Ernst Grundmann Posted January 6 Share Posted January 6 Dear people. First, I wish you a very good new year. As the "electronics man" of our club most members come to me if they have problems with their electronics. We have a strange problem I want to ask for your help with. A club member and good friend of mine is building a scale P51. For the ailerons and the flaps, he bought four new digital servos. They have a blue aluminum case with a black plastic bottom and top. On the label is printed: 30Kg SERVO DC 4.8 ~ 7.4V Deao DSSERVO DIGITAL SERVO. They are connected to a Futaba R7008SB receiver, and a Futaba T16 transmitter is used. When he moves the servos from full left to full right a couple of times, all seems to work correctly. But when he stops moving them, the servos don’t go to the rest position in one go. They move until about ¼ of the rest position and then go to the rest position in 4 or 5 steps. The ailerons and the flaps are not yet connected; the servo’s have only the servo arm mounted. All four servos show this effect. We think it is not the transmitter and/or the receiver because if other servos (S3001) are used, they work normally. Does anybody recognize this effect and have a solution for it? We would be very grateful for any help. Quote Link to comment Share on other sites More sharing options...
kc Posted January 6 Share Posted January 6 No doubt the electronics experts will reply - but my guess is the battery is not enough for the job. Digital servos draw more current than analogue. But of course the possibility of fake servos should be considered. Quote Link to comment Share on other sites More sharing options...
Jon H Posted January 6 Share Posted January 6 I think the issue is with the quality (of lack of quality) of the servo. A quick google search reveals them to be bangood/amazon whizzers and prices as low as £10 A servo of this spec (30kg) with an unknown brand for that sort of money is a massive red flag and would not touch them with a 10 foot pole. I assume this mustang is a reasonable size? 1/5 or 1/4 scale? If so i would favour good branded servos from Futaba, Hitec, savox etc of a lower spec. A 10-12kg digital from any of these brands would easily handle a P51 of 1/4 scale and at 1/5 scale 6.5kg-10kg would still be more than man enough for the job. Not only is a branded servo likely to be more reliable and thus much safer, its also far more likely to meet the specs listed on the box as 30kg seems optimistic to me. The brands i mentioned all have ranges of standard and HV servos. Personally i used the standard stuff as my warbirds up to 1/5 do not need HV, but if you use lipo/fe power then HV is available. Its not worth risking the model or anyone it might hit by using cheap gear with questionable reliability. 2 1 Quote Link to comment Share on other sites More sharing options...
Paul De Tourtoulon Posted January 7 Share Posted January 7 As Jon says, you can't get a decent 30k metal geared servo for £10, I bought a build but not flown 2.08m Stuka, these had HK metal geared servos ( all binned ) servos, real rubbish, and a friend gave me some ebay copies with fake red labels and they were even worse especially when you put them on a Y lead. Quote Link to comment Share on other sites More sharing options...
Phil Green Posted January 7 Share Posted January 7 17 hours ago, Ernst Grundmann said: When he moves the servos from full left to full right a couple of times, all seems to work correctly. But when he stops moving them, the servos don’t go to the rest position in one go. They move until about ¼ of the rest position and then go to the rest position in 4 or 5 steps. The ailerons and the flaps are not yet connected; the servo’s have only the servo arm mounted. All four servos show this effect. We think it is not the transmitter and/or the receiver because if other servos (S3001) are used, they work normally. Simple faulting techniques will isolate the problem... First you need to prove whether it is a tx/rx problem or (far more likely) a servo problem, so eliminate the tx/rx and use a servo tester. Switch between neutral, and +/- full throw and if you observe the same symptoms then the tx & rx are not to blame. Repeat the same process using a beefy regulated power supply (say 10A). If the symptoms remain, you have duff servos. If the symptoms disappear, then your rx battery is the problem. 1 Quote Link to comment Share on other sites More sharing options...
Ernst Grundmann Posted January 7 Author Share Posted January 7 The battery as a possible cause was our first thought, but the battery is fully charged and in good condition. What is left are the servos themselves. They cost about £18 each, so they were rather cheap. That may very well be the issue here. Thank you for your input. I think my friend should buy new and better servos. Indeed, a crash is much more expensieve. 1 Quote Link to comment Share on other sites More sharing options...
