I am currently putting together a Hangar 9 Meridian which I have decided to build as electric. Everything has so far gone well until I get round to the servos. I decided to go for standard size metal gear servos. The first lot I bought were tower pro and none of them centred properly so I bought some others which did exactly the same. It's definitely the servos and not stiff linkages as I have tried them just plugged into the receiver and they have a different centre each time you move them. I have used these servos in lots of models before and not had this issue. What use is a servo that centres in a different position each time you use the controls. Have just ordered some futaba s3003 servos but with ball race conversion. Hopefully these will be better.
My first thought is if you have the same problem with two different sets of servos I would look somwhere else for the culprit such the servo tester if you are using one or your transmitter stick pots may be worn.( I don't think that would affect every channel though) perhaps a weak transmitter battery.
Check your system with another transmitter to isolate the problem.
Thanks for the reply Roger. I use my DX9 transmitter currently for 35 fixed wing and 6 helis so I know its not the tx. I have tried another receiver to rule that out. It is definitely the servos. I can plug a known good servo into the receiver and it works and centres fine. I must have just been unlucky and received two bad batches of servos from different suppliers. You would think they would have failed quality control on this issue though.
BEC is good Rob. Everything is good except the servos.They even have centering issues on my servo tester. Have tried all different setups including plugging these servos into a different model. The troubleshooting confirms it is the servos that are faulty.
I have some MG 995 servos and also found the centering to be poor. I would not fly them in an aerobatic plane but in a big gentle trainer they should be okay.
I have a half circle plastic protractor with about a 1/2" dia notch at the center point. I put a long pointer on the servo, lay the protractor on the table and hold the servo against the edge, rotate the protractor to read 90* using a known good standard. I have a servo driver with digital readout down to 1 micro second.
Using this setup has shown me that different servo brands and type can have HUGE differences. Some turn 30* each way going from 1000-1500-2000 micro seconds (my standards) some turn 45* I have a couple that turn 55*.
Most new servos travel the same amount both directions, I have a few that go maybe 10* more in one direction, like a built in differential.
Most new servos center well, less than 1/2* diff. every time.
A few servos will stop before reaching the center, and some will overtravel and stop on the other side of center.
I must have well over 150 servos, so I had to come up with a way to test and understand them. I like to have matched servos in a plane, especially when using 2 servos on ailerons or flaps or split elevators. 2 servos on 1 surface and you must have matched servos.
Centering issues were very common back in the 70's, many cutting edge aerobatic planes could only use the best servos available. Those may have cost $100.00 each, I was only making $3-$5 per hour through most of the 70's, that made them out of reach. Also the control pots would build up little piles of carbon dust from the wiper scraping at the resistive surface. Giezendanner carbon button pot wipers cured the most of that problem.
The famous Hobie Hawk glider, was put down as wandering through the air, it was the fault of the servos and the rest of the control chain of 1974 when it came out.
I have two of them, they fly fantastic with modern tight centering equipment. They are simply very sensitive to small control inputs.
Our equipment today is so good, that we are startled when it doesn't work right. Man it's great to have these modern radios
Iv seen some vids and post about Tower Pro servos not being very good. This post just adds to their stigma. They're some of the cheapest servos on the market. I personally wouldn't run them in my planes or expect to much from them.
Just looks like a 'crap servo' issue to me.. Only fix would be chuck them in the bin and get different servos, unless the seller will change them under warranty (they should)
Slop in the drive to the feedback pot is a definite possibility but what if anything you can do about it is a different question. Hard to imagine that's it's just a dirty pot given that you have several servos with the same issue.