Engine Doctor Posted June 27, 2021 Share Posted June 27, 2021 I had strange thing happen yesterday. I was fitting servos and radio to a little mustang given to me by a club mate. All servos are new 9 Grm TowerPro jobs . All tested prior to fitting in model with a servo tester , all smooth and working well. Fit rx and connect the ailerons servos via a "Y" lead . One servo works fine the other refuses to move and acts like it has no power and completly dead. So unplug from Y lead and connect to servo tester and it works fine ? Ah the proplem is the y lead I think so swap out the y lead. still the same . Now I connect this servo to the tester and it works fine , connect to rx and nothing . so changed for another servo and all is fine . So the question is , why would a servo talk to a tester and not a rx ? All leads checked for continuity etc and all fine . Not worried about the servo as they only cost pennies but it is an odd one . Any Techies out there that might offer a answer ? Quote Link to comment Share on other sites More sharing options...
Cuban8 Posted June 27, 2021 Share Posted June 27, 2021 (edited) A guess without actually taking measurements, but possibly a difference in the control signal characteristics between rx and tester. Maybe a bad or wrong component in the servo that's highlighting a small difference in for example, signal pulse amplitude. Perhaps a bit low from the rx compared to the tester but within spec for a working servo? Edited June 27, 2021 by Cuban8 Quote Link to comment Share on other sites More sharing options...
Alan Gorham_ Posted June 27, 2021 Share Posted June 27, 2021 Is your receiver Spektrum by any chance? If so, then the issue is to do with the drive current of the logic signal to the servo(s). You may need to fit one of those online signal boosters (yuk) if you can't simply plug the aileron servos into separate Rx outputs. Quote Link to comment Share on other sites More sharing options...
Cuban8 Posted June 27, 2021 Share Posted June 27, 2021 (edited) 7 minutes ago, Alan Gorham_ said: Is your receiver Spektrum by any chance? If so, then the issue is to do with the drive current of the logic signal to the servo(s). You may need to fit one of those online signal boosters (yuk) if you can't simply plug the aileron servos into separate Rx outputs. Strange, but I've never had a problem driving more than one servo from a single spekky rx output. I'd investigate or simply out the suspect servo. Edited June 27, 2021 by Cuban8 Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted June 27, 2021 Share Posted June 27, 2021 Is it possible that the receiver is outputting in low frame rate and the servos can't handle this? Quote Link to comment Share on other sites More sharing options...
Dad_flyer Posted June 27, 2021 Share Posted June 27, 2021 I would suspect the same as @Alan Gorham_. Receivers are often/usually 3.3V logic, but the servos are powered at 5V. This works perfectly well if both are operating within the actual specifications of the logic, but a slightly low output from the receiver and a slightly high threshold on the servo logic would mean that the servo does not register anything. To be sure of it you would need an oscilloscope to check the receiver output, or a logic level converter to check the servo threshold voltage. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted June 28, 2021 Share Posted June 28, 2021 14 hours ago, Cuban8 said: simply out the suspect servo. Given the cost of these types I think that's the easy route. Or, put the ailerons on two separate rx channels. The problem might even be a fault in the in the 'good' servo (near short between signal and 0v?) causing the RX output to be mangled enough to not drive the 'bad' servo. Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted June 28, 2021 Share Posted June 28, 2021 (edited) It's hard to beleive, but the lower cost, identical servos, are not identically matched. The " supply" from a pack, is finite, so can provide " so much " power, but in our case, does not increase if need be. On the day the servos were made, it could be hot, cold, wet, dry, and maybe a draught in the workshop ! As solder is heated, this will liberate zinc as it does its work, and On cooling, a slightly different alloy remains on the circuit board touching wire and making connections. Slight differences in current flow will be apparent on lower cost components, and electrons will find the easiest path through one servo of a pair on the same line. We swap out lame low cost servos, but though not cost effective, we could balance the supply between the two, that they operate, as in your case, they do work singularly. You will have noticed the change in solder when you come to de-solder a joint. Intially, you can melt the fresh solder onto your iron, it flows freely and appears shiny. When you come to de-solder, you have a different animal, it is more difficult to melt, it has a higher melting point, and is physically harder. Solder changes its properties as it melts and re-cools, and just becomes harder, and has a measureab!e change in resistance. One servo had an higher resistance than the other. Edited June 28, 2021 by Denis Watkins Quote Link to comment Share on other sites More sharing options...
Nigel R Posted June 28, 2021 Share Posted June 28, 2021 (edited) My first thought was a duff component pick for one of the servos, as sometimes happens in a reel of resistors, but as you say Denis a solder problem could well do it, too. Could also be a weakness in signal drive side (i.e. RX). Many causes, needs narrowing down to a particular piece of kit before much better diagnosis could be offered by the assembled minds. Edited June 28, 2021 by Nigel R Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted June 28, 2021 Author Share Posted June 28, 2021 (edited) Hi thanks for all the discussion. I suspect a faulty component in the dodgy servo as has been suggested. All the other identical servos work fine so drive from Rx is obviously OK .Rx is FrSky . The offending servo would not work if it was plugged directly into a Rx .As posted in the OP the offending servo has been ceremoniously promoted (thrown)to the bits box for gears and trial fitting etc. Edited June 28, 2021 by Engine Doctor Quote Link to comment Share on other sites More sharing options...
