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Old 09-22-2012, 05:28 AM   #2
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Originally Posted by hayofstacks View Post
I think it is time to dispell some of the myths that revolve around older planes, am radio's, and newer spectrum radio gear.

I have seen written many times, that am servo's are not compatable with spectrum receivers and other radio gear. Upon completion of my second am radio conversion, I would like to hear where this rumor comes from.

First plane I converted was an old 25ish sized phil craft ugly stick. I pulled an am radio out of it, replaced the battery pack, swapped in a newer parkflyer fm receiver off of my slowstick. I did several antenna down range check, and found everything on the model to work perfectly, farther then I can see it, with antennea down.

I used the fm receiver for two reasons. Plug and play with the origonal am receiver, and because I had doubts about if the spectrum receivers I upgraded to would work. After thinking about it, I went from a fm receiver to spectrum stuff on my slow stick. So I knew if the fm servo's would work, then so would am ones.

The old covering on my ugly stick was solarfilm. After a couple trips to different hobby shops, I decided that solarfilm is quite a bit thinner, and less heat resistive. I also could not find any trim sheets or stickers for it. I have had several holes open up on the wing in flight. My first patch I thought was caused by a level landing off feild when the motor cut out at low altitude. After inspecting the wing, it has many cracks and small holes in it already. The monocoat trim sheets I attempted to patch the wing with, work very well, but the solarfilm is too old and melts, even under the trim sheets.

This led me to another plane. We have three generations of airplanes in our garage. My grandfather. Was a silk and dope man. He also way overbuilt all of his planes. Making them very heavy, and many, the silk has rotted away. I found a foam wing and built up fuse with a newish looking .40 on it. We pulled it out, it fired right up, and the motor ran well. The biggest problem was the servo's. It used a pre 1991 am radio with the three prong futaba servo connevtors. I was getting ready to swap all of the servo's out when it hit me, I have about 5 bad servo's with good leads.

We had a 5channel "full range" receiver in my dads recently trashed trainer. I cut off all of the servo leads, removed radio and flight pack. Soldered on some "new" sevo leads, bound the receiver, and the plane is now ready to fly. After regluing the elevator stab, it should come out to be a very nice flying model. It is a high wing, symetrical foam wing covered in some sort of vynil meterial. Coming in at about 4lbs, this .40 fox motor should power it very nicely.

But back to the origonal question. Where did the am servo's not being compatable with spectrum receivers come from? I am guessing the radio gear to be somewhere between 1985-1991.
That's interesting. I just wrote an article on this very subject as the editor of the RRCC (Racine Radio Control Club) September newsletter. A copy of that article is attached.

The conclusion of the tests on my oscilloscope show that the servo's I tested worked OK with a receiver servo signal that ranged from zero volts to a minimum of about 1.7 volts DC at the pulse voltage peak.

Most newer receivers use what's called "CMOS" integrated circuits for driving the servo signals. These CMOS chips have an output voltage that swings from zero volts to what ever the receiver uses for its operation. And that voltage swing is well within the requirements to drive the servos that I tested.

Some integrated circuits are known as "TTL", that have very different output voltage swings, usually from 0.7 Volts as a low signal, and about 2.5 Volt as a high signal. Using this type of TTL circuit would be very risky on a servo. It could work, only marginally. That said, TTL stuff dates back to perhaps the 1970's or so, and were replaced with the CMOS stuff.

Take a look at the attached PDF file.

Attached Files
File Type: pdf Servo Compatibility - WF.pdf (546.0 KB, 146 views)

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