Radio Interference Problem

[rcboater]

I'v run into an odd RF interfenece problem in my latest project.

I have a plastic model that I've converted to twin screw RC. I used my standard plastic model conversion process:

1/16 inch diameter brass rod for the shaft(s), running in the matching brass tube. A stanard model airplane wheel collar keeps the shaft from falling out of the tube.

The prop is a Raboesch brass 20 mm.

Nylon HO scale locomotive universal joint to connect the brass shaft to the motor.

Motor is a standard servo, removed from the case, and attached to a home made plastic motor mount.

Standard 2 channel AM radio, using a 6 volt rechargable pack instead of the four AAs.

I've used this setup in a number of Lindberg tugs and other models in the past-- it has always been very reliable.

In this model, I'm getting a lot jitter - classic RF interfernece symptoms- when I run the model. It happens when I apply power, so it seems to be coming from the drive train. It is the same kind of interference I see if I hit the trim pot with a metal screwdriver while making adjustments-- that metal on metal contact causes a little servo glitch. That's why I use non-ferrous (brass) for all the moving parts-- I know that metal on metal contact can casue RF interference otherwise.

At first, I thought it might be the antenna lead or the radio itself. I replaced the Hitech 2 channle receiver with a Futaba unit on a different channel, and temporarily laid the antenna wire fully extended out on deck to improve reception. That didn't help-- I still get a lot if interference-type of hits.

The one thing that is different in this model from my others is is that I am using two motors this time. Instead of a single servo, I have two of the same type, one for each shaft. I spliced the servo leads together so both are driven off of the throtle channel on the 2 channel radio. (A home made Y cable, in effect.) I know that you can drive two servos off of one channel-- airplane guys do it all the time for aileron control on bigger models. They use a Ycable adapter to allow them to conenct two servos to the same channel on the receiver.

I have one last thing to try-- I once had an old rudder only Lindberg Fletcher that suffered from interference when the cheap can motor was onl. I made a little metal box out fo sheet aluminum, and placed it over the moter like a cover. That kept the interference from the motor from getting to the receiver. I am going to try that next--it is the last thing to try, short of replacing the entire drive train.

Any ideas/comments?

[kennylibben]

i'm afraid you pretty much discussed all my ideas. Just make sure the end of the antenna is far from the servos...and is there anyone else around that might be using that frequency? i'm not very familiar with AM... perhaps some sort of radio tower nearby is causing it... like a CB tower or something.

Make sure your on ground frequency too.

Hopefully ARH or Dave or someone else will know.

[Dave Wooley]

Hi rcboater. The problem could be related to splicing two servomotors together. Remove or if possible disconnect one of the servomotors and run the model with a single motor. If you experience no interference repeat the procedure with the other motor. This way you can isolate the motors as a possible source of interference. Electric motors especially the simple "can" type often require suppressor across the terminals which has the effect of dampening the interference generated by the simple brush / commutator contact. Another possible source of interference is using dissimilar crystals, they may be the same frequency but are of different manufacture say Hi-tec TX and Futaba RX, or using the crystals the wrong way round both are common faults. You didn�t say which frequency you were on which can also be a problem? Finally and as Kenny has mentioned keep the RX aerial routed well away from the motors. And never be tempted to reduce the length of that aerial.
Dave Wooley

[Greg W]

Just to isolate any source of noise try disconnecting the shafts from the motors to see if the shafts are creating the noise.Greg

[MechTech]

One problem I've had before; a bad servo. Long story, but I've had a perfectly functioning servo cause the same symptoms. If your using the circuits from the servos to drive the servo motors, one of them could have gone bad too. I bought a radio on 2.4 GHz (no typo) and you still get the same problems! Hope that helps. - MT

[rcboater]

I solved my interference problem-- by wrapping the receiver in aluminum.

I had some thin sheet aluminum (flashing) left over from doing some roof repairs.

I took a small piece, and folded a simple box to cover the Hitech 2 channel receiver. I left it open at one end, so the servo wires could get out. I then put the receiver back in the model.

That solved the problem-- I no longer got glitching on the bench, and at the pond I was able to go way out and still have good control. (I used to have glitching if I went more than 20 feet offshore before.)

Thanks for all the ideas and feedback, folks.


(I do think that one of the serovs may be the cause of the noise-- I don't see how the all-brass drive train can be causing the problem. If the problem comes back, I'll look at the servo motors first...)