DMcQuinn

My Feedback: (13)

Help me understand this failure. After 5 years (perhaps 20 or 30 flights per year) my throttle servo went berserk, moving erratically. I shut off the gas engine (3w-100) using the ignition switch to diagnose what had gone wrong. With the engine off, the servo works fine. The servo is a JR 517 (older model "standard" servo, analog, non-ball bearing, I think). All of the other 8 servos in the plane work fine when the engine is running (these are all digital servos). I plugged the throttle into the aileron channel and the servo still went berserk, I plugged ther aileron into the throttle channel and the aileron servo worked fine when I move the throttle stick.

So obviously, the servo is bad, not the transmitter or receiver.

So I got another used JR 517 servo and plugged it into Ch 1 of the receiver. It worked fine until I started the engine. This servo also went berserk when the engine was running regardless of what channel it was plugged into. I tried a third servo (this time I went to a high end digital DS8411) and all is fine. I am thinking this is just a weird coincidence and that I have two bad JR 517 servos. The 3W motor was running especially bad today, running on only one cylinder, so perhaps the resulting vibration was just too much for these low-cost servos.

The receiver is a high-end 10 channel PCM (JR 950S I think) and all the other servos work fine engine running or off. The plane is 5 years old and has been working OK before today (other than the engine which has been acting up). I have never experienced any ignition related interference that I know of. And if there was interference from the ignition, I would expect it to affect all channels. With the pcm receiver, I would expect maybe a fail-safe to occur (it did not). The setup is very clean for a gasser. ignition unit, ignition switch and battery up front and everything else rearward. There is at least a foot between the ignition system and any receiver component.

Both receiver batteries show 6.0 volts under 1 amp load. I use two 5-cell NiMh packs (2500 mAh each).

TLH101

My Feedback: (90)

Plug the servo into a different channel, and see if the problem goes away. If it does, receiver is bad. If not, servo is bad. Its possible that a pin in the receiver is loose from the circuit board, and loosing contac through engine vibration. You might try holding the plug tight in the receiver and see if that stops the problem.

DMcQuinn

My Feedback: (13)

servo behaves the same way plugged into aileron channel (ch 2). and the aileron servo behaves fine when plugged into the throttle channel. So I'm pretty sure that it is in the servo. Cannot explain why two servos in a row did the same thing. I tried wiggling the connectors at the connection to the receiver while the problem was occurring but nothing changed while wiggling.

I put the "bad" servos on a vibration test at home and they work fine. I built a little test table with an electric motor and a cam to shake the table. put the servo on the table and operated it while it was being shaken by the electric motor. No problem. The electric motor only runs at one speed so it vibrates the table at a different frequency than the gas engine. I was hoping to duplicate the problem on the bench.