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Receiver Problem - Uncontrolled Servo movement

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Old 10-02-2003, 11:01 AM
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lr172
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Default Receiver Problem - Uncontrolled Servo movement

I have a Futaba Radio setup with a 127F Receiver. I have used this reliably over the summer (maybe 50 flights or so) with no problems. I am new and this receiver has seen a couple of crashes.

I had a problem on yesterday's first flight. Everything checked out OK on my preflight, however, on takeoff the receiver got a mind of it's own. It was generating movements that I wasn't asking for. I was able to keep control for about 10 seconds and ultimately got it slowed down enough that the crash wasn't fatal. It took almost 30 minutes to find it.

As I was walking back to the flight line with the pieces, all three servos began to move seemingly randomly. The movements were back and forth, with a fair amount of travel in each direction, and definately too much to have been someone else transmitter. It seemed that all three servos were moving. Although the movemements of the three servos were not in unison, they basically started and ended their series of movements at the same time. After about 30 seconds this stopped. It did this intermittantly for a few minutes; Sometimes just a quick jitter and sometimes a more lengthy set of movements. When I got back to the flight line, I noticed that my transmitter was still on. I did not test to see if this problem continued after the transmitter was off.

Before this day's flying, I had done two things that we different. First, I had disconnected and reconnected the battery connector which I later found had a somewhat compromised connector. However, in the post-flight checks, I could not cause it to fail or lose voltage at the receiver. Could a loose battery connection cause this type of problem?

Second, I had moved the receiver to a position in front of the servos and had the antenna wire layed over the servo tray. The wire was sitting right next to the Rudder servo. The wire exits the fuselage behind the servo tray. I would say that 2/3 of the wire was outside the plane.

There are very small fractures in the receiver case, likely from a prior crash. However, the receiver has worked without incident for at least 15 flights since that time. I have no real way to check the transmitter, however, I haver never experienced a problem there either.

I have range checked this TX/RX in the recent past with good results (100 ft. with att. down).

I really appreciate any guidance you can provide in troubleshooting this issue.

Thanks,
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Old 10-02-2003, 11:31 AM
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Default RE: Receiver Problem - Uncontrolled Servo movement

Considering the RX has been crashed before, it should be suspect. You should also check to see what the battery reads on a loaded voltage meter.

During your range check, did you check with the engine running? Perhaps the RX has a problem that is caused by vibration? This could also point you to a bad connector.

Let us know if you find out.
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Old 10-02-2003, 11:56 AM
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Default RE: Receiver Problem - Uncontrolled Servo movement

Thanks for the input. Yes, I did check the voltage after the fight with a loaded meter and it was 4.9 or 5.0 volts on a 4 cell pack. I have not range checked with the engine running. However, the servos were going at it pretty agressively for almost 20-30 seconds while in my arm walking back to the field. It would not seem like an intermitent open. Although, I would imagine that a the connector could cause resistance or reduced voltage, I can't imagine that it was cutting in and out while the servos were going to town. Can voltage fluctuations cause this behavior?
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Old 10-02-2003, 01:12 PM
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Default RE: Receiver Problem - Uncontrolled Servo movement

If you had sufficient crash force to crack the case it is almost a given that there is internal receiver/crystal/component damage.

Suggest you do yourself a favor and save an airplane by sending the whole system in to a reputable shop to be checked.

Jerry
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Old 10-02-2003, 01:34 PM
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Default RE: Receiver Problem - Uncontrolled Servo movement

I had the same problem with my trainee's 127 reciever. never been crashed, just sometimes would go haywire with the elevator servo. If you send it to a shop they might not incur the same problems you are having, but it might help. Just dump the reciever. I know we arent the first to have problems with this reciever.

Joey
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Old 10-02-2003, 02:50 PM
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Default RE: Receiver Problem - Uncontrolled Servo movement

You did not say whether or not your transmitter was on when you got the odd movement. If not, this may be a normal reaction. If your transmitter is not on, the AGC (automatic gain control) goes wide open and; if there is spuriouse rf radiation--even if not on your frequency--the reciever will react to it in the manner you mentioned. That is why you are cautioned to always have your transmitter on before you turn on your receiver and turn off the receiver before you turn off the transmitter. Of course, the crash could have damaged either the crystal that determines your frequency or the ceramic filter that is in the front end of your receiver. Either one being damaged could cause the symptoms you experienced. In general, the 127 is a very reliable reciever as evidenced by the number of them out there performing flawlessly. Any receiver can be damaged in a crash or hard landing if the receiver is not shock buffered properly and all brands are equally effected by this problem.
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Old 10-02-2003, 10:21 PM
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Default RE: Receiver Problem - Uncontrolled Servo movement

The transmitter was on while the problems occured. The problem started at rotation and I inadvertantly left the transmitter on while searching, so it was on as I was walking back. The cracks are very small and only at the casing. When the plane crashed, the receiver was wrapped in 1/4" foam. However, I realize that may not be enough.

Do damaged crystals exhibit this type of behavior?
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Old 10-03-2003, 11:48 AM
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Default RE: Receiver Problem - Uncontrolled Servo movement

I think crystals either work or don't work, but needless to say it's one thing to consider.

Do you have other equipment to swap in and do some testing? There is really no way to solve this without trying other RX's, crystals, etc one at a time until you find the bad component.
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Old 10-04-2003, 12:51 PM
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Default RE: Receiver Problem - Uncontrolled Servo movement

Unfortuneately, a damaged crystal can appear to be working normally. If the broken leads are making contact, things work normally but just a bit of vibration or attitude change can cause intermittent operation or complete failure. The current in the crystal is so small, the added impedance of a touching but broken connection can appear to be working correctly while it is touching.
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