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Old Jan 20, 2013, 07:49 AM
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Originally Posted by 600Bob View Post
Would you run 6v servos on an unregulated 5 cell NMXX battery?
Yes that 5 cell Nicad or NMH battery pack would work OK.
But one still has to be extra careful that the linkage to the servo works Ok and is free and not binding up somewhere. Plus the servo has to be free for its entire travel without jamming up against something at the ends of its throws. For a throttle it would be at both ends for idle or below idle and for full throttle. If the throttle is running up against its stop at full throttle, it will burn out the servo.
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Old Jan 20, 2013, 07:58 AM
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Originally Posted by grosbeak View Post
Yo, OP here... see posts 12 and 13. Channels 3 and 7 were linked in the Tx.
Yes I saw that, thanks, but did it cure the smoking throttle servos?

I've had that problem too with my Futaba 10CG -- factory programmed mixes which I didn't know were there, and were screwing up my throttle curve in heli mode if I ever moved VRa knob. But I don't understand how any mix -- intentional or not -- could cause a servo to smoke
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Old Jan 20, 2013, 08:23 AM
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Originally Posted by abenn View Post
Yes I saw that, thanks, but did it cure the smoking throttle servos?

I've had that problem too with my Futaba 10CG -- factory programmed mixes which I didn't know were there, and were screwing up my throttle curve in heli mode if I ever moved VRa knob. But I don't understand how any mix -- intentional or not -- could cause a servo to smoke
I agree - I don't understand it either. Consider the following:
  • Plane (including throttle servo) works fine for two years for previous owner
  • I install my receiver and turn on the radio and a throttle servo promptly burns out
  • I try another throttle servo and it promptly burns out
  • I notice that advancing the throttle is turning off the ignition
  • I find that channels 3 and 7 are linked in the transmitter
  • Only the throttle servo was linked to ch7, and only the throttle servos burned out

Pretty coincidental, but you're right - there's nothing conclusive there.

Normally ignition kill switches on the transmitter are on or off. With the kill linked to the throttle, however, perhaps the voltage to the ignition was being reduced incrementally as the throttle advanced, until it reached the 50% mark where the ignition was shut off. Then again, perhaps it still functioned as an on-off switch. In either case I don't know how that could have affected the throttle servo.

I suppose I have two choices - plug in the new throttle servo and hope for the best, or continue checking the system for another cause. I hate to say it after the elation of my discovery, but I think I have to keep checking.
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Old Jan 20, 2013, 08:49 AM
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The clue in this is that your ignition light went out when advancing the throttle... that probably means that the flight systems power was browning out due to what Earl pointed to as either a stalled servo or a servo short and major battery drain.

I suppose it is possible for the servo to be confused by two variant signals assigned to it and that should be ruled out by testing the servo in another channel for proper operation. Certainly remove the servo arm and by hand move the throttle linkage and verify easy operation without undue strain or binding.
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Old Jan 20, 2013, 09:58 AM
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Originally Posted by AA5BY View Post
The clue in this is that your ignition light went out when advancing the throttle... that probably means that the flight systems power was browning out due to what Earl pointed to as either a stalled servo or a servo short and major battery drain.
I don't think so - the throttle channel and the ignition kill channel were linked in the transmitter, so any action taken on channel 3 was repeated on channel 7. It was precise an repeatable - every time I advanced past half throttle, the ignition LED went out. And every time brought the throttle back down to the halfway point or below, the LED came on again.

Quote:
I suppose it is possible for the servo to be confused by two variant signals assigned to it and that should be ruled out by testing the servo in another channel for proper operation. Certainly remove the servo arm and by hand move the throttle linkage and verify easy operation without undue strain or binding.
When I tried the spare servo in the Rx, it was connected neither to linkages nor extension - I held it in my hand and plugged it directly into channel 3 on the Rx - it fried anyway. I have checked that servo in other channels and it not respond. And I have tried other servos in channel 3 and they worked fine.
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Old Jan 20, 2013, 10:25 AM
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Ok... I missed that there was a mix to the ignition relay... yeah, that is what was happening with it.

The DS-821 has been released in a HV version. I don't know if that is to enable use with unregulated LiPo or because of problems with 6v use. LiFe is generally regarded as similar to 6V use because the top charges of both 6v NiMh and LiFe are quite similar.

Maybe coincidence got you. A friend at the field crashed two planes yesterday... so two is a hard number, eh.
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Old Jan 20, 2013, 10:25 AM
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I would suggest using a voltage regulator on the radio system to drop the voltage down to say 6.0v or less, maybe 5.5v instead. You may be trying to use servos that just are not capable of working at unregulated voltages out of a Lipo or LiFe battery pack. Just because a manufacturer shows specs for a servo at 6.0v does not mean it can handle 7.2v or more volts from the battery packs at peak charge voltages.
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Old Jan 20, 2013, 11:03 AM
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Originally Posted by earlwb View Post
I would suggest using a voltage regulator on the radio system to drop the voltage down to say 6.0v or less, maybe 5.5v instead. You may be trying to use servos that just are not capable of working at unregulated voltages out of a Lipo or LiFe battery pack. Just because a manufacturer shows specs for a servo at 6.0v does not mean it can handle 7.2v or more volts from the battery packs at peak charge voltages.
A charged LiFe/A123 pack will put out the same voltage as a charged nimh or nicad. Lipos are totaly different. They are 8.4 fully charged. He is using a LiFe.