Phil Green Posted January 7 Share Posted January 7 5 hours ago, Ernst Grundmann said: The battery as a possible cause was our first thought, but the battery is fully charged and in good condition That wouldnt be enough for me Ernst, I would have to find the actual fault...to be sure, I would take some ESR measurements and run a couple of measured discharge cycles 🙂 What kind of battery is it? what cells? Switch? Cabling gauge? 5 hours ago, Ernst Grundmann said: What is left are the servos themselves. They cost about £18 each, so they were rather cheap. That may very well be the issue here. Thank you for your input. I think my friend should buy new and better servos. Indeed, a crash is much more expensive. Again, I'd humbly suggest that he finds the actual problem, not just spend more money without isolating the culprit... 🙂 Quote Link to comment Share on other sites More sharing options...
Jon H Posted January 7 Share Posted January 7 41 minutes ago, Phil Green said: Again, I'd humbly suggest that he finds the actual problem, not just spend more money without isolating the culprit. everything apart from the servos themselves has been isolated and tested as good so i would say he has found the actual problem. You also do not have to spend any money as returning these servos as faulty will get you your money back. Quote Link to comment Share on other sites More sharing options...
FlyinFlynn Posted January 8 Share Posted January 8 (edited) 15 hours ago, Jon - Laser Engines said: everything apart from the servos themselves has been isolated and tested as good so i would say he has found the actual problem. You also do not have to spend any money as returning these servos as faulty will get you your money back. Unless you can get a return label good luck with sending four 30kg servos back to China. Looking at their web site, the company doesn't look like a bad company, perhaps shoot them an email to explain your problem and see what they say? What is the actual model number of the servos because the company don't seem to list a 30kg servo anymore.. perhaps old stock? Are you sure they are 4.8~7.4V.. the majority of servos listed seem to be 6.0v + Edited January 8 by FlyinFlynn Quote Link to comment Share on other sites More sharing options...
Jon H Posted January 8 Share Posted January 8 55 minutes ago, FlyinFlynn said: luck with sending four 30kg servos back to China. true, but surely that is another reason not to buy them. At least you can take/send a branded servo back to the shop you bought it from. 56 minutes ago, FlyinFlynn said: Looking at their web site, the company doesn't look like a bad company Flash website and shiny anodising on the product is easy. Not much good if the electronics suck. However many people shop with their eyes and bling sells. Quote Link to comment Share on other sites More sharing options...
kc Posted January 8 Share Posted January 8 If goods were bought by credit card then complaining to the credit card company should get your money back if the goods are faulty - for those of us in the UK. Hopefully the Netherlands would be similar. Quote Link to comment Share on other sites More sharing options...
Ernst Grundmann Posted January 8 Author Share Posted January 8 We have done some measurements. The battery is a 5 Sub-C cell XCell 2000mAh NiMH battery. They have been used only for one season and are in very good condition; we tested them. The current drawn from the battery with those four servos connected and moving is no more than 0,7A continues, and 3A peaks. This is when the servos stop and go directly in the opposite direction. The servos are not connected to anything, so they don't have to "work." The servos are moved from full left to full right several times, and the battery voltage stays nice and constant. At the receiver, there is a very small drop of no more than 0,124V (124mV) when the peak current of 3A is drawn from the battery. Our conclusion is that the servos are of poor quality, and better servos are already ordered. Getting money back is a great hassle and not worth the effort. The servos will probably be used for unimportant things like opening the canopy. Again, thanks for all your ideas. Quote Link to comment Share on other sites More sharing options...
FlyinFlynn Posted January 9 Share Posted January 9 23 hours ago, Jon - Laser Engines said: true, but surely that is another reason not to buy them. At least you can take/send a branded servo back to the shop you bought it from. Flash website and shiny anodising on the product is easy. Not much good if the electronics suck. However many people shop with their eyes and bling sells. Yep...you pays your money and you take your chances.. I was basing my comment on the fact that they advertise a range of servos of differing technologies and capabilities. They are not an aliexpress shop with a name like 'shop28456295462'. Anyhow, the decision has now been made. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted January 9 Share Posted January 9 21 hours ago, Ernst Grundmann said: Our conclusion is that the servos are of poor quality, and better servos are already ordered. Getting money back is a great hassle and not worth the effort. The servos will probably be used for unimportant things like opening the canopy. Again, thanks for all your ideas. Hello Ernst . If servos are faulty then dont even use them for " unimportant things" ! A short etc could let the magic smoke out and bring down the model . Use them on a car or trainset but dont use on a model aircraft. 1 Quote Link to comment Share on other sites More sharing options...
Martin McIntosh Posted January 9 Share Posted January 9 Certainly faked copies. I had the same problem with so called Tower Pro ones. If stopped by hand they would not restart without a Tx input; no wonder that the autogyro they were in behaved strangely. Bin them. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.