Servo Centering - Receiver vs Programmer
 

In trying to get lock-to-lock steering on my SCX10 with a Hitec 7950 servo and stock aluminum horn, I bought a HPP-21 programmer. I plugged the servo into the programmer and went through the steps to get 180 degrees of movement and then trimmed down the EPA to work on my truck.
I then plugged the servo back into my Spektrum receiver. As soon as I turn on the DX3E radio, the servo moves to a new ‘center’, several degrees from the center the programmer had, making all my tweaking irrelevant.
In testing, I went back and forth with the servo plugged into the programmer and then into the receiver and found ‘center’ to be at least 10 degrees different. Enough that I can’t trim the radio far enough to reach the same center as the programmer.
So how can the centers be so far off? If each device is sending the 1.5mS signal the centers should be very close, if not exactly the same.

I'm assuming you reduced the EPA's equally via the programmer?

Frankly, I would change the throw of the servo with the programmer, then dial the EPA's back using the TX.

I agree though that "0" should be "0" using either method.

I wound up putting the servo back to stock until I figure out the center issue. I am thinking my next step will be to isolate the transmitter/receiver. I'll plug the servo into another radio setup and see where it thinks center is.

Learning a few things with the help of Google (imagine that)

Not all receivers send a 1500us signal to the servos. It appears that my Spektrum receiver 'center' is probably around 1600-1650. I'm going to play around with it to get a better idea. (Anybody know how to accurately measure the PWM sent by the receiver?)

Also, learned that my DX3E setup is not capable of sending the full range signal of 900 to 2100us. It cuts it a bit short in both directions.

However, I think I know what I am going to do to attain my goal. Use the programmer to set the servo resolution to 'normal', max out the EPAs, and then use the 'steering rate dial' on the DX3e to get my lock-to-lock steering. Next step will be to drop-kick the Spektrum gear and try another vendor (Airtronics or Futaba).

Use an oscilloscope to measure that.

It's cool to know the numbers, but I'm not sure it buys you much. Set the end points on the servo programming to make the horn swing far enough. Adjust the transmitter's settings for center and both end points and go have a good time driving your truck!

For whatever it is worth, I ran home on lunch break and tinkered.

I started with the servo plugged into the receiver. I made a mark on the servo horn and the case to represent the Spektrum 'center'. I then plugged the servo into the programmer. With the programmer I learned that the Spektrum center is somewhere between 1580 and 1600us. I then told the servo that is its new center.

I changed the resolution to Normal, monkeyed with the EPAs, set the deadband to 2, and Overload Protection to 10%.

Plugged the servo back into the truck, dialed in the EPAs on the radio and now have as much steering as my front axle will allow. The truck requires very little steering trim as well. "thumbsup"

I like the way you worked that out!
Sounds perfect now.

+1.....more than one way to fix an issue. I usually go to a longer servo arm/horn to gain travel but that may also create other issues.:roll: