All Servos Blowing Back/Stalling Champion SRS

  • Hello,

    I currently got 2 Krill 41% Extra 330SCs. One for 3D and one for IMAC flying. Both have the Champion SRS, both are flown with the PowerBox Core Radio, both have MKS high torque Servos on all surfaces (HBL380s on one and HBL3850s on the other). Both use two PBR-26D receivers per airplane. Everything was working fine on both of these airplanes up until today. I made several flights today and noticed on my 3D airplane that the aileron rolls were very slow in the air even though I was flying on full rates. It just didn't feel right in the air. I knew something wasn't right because in the previous flights before today, the airplane had much faster roll rates and elevator/rudder response. I soon figured out that my ailerons weren't deflecting to the 100% rate in flight even though they did on the ground. They were deflecting only up to 30-40% during the flight while rifle rolling which didn't happen before a few weeks ago. As soon as I landed the ailerons/rudder/elevator were traveling at 100% on the ground but not during flight. I then decided to apply pressure with both thumbs to each control surface and noticed that each surface was stalling/blow back while moving each control stick on the radio. This was occuring on everything including both ailerons, rudder, and each elevator half. At first I thought I had a servo problem/cable problem but it's happening on all surfaces when pressure is applied with your hands to create resistance just like in the air. I didn't have this problem before. I'm running everything at 7,4 volts with 2 Pulse 2S 5000mAh Lipo batteries which are fairly new and were purchased earlier this year. The only thing I changed in the CORE before flying today was I eliminated the dual rates on my airplane as I prefer max travel on my planes for 3D. I thought this problem was only on this airplane but decided to check my other 41% 330SC for IMAC flying on the ground and I got the same results! When pressure is applied on each surface while deflecting, every surface stalls on me! With a little pressure, every surface stalls on me and it would definitely happen in the air but I can't fly and test it out because this is a very serious problem. I'm running 3 Servos per wing and 2 on each elevator/ rudder. I don't know what's going on all I know is that both planes are getting blowback/stalling on each surface during flight and after landing when pressure is applied on every surface. The Max current shown on the champion is only 8.4 to 9.6 amps which is way to low for this big of an airplane.

    Thank you

  • I did connect a brand new MKS HBL380 into both airplanes using a different port in the champion. Same issue. When you hold the servo arm with force, the servo simply wont move to the desired position unless you release the tension/pressure from your hand. I also put 2 brand new Pulse 5000mAh 2S 15C/30C batteries today just to see and nothing changed at all. It looks the amount of current flowing into the servos stops at a certain point and that's why they're not fully deflecting when pressure is applied. They do move to the full position when you release the pressure .I really can't hear that much whining/buzzing in the servos when force is applied. The servos should be whining like crazy when force is applied but they're not. I'm uploading a video of this exact symptom right now. This is demonstrated on the ailerons but occurs on every single control surface when you do the same thing.

  • ...sounds and looks may like a servo torque inrush current limit issue. The sense of these kind of limiter is to protect the servo.

    A high peak inrush current is always produced at the start of the servo motor/travel.

    Are these servos programmable?

    I´m using very much the Hitec D-servos line. The Hitec programming tool for these servos allows me to setup different protection (current) levels.

    I agree with mhs try to duplicate this problem with a separate standalone servo tester.

    May you could change the Champion SRS output voltage to the lowest level and run the test again.

    See if you get a different result or not.



  • I don't have a servo tester at the moment and I don't even know where to buy one at. Also, none of these servos have stripped gears. They are all fairly new servos. The only thing I noticed is when I plugged in a brand new HBL380 into each champion SRS, the same issue was occuring when I applied resistance to the servo arm. That's with a brand new servo so I doubt that the servos are the problem in this case. I checked with MKS and their servos are not programmable.

    I also tested each champion with 6 different Lipo batteries and no matter which one I used I got the same issue regardless of what battery was being used whether it was a brand new lipo or an older one (5-6 months old). I also doubt that the batteries are the problem in this case.

    I also checked the flight recorder data after performing these tests. Battery 1 showed a max current of 9 amps and battery 2 showed a max current of 9.4 amps. The Champion is able to handle 4x20A which is nowhere near these levels that both of mine are at.

    It sounds like there might be something with the Champion itself not delivering enough current to the Servos. Everything was fine until yesterday so I that's what I'm thinking at the moment. I can try to reduce to voltage to 5,9 from 7,4 and see if anything happens.

    Richard, what do you think about this problem?

  • Here is another video I filmed today to show how little pressure is needed to stall the elevator and rudder servos. The first link shows the elevators only being tested and the second links shows both the elevators and the rudder being tested at the end. Absolutely not safe to fly with this going on.…TjoP93Mc/view?usp=sharing…Qw6n1zBq/view?usp=sharing

    I also attached an image of the flight recorder data after these tests were done to show the current values.

  • Hello,

    There are two things:

    - It sounds like the servos are not matched and fighting against theirselve. Even a 60kg/cm servos can be turned down to 0kg/cm

    - You always test it with zero or full deflection - if your travel is to large the servos maybe already in the mecahnical limit which looks like I saw it in your videos

    - Are you using bus adapters or are the servos connected directly to the PowerBox

    - You can see in the Powerbox screen what the output voltage does at the servo

    - What wires did you use?

  • Hello Richard,

    The servos are matched very close to each other. The buzzing is coming from the weight of the control surface pushing on the servos at the center points. I've checked this a few times. For example, the elevators have only one servo per half. So no fighting there because there's only one servo. The issue is happening everywhere regardless of how many servos there are per surface. The ailerons have 3 Servos for a total of 123kg of torque but look at how weak the are.

    I don't think that travel is an issue because the problem never happens at both extremes or the center when the control stick is stationary. You can apply resistance to it when it's at both extremes and it won't move and you can hear the servos begin to make noise as resistance is applied. It begins to happen when you move the control stick, the servo begins to move, and apply pressure that's when the servo is very weak and stalls and no noise of current can be heard with this.

    No PowerBus is used in this airplane. The servos are connected directly into the PowerBox using Powerbox wire that I cut myself to the appropriate length. I used the Three4One harnesses on the wings and the One4One harnesses on the elevators. The rudder servos are plugged directly into the PowerBox.

    I took a 6 inch PowerBox wire and plugged it into an empty port. I check the voltage with a volt meter and it showed 7.32 volts no matter where I plugged it in.

    The problem here is that everything was working fine until something happened over the weekend and caused the servos to become very weak. I was flying this airplane a lot over the summer and nothing was happening with the servos until a few days ago. If you listen closely, the servos aren't buzzing/whining when you apply pressure to them. They simply stall and move to the desired position once you take you finger off the control surface.

  • Hello,

    I have no idea what this is- if the voltage is stable at the powerbox (which is the intention of the regulators) I have no other idea what causes the servos to be so weak. The only thing you can do now is to exclude step by step each part.

    Use a receiver, connect the servos directly with a battery. This tests the servo and wiring

  • Richard,

    I already tested a brand new MKS HBL380 Servo plugged into the Champion and I got the same results, a brand new out of box very weak servo and it happens at the exact same scenarios as shown in each video above. So I don't know for sure that it's the servos or wiring. Also it would be usual for all 12 servos on an airplane to fail along with the wire itself. I can talk with Adam at PowerBox Americas and see what he says. And we'll go from there.