So if a fully charged 5 cell nimh will give you 1.45v per cell = 7.2v and a 2 cell LiFe will give you 3.6v per cell = 7.2v. Although a LiFe battery will drop pretty quickly to 6.6v and stay there for a while.

Guess I've been lucky all these years using 5 cell nimhs and nicads on my JR and Hitec servos.
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Old Jan 20, 2013, 11:48 AM
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Originally Posted by grosbeak View Post
...[*]Only the throttle servo was linked to ch7, and only the throttle servos burned out ... I suppose I have two choices - plug in the new throttle servo and hope for the best, or continue checking the system for another cause. I hate to say it after the elation of my discovery, but I think I have to keep checking.
The receiver and all servos connected to it get the same voltage across their red and black wires, since there's a common buss in the receiver. Only the white signal wires are different for each channel.

So, if only one servo is burning out, and it's not stalled because the second one burned when it wasn't connected to anything, it surely must mean that it's getting some volts it's not expecting through its signal wire -- or the red/black leads to that servo are reversed (not the same as plugged in the wrong way), or two servos were faulty. If the excessive volts were coming through the red/black wires, all servos and receiver would be affected.

If I were you, I'd try to check the voltages across the red/black, red/white, and black/white wires of your throttle channel before connecting another servo, and see if there's anything strange happening when the ignition kicks in or out. Then I'd connect the throttle servo without any of the ignition system connected, to positively rule out a receiver or servo issue. Not sure where I'd go after that -- it would depend if the voltage check told me anything
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Old Jan 20, 2013, 02:57 PM
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I'd suspect that you somehow reversed the voltage to the servo that was burned out. That is the only solution that makes sense with what else you said. I do not know of any servo that will withstand having the positive and negative leads reversed.
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Old Jan 20, 2013, 07:02 PM
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Originally Posted by abenn View Post
The receiver and all servos connected to it get the same voltage across their red and black wires, since there's a common buss in the receiver. Only the white signal wires are different for each channel.

So, if only one servo is burning out, and it's not stalled because the second one burned when it wasn't connected to anything, it surely must mean that it's getting some volts it's not expecting through its signal wire -- or the red/black leads to that servo are reversed (not the same as plugged in the wrong way), or two servos were faulty. If the excessive volts were coming through the red/black wires, all servos and receiver would be affected.

If I were you, I'd try to check the voltages across the red/black, red/white, and black/white wires of your throttle channel before connecting another servo, and see if there's anything strange happening when the ignition kicks in or out. Then I'd connect the throttle servo without any of the ignition system connected, to positively rule out a receiver or servo issue. Not sure where I'd go after that -- it would depend if the voltage check told me anything
Here are the results of some voltage tests I did on the receiver.



A few notes...
  • At no time did turning on the ignition change the voltage in the receiver
  • Tests 1 through 6 were performed on channel 3, which I had assigned to an undefined auxiliary function shortly before. Channel 5 was assigned to the throttle at that time
  • Tests 9-10 and 17-18 showed the voltage creep up with repeated cycles of the throttle from no throttle to full and back again
  • Tests 15-22 were conducted with the throttle function returned to channel 3, and the undefined auxiliary function assigned to channel 5.
  • Tests 15-18 showed unusually high voltage and I'm puzzled as to what might be the cause.
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Old Jan 20, 2013, 08:08 PM
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In any case I think we have a cause for the servo failures.

The receiver, a Futaba R6008HS, was in HS mode the whole time. None of the other servos were bothered by that, but the original Futaba throttle servo and the spare test JR DS821 could not play nice with HS mode and offed themselves out of spite.

I have just changed the receiver to normal mode. This is an IMAC place and normal mode will be fine.

However...

Given the chart in the previous post the question still niggling at me is, why is channel 3 putting out such high voltages in tests 15 to 18?
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Old Jan 21, 2013, 01:53 AM
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So far as I'm concerned, it's a mystery -- how can a receiver put out a voltage that's higher than the supply voltage, and why should it increase each time you cycle the throttle stick? Hopefully one of our contributors is an electronics engineer, who might be able to explain. Or perhaps it would be worth a call to your Futaba distributor to get a replacement receiver.
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Old Jan 21, 2013, 06:46 AM
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That was interesting. I had forgotten about the RX High Speed mode. Hopefully changing it to regular mode will solve the problem. That voltage creeping up may be the load changing. You should try to measure the current draw during the tests. If the current draw goes up the voltage usually drops, and if the current draw goes down then the voltage usually goes up.
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Old Jan 21, 2013, 07:14 AM
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Originally Posted by abenn View Post
So far as I'm concerned, it's a mystery -- how can a receiver put out a voltage that's higher than the supply voltage, and why should it increase each time you cycle the throttle stick? Hopefully one of our contributors is an electronics engineer, who might be able to explain. Or perhaps it would be worth a call to your Futaba distributor to get a replacement receiver.
One way the voltage can go higher is if there is an oscillation happening in the circuitry. That would induce an AC voltage component where a capacitor could then provide an RMS (root mean squared) voltage increase.

It would be interesting to put a scope probe on Ch 3 to see what it looks like.
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