Cuban8 Posted June 28, 2021 Share Posted June 28, 2021 Having earned my living by sorting out similar problems in industrial equipment, I know only too well that it's easy to misdiagnose a fault either by a hurried educated (wrong) guess or simply not having the full facts to go on even when you have the bit of kit right in front of you. Often the trickiest of faults to track down could manifest themselves in relatively simple kit where one scratches one's head and says "well there's not much to all this stuff so why on earth is it doing what it's doing"? Best place for the servo is in the bin as you've done...........forget it and move on. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted June 28, 2021 Author Share Posted June 28, 2021 Hi Cuban 8. Agreed. I had already dumped it as its not worth risking a model for but thought I would ask the question. They only cost ppennies so not worth the risk . Quote Link to comment Share on other sites More sharing options...
Mike T Posted June 28, 2021 Share Posted June 28, 2021 The bigger question for me would be "what use is a servo tester that tells me a servo is OK when it (likely) isn't? Quote Link to comment Share on other sites More sharing options...
Nigel R Posted June 28, 2021 Share Posted June 28, 2021 They can subject a servo to continuous complete movement sweeps for (say) a half hour. Can be used to set up surfaces / pushrods / horns / etc. Drive retracts with it while you're building. I'm sure there are other uses. It's a five pound widget, it won't work miracles. 1 Quote Link to comment Share on other sites More sharing options...
Mike T Posted June 28, 2021 Share Posted June 28, 2021 Yes I know about their functionality (I didn't come down with the last shower ?) Maybe @Engine Doctor should fish that servo from the bin and soak-test it to see if he can get it to fail on the tester? Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted June 28, 2021 Share Posted June 28, 2021 52 minutes ago, Mike T said: Yes I know about their functionality (I didn't come down with the last shower ?) Maybe @Engine Doctor should fish that servo from the bin and soak-test it to see if he can get it to fail on the tester? A servo tester should be called something else, perhaps servo vague indicator But it does show your surfaces working and centres the arm But it is " off load " So is electronically testing very little on that side. " On load " would be in the air, checking feedback from surfaces pushing back. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted June 28, 2021 Author Share Posted June 28, 2021 1 hour ago, Mike T said: Yes I know about their functionality (I didn't come down with the last shower ?) Maybe @Engine Doctor should fish that servo from the bin and soak-test it to see if he can get it to fail on the tester? Have since done that , and it works faultlesly. Plug it into Rx and its dead ? It's staying in the bin. Quote Link to comment Share on other sites More sharing options...
Geoff S Posted June 28, 2021 Share Posted June 28, 2021 Interesting (in the Chinese sense). Have you tried the servo in another receiver? I think you've done the right thing by discarding it btw but it might be interesting to see if it can work with a receiver. Geoff Quote Link to comment Share on other sites More sharing options...
Sparks Posted June 28, 2021 Share Posted June 28, 2021 Hi, are you powering the receiver and servo tester from the same battery? Could be that if you are powering the servo tester from a higher voltage than the receiver, it might be enough to make the servo work. Just curious, anyway sounds as if the servo should stay in the sin bin. 1 Quote Link to comment Share on other sites More sharing options...
Don Fry Posted June 28, 2021 Share Posted June 28, 2021 1 hour ago, Geoff S said: Interesting (in the Chinese sense). Have you tried the servo in another receiver? I think you've done the right thing by discarding it btw but it might be interesting to see if it can work with a receiver. Geoff Yes, very interesting. But even if you can get it to work, you are quite right such experiments stop before takeoff. Quote Link to comment Share on other sites More sharing options...
Mike T Posted June 29, 2021 Share Posted June 29, 2021 23 hours ago, Engine Doctor said: Have since done that , and it works faultlesly. Plug it into Rx and its dead ? It's staying in the bin. OK. But fish it out again, strip out the gears, keep the case and screws, then bung the electronics back in the bin... ? Quote Link to comment Share on other sites More sharing options...
Martin McIntosh Posted June 29, 2021 Share Posted June 29, 2021 Just joining in but have some positive input. Firstly. if the TP servos did not come from a reliable source such as SWM then they are almost certainly fakes. I had some in an auto gyro and if stopped by hand they would not restart without a Tx input. Not the first time I have had problems with the `brand` and certainly not alone. The pot centre connection is made by a tensioned split leaf and is a copy of the HiTech idea but from inferior material. When the connection fails the servo drives to one end and burns out if you have not already crashed. I have some sets of HK MCR retracts but the latest ones will not even respond to an Orange Rx but are OK on Spektrum etc. They work fine with a servo tester and appear identical to earlier ones which are fine. On checking with a `scope I found that Orange only outputs a 3V signal and the other sources 5V. On taking this up with HK and suggesting that there may have been a component change, intentional or otherwise, they simply sent me another duff set to shut me up. Regarding the solder joints, by law our stuff should only be manufactured with lead free solder which is just awful to use. Military, medical and aviation ban this rubbish because it is so unreliable. I would say that the Y lead in use may have caused a voltage drop sufficient to affect a servo which was teetering on the edge of responding or not. Ditch them all and buy some proper ones. Quote Link to comment Share on other sites More sharing options...
Paul De Tourtoulon Posted July 1, 2021 Share Posted July 1, 2021 (edited) Oh dear,,, I posted the same problem on a British forum now closed down, the blue servos wouldn't even work on my Futaba 2.4 or 41 mhz gear the same servos and the same problems, so I changed the servos ( not my plane) and put some decent ones in the wing with the Y lead.,,, ps I did get a good answer from one of the 'expert' members (PDR ) about the signal coming from newer radios,, pps they all worked separately on any radio or servo tester,,, Edited July 1, 2021 by Paul De Tourtoulon pps Quote Link to comment Share on other sites More sharing options...
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