch++) {
digitalWrite(ppm, HIGH);
digitalWrite(ppm, LOW);

The fist line is a note. The second line is a loop that does this process 8 times. That is 7 channels and the Sync pulse. So our first actual step sends the pin high. Then it delays for ppmPulse. This is the 256 in Ron's photo. In Phil's sketch, he uses 300. You can see it assigned near the top. Then we bring the pulse back to low. And finally, we send the value we have calculated for that channel at that point in time. In Ron's example, this would work out to 1280 for channel[0] or the first channel we generally call Channel 1. Computer math generally starts at zero to confuse us regular folk even more.

So that's the PPM pulse we are trying to build to put on Pin 0 or 13 in Phil's sketch. This signal goes to the RF module where some other magic happens. Don't you want one of those cool testers? Go build one! Phil has a nice video and a basic tutorial. You could also buy one of the cheap $20 0pocket Scopes on Amazon or eBay but where's the fun in that
Last edited by Scott Todd; Aug 05, 2018 at 10:13 PM.
Aug 05, 2018, 10:01 PM
Thread OP
So where does the pulse width come from? Phil uses the Analog inputs on the Arduino as voltage dividers. This is exactly what they are designed for. He has a clever routine that lets you calibrate the sticks after you do a fresh upload. It only has to be done right after the sketch upload and not again.

for (ch=0; ch<6; ch++) {channel[ch] = map(analogRead(ch),stickcalLo[ch],stickcalHi[ch],-500,500); }

So again he loops thru 6 times. These will be the proportional channels. Each loop, he reads the stick or pot value and maps it between -500 and 500. This gives a center of roughly zero, depending on the linearity of the stick pot. He declares the nominal pulse width as 1200 and the ppmPulse as 300. The stick or pot values get added to nominal pulse during the "format the frame" part of the code. So if the stick is centered, 0 + 1200 gets assigned to the channel for that time thru the loop. He adds the 300 in for proper formatting and then goes to the next channel.

This all gets sent to the PPM pin near the end of the sketch. Then it all repeats, very fast. You don't need to know all this to convert an old transmitter but it doesn't hurt. I know it seems like a lot and may be confusing. But again, its a good placeholder and may help someone. Walking thru the sketch and studying Ron's picture should help. You can Google it for other explanations and there are some YouTube videos showing the PPM stream on scopes.
Last edited by Scott Todd; Aug 05, 2018 at 10:17 PM.
Nov 12, 2019, 07:24 PM
Thread OP


I have got a few questions about cleaning. Mostly people want to know how I make them look new again Of course its a secret.

Take EVERYTHING off the case. Then I use Dawn dish washing detergent. Hey, if its good enough to clean oil off wildlife...And you need to discover Magic Eraser. Ask you spouse. Its a sponge in the cleaning section of the grocery store. It truly is Magic! Now start cleaning. Be careful with labels, The magic eraser will take the ink/paint right off them. Go easy. Work the edges of the aluminum badges and you will get all the grime from there. Clean the case inside and out really well. I typically spend about 30 minutes on a case. After thorough rinsing and letting it dry, you will see spots that need more work.

I usually take the pots apart and wipe them with alcohol on a q-tip or similar. Then they get a small dab of contact lube. I use some 30 year old Silicon based stuff but about anything will work. Amazon can be your friend.

I spray the gimbal assembly's with alcohol and use an old toothbrush to get the corners. Don't forget the plastic stick ends where your fingers go. I like to take the entire gimbal assy apart but that's just me. I want EVERYTHING clean like new.

The main slide switch used for power is often in rough shape. Blast some contact cleaner in there or take it apart. Its usually OK to re-use wiring but I NEVER re-use any of the power wires. That corrosion or black wire disease crawls right up them.

Be careful with the meters. Just some water on a cotton towel can usually make them shine again. Otherwise try a little plastic some people have good luck with floor wax but I use Novus 1 from my Pinball restoration days.

I would recommend finding a cheap transmitter on eBay to practice with. You can often find old ones for less than $15. Its worth it to NOT ruin your prized Kraft (or whatever). Good luck and let us now how it goes.
Last edited by Scott Todd; Dec 11, 2019 at 12:33 PM.

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