Hi guys,

I'm having a bit of trouble with one of my jets that all of the sudden it decided to start frying my gear ( ch 5 ) servo. I've been flying this particular Jet this year non stop and the last outing it began frying my gear servo. I've replaced it now 4 times. Today I did a few tests and I gave up. I normally load my birds with a pair of 2700 Nicads on 2 separate channels ( no regulator ) with 2 separate heavy duty switches and it has never been an issue.

I even eliminated one of the batteries and switches to see if that was the issue and still after 4 or 5 minutes you see the servo case starting to melt.

Anyone care to give me some ideas as to what I need to check next please ?

Is it possible that the RX has gone bad ?

Can a RX for some reason or another go nuts and just take out one channel like it's doing ?

I'm really running out of options here and don't know what to do.

I would have tried swapping out the RX after the second servo fried. I assume you've already determined the servos not binding on the valve? You must be pulling a lot of current to melt the case. Sounds like an RX problem to me. Do you have a whattmeter or similar to check the current being pulled by the system? Good luck.

Marty

Sounds right.....either RX or that gear servo is binding somewhere/somehow.

Get voltmeter and check the voltage in gear up/down/during retraction on the old and new RX.

If it doesn't change then its because of the gear

Dont know what kind of retract valve you are using and I assume it is not binding, but is there some kind of spring in there that the servo must fight against all the time to hold position.

Hey Johnny,

What is the servo type that is melting?

There really is no way just one channel on a receiver can do this. It is a main power bus across the board, so what ever voltage your retract servo is seeing, it is the same as what all the other servos are seeing.

Is this a mechanical retract system or pneumatic system? Being mechanical there could be some serious binding issues, of course you know that.

If it is pneumatic, I would guess you are using one of the regular micros and it might be binding. Either weay my guess is something in the linkage is the problem causing the servo to draw too much current.

Also one other question, where is the servo located? I am guessing it is in one of your jets. Is it possible it might be in an area of the fuselage next to a bypass or tailpipe that might have a small pin hole in it and hence hot exhaust gases are hitting the servo case. This would be more of a possible reason as to why it just started happening.

More info and we'll figure this out.

Thank you for all the tips guys. Let Me start by showing you where the problem is happening.

This picture shows you where the RX and retract valve is located. The Valve is a BVM High-Flo and the last servo ( currently melted in the picture ) is a DS 368 (metal gear ), RX is a JR 955S in a BVM Mig-15.

FYI the previous servo heated up NOT CONNECTED to the valve.


I didnt want to change the RX until I got some info from you all, what John Redman says makes perfect sence otherwise all my other servos would be heating up as well and that is NOT the case.

This is where is located ( all the way up front ) NOT over the pipe or turbine.

whell:
if one servo heated up not connected (where all the servo same brand-type??)

the problem may just be that the RX (TX in second time..) is giving an incorrect signal to the servo, and he try to go in some impossible position..
what john say is absolutly perfect, and the last source of error is pratically the signal wire from RX to servo..


the output electronics of chanel 5 may be just broken..or giving something wrong...

Alessio,

I honestly dont think that different brand of servos could cause this. A Lot of my jets have several brnads, not only JR for peace of mind. I like to use Volz, Multiplex, Airtronics, some Futaba and mainly JR servos. This problem so far has burned 2 Futaba's and 2 JR's.

Before I change the RX I would like to know if there's anything I could do, how do I check the voltage from one channel ?

Johnny,

Well that blew my theories away. Damn, you could have mounted it on the tailpipe for the picture, then I would have felt better.

This is incredibly odd and I will talk with the JR service boys in the am for you on this. Only other item is a receiver and I still can't see how it is bad, but there isn't much more in the system.

I'll get back with you tomorrow.

Check to see if your programmed end points do dont pass the mechanical limts of the servo's travel.


Since the servo has gotten hot with no linkage hooked to it, it may be an issue with the "deadband width". Meaning it is allways "hunting" for true position. Or a very tight set of gears. Thus the servo motor is allways running to correct and getting hot.

Internal short circuit may also be generating the heat.

Test the servo for internal gear binding. Try using the DS-368 on a proportional channel ( elevator, rudder or aileron) and check it's movement and see if it still heats up.

Bob

jetflyer

Hanger Nine makes a real nice amp meter. it connects between servo and receiver. it will tell you immediatly either voltage or amperage that servo is drawing. I used it the other day for setting up dual inline 8611's on buddies F-84. We eye balled linkage as close as ball and then used matchboxes to fine tune it. I plugged up the amp meter and sitting at neutral the two 8611's togther were pulling .45 - .51 amps. So much for the eyeball theory. With the amp meter install i could fine tune the servo's neutral and endpoints down to .13-.17 amps. VERY COOL TOOL not only for determining what a servo is getting voltage wise...but also how much amp it will draw.

