Old AM servos on new FM Radios
 

I was very active in RC in the '80s. Put my stuff in storage due to work demands until I retired last year. Problem: will the old Futaba S26,S28, S38 servos work on the new fm radios? My old radios all AM are not legal. I have several aircraft that have flown for years under the old Am units.
I got a Tower 4 channel unit and refurbed the airplanes (be sure to replace the plastic control links, they get brittle with age). I put J plugs on the servos. I adjusted the controls (I thought). When I flew the first time the trims were a mile off. I bearly got it down so I did a readjustment and flew again. The aileron was nuts. It was so far out of trim that I bearly had time to cut the engine before the crash (totaled the plane).
Lesson learned: The electrical zero center of the servo is not the same as the AM unit. I analyzed the situtation with my pluse meter and found that FM center is about 1.45 ms as compared to 1.20 for the AM unit. Also, I have a feeling that the throw rate is also different. (So even engineers screw up once and a while!!! )

Comments and Suggestions Please. :D

Old servos..
 

The old Futaba G series servos will work fine with the new FM radios..except for the one little thing you already found out, the center is off.

Futaba used to make a G to J sevo adapter cable that looked like a short aileron extension...I don't know if they still sell them or not. I have a few still laying around..I had Futaba do the narrow band upgrade on some old FG radios back around '91 when the spec became law and they threw some in with each RX they sent back.

So don't let anybody tell you they won't work (I've seen that opinion posted here), I'm sure Futaba wouldn't have went through the effort/cost of making these adapter cables if the servos they were adapting wouldn't work with the RX's they were adapting to ;)

Old AM servos on new FM Radios
 

I have a couple of very olf Futaba servo's which I still use with my modern PCM/FM radio gear.

I think the centre position used to be 1.3mS, whereas modern servo's are 1.5mS.

They can easilly be converted by cutting off the old connectors and fitting new ones, or soldering on a new cable end if you don't have the right crimping pliers.

Then the servo needs to be repositioned, twisted round a little bit, to move the centre in line with modern radio gear.

Hope this helps.

Center RESET
 

I found a hobby shop(Dymond Modelsport USA they have a web site) in Oshkosh WI that sells the J male and female plugs (1.80) so I soldered J plugs on the servos and my test equipment (should av done that before the flight). (I would hate to be in the Shuttle engineer's shoes now)
Didn't mind loosing the Plane as much as looking like an ass at the field.

When you say recenter the servos do you mean using the spline shaft/arm or resetting the internal gears???

Also, are you saying that the old servos will work on new PCM FM radios??? I thought that PCM radios required a special PCM servo. That is neat. I got piles of old Futaba servos.

So I guess my Gold AM Futaba radios are trash, huh? Damn, they were great radios.
I was so busy in the early 90s I missed the conversion. :rolleyes:

Just...
 

Recenter them on the servo wheel splines.

And sure you can use them on PCM...the PCM/PPM bits get sorted out before the servo...

And don't trash them old Gold radios....mine all met the narrow band requirements, it was the RX's that got replaced. I don't think you can still get a Gold Sticker on them....just not sure what the rule is on them now. Mine still work at my field ;) but I don't take them on the road...

Old AM servos on new FM Radios
 

PCM/PPM FM/AM don't matter, a servo is a servo. Excepting the newer digital servos, which are servo's with brains. PCM has to do with what goes on between the radio and the receiver, nothing whatsoever to do with the servo's.

Old AM servos on new FM Radios
 

The old servos did center with a narrower pulse width. usually this is not a problem with using the 1.5 millisecond pulse width now used by most manufacturers. If it is it will be noticable at full travel in one direction or the other due to the internal pot hitting its end point. If you open up the servo, you can usually reposition the pot so that it accomodates the 1.5 millisecond pulse and has equal travel in both directions. I have never had to do this on any I converted but some were close to hitting the end points on the internal pots.

Old AM servos on new FM Radios
 

FYI:

Check out Radio South for narrow band conversions. http://www.radiosouthrc.com

BTDT!

Later!

Old AM servos on new FM Radios
 

Thank you for the Info. Maybe I can at least salvage the xmitters. Be cheaper to buy new receivers probably. :)