I know this won't fix your problem but it will show you your problem.

I hope it helps you some.

Jetfreak

Hey Johnny,

If you know someone with a Hitec Digital servo programmer (that they're willing to risk for a second) try hooking it up to the offending channel. It has a screen that displays what the RX is sending the servo for a pulse width... It should read somewhere in the range of 1000 to 2000, roughly...

Kelly

Ok, I'll see if I can find one of those meters or a programmer. John, I'll also look forward to your response from your tech dept.


Thanks for all the help guys, now I see why a lot of guys that loose planes have no clue as to what happened, this could very well be one of the many reasons a bird goes down without ever knowing. I was lucky to find the issue on a non flying surface.

I'll hold off swapping RX.

Johnny,
You don't have to swap RX just plug that servo into a different receiver with a similar power setup and see what happens. If it does not happen with the servo I would swap receivers as that could be the last possible cause.

Patrick.

You don't have a voltage problem--The receiver passes on whatever voltage you have from your battery to the servos---The maximum amount of voltage you will have to the servos will be close to equal to the voltage from the RX battery pack. (A slight drop is normal.)

You either have a pulse width problem from the RX or a severe binding problem...Take the servo loose from the valve---If the servo fails then you have an RX problem...If not then you have a binding problem...

Kevin

If you've replaced with a new servo 4 times already, it Sounds like there is a short in the receiver on channel 5. Maybe a weak solder joint failed due to heavy voltage (6V packs?) If not, your servo motors are burning up for a reason previously mentioned.

Johnny

I had just about the same problem, two SH81s burned the moment I turned the switch on the airplane, what the? it was working just fine before, after talking with a friend we end up concluding that my battery a 5 cell pack was gaining strength and like Gordon said " Don't look at a 5 cell pack battery like a 6 volts batt. but instead look at it like a 5 cell pack batt. that could go as high as 7.5 on voltage.

My servos were attach to a BVM smooth stop valve, no binding at all, when the servos burned was when I turned the switch on.

I solve my problem by adding a regulator to the system, since then not a single glich. I have a 4.9 regulator, I have it because it was given to me by a friend, but I will be getting a 5.9 volt regulator in the future because I know the HS81 are OK with the 6 volts.

I hope this helps.

Johnny

Connect a big servo to the retract channel to see if it works OK, of course, use an old one in case you loose that one too, if it does OK a regulator may be your answer.

I agree with cars II.

I use the powerbox sensor for my Jets in combination with 2 lipo power packs 7,4V.

http://www.powerbox-systems.com/e/po...nsor/start.php

This system regulates the voltage exactly at 5,9V!
So no worries of high voltage anymore!

Hope this will help.

Henk

JH, The servo looks close enough to the rx to have no extension on it?? if there is an extension lead replace that too, maybe internal short in the connector(s). V..

How could it possibly be a short in the connectors? All that would do is melt wires, NOT the servo case or motor. Johnny, I think that somehow the receiver is somehow commanding the servos beyond their end-points, as Kevin pointed out. Sounds like you have already eliminated binding (heck, one failed not even connected to the valve, right?). Therefore the receiver has somehow failed and caused erroneous pulse width commands or your transmitter is commanding higher or lower than normal servo positions.

P.S. And I don't think its high voltage off of a 5 cell pack either. Yes, they can be quite high on voltage fresh off of a charge. But the instant you pull some load, that's gonna drop like a dead stick BVM F-4! And if you are melting servos, believe me you ARE pulling some load! Besides, I used un-regulated 5 cell packs for years, no issues EVER! It's the receiver or the transmitter.

Johnny,
I agree with Woketman. A servo will only draw the amps that it needs to get the job done. I would look to the radio and the receiver. It looks like the servo is receiving a signal from one of the two to go beyond it's mechanical end points. You have eliminated everything else. Please post your results when you nail the problem. Good luck!


John Ligons

AMT USA, LLC.

Johnny,

My bet is that the RX is putting out a pulse width that is driving the servo up against it mechanical limits. Have you tried changing the programming such that the servo is in the middle of the travel and then turn on the system? Also, to check the pulse width, you can use an FMA servo lab:

http://www.fmadirect.com/detail.htm?...901&section=52

This thing is the greatest thing since sliced bread and I use mine all the time. You can find the limits of the servo travel when its installed and then connect it to the RX and set the ATV (end points for you JR guys) to those values and never overdrive the servo. Its also great for setting up gyros, exercising servos, etc.

Also, you need a Hanger 9 amp meter to see how much the servo is drawing...

http://www.horizonhobby.com/Products...?ProdID=HAN172

Two things you should not be without....

Bob