HS-7950TH Servo with spektrum receiver

[HumboldtEF]

So I've just been having bad luck with electronics this week, First I had the steering channel konk out on my Acsenders receiver after a tiny fall at a comp. I was easy enough to track down and confirm the servo was fine and the receiver was at fault, the steering channel had a short or something.

Now my next surprise this week was when I installed all new electronics (except the motor) onto my freshly built SCX 10.2 kit. I picked a Holmes TorqueMaster BR-XL ESC, a Spektrum SR310 Receiver, and a HiTec HS-7950 TH.
What is happening is that when I steer quickly there is voltage feeding back to the motor and it will throttle up for a short moment every time I quickly steer.

When I swap out the servo with a Holmes HV500 its fine.

I swapped the steering and throttle channels and get the same effect

Swapping out just the receiver (also Spektrum) gives the same results

When I use the original HiTec servo and swap the ESC for one with an external BEC the throttle glitches go away but it has another issue, it gets stuck at like 30 degrees for a solid second before centering again.

Maybe this Hitec servo needs the extra power from the BEC? This servo should function off of 5v or whatever the receiver gives it right? Its specs list 4.8v, 6v and 7.4v.

I found a service bulletin on HiTec's site mentioning that "very long servo extensions" (which I'm not using) can cause issues especially with Spektrum and fail safe receivers
Bulletin found here HS-20110524 HS-7940/7945/7950TH Jumping/Glitching



I've already reached out the HiTec who I hear has great customer support

Another thing happening with the servo is that it buzzes a whole lot when the wheels are off the ground like its really tring to center but its under no load (its centered properly). To stop it making noise wiggling the tires helps very little instead flicking the steering wheel lightly can fix it much easier, I feel like the steering needs a larger dead zone if thats possible to program, or maybe this is part of the servo going bad.

EDIT: I guess the next step should be to test these with a different brand of receiver.

Its a bit ironic that I was going to go with a Holmes servo but was talked into Hitec due to their better reliability and wouldn't you know it I started off with a dud.

Just to cap this off how much easier is it to troubleshoot with a servo tester?

Also I started this thread not knowing what was going on and I've slowly pieced it together as I wrote, so its less of a tell me what I need to do and more of a theoretical whats happened?


EDIT: I just got off work and I tried a different brand receiver (Traxxas TQi from my TRX-4) and everything works flawlessly even when I connect a BEC. I also tried a 3S battery (with all the original parts) and it fixed the issue as well, in the Bulletin listed above that was regarding 2S batteries only.
2ND EDIT: nope 3S didn't actually help once I got out on the rocks.

So I have to assume Spektrum Receivers simply don't get along with HiTec servos on 2S(or this one at least).

I'll just trade my TRX-4 for its servo (Holmes HV500) instead of trying to find a way for them to play nice with each other, although I was planning on running 3S anyway so maybe not.

If anyone else can chime in on the subject I'd appreciate it, I know a tech thread holds a bit more weight when there's comments from more than one person.

[svt923]

What is your BEC voltage set at?

Supposedly, Spektrum receivers are more sensitive to voltage fluctuations that other brands. If you are still at the factory 5.1V, the servo may be causing some voltage drop from the BEC and affecting the receiver. Try upping the BEC voltage to 6.3 volts or so and see if that helps.

If not, you may consider directly powering you servo from the BEC and using the ESC's internal BEC to power the receiver. The Holmes bypass harness makes it easy to hook up but you can make your own if you can install servo connectors and solder a wire.

[Alexander_0_1]

I have a hitec 7950 in my ascender, with the stock radio, which I think is a spektrum radio/receiver. I used a heyok pug in and play BEC. I've never experienced any of the issues you've mentioned.

I'm also using the dynamite 2s hard case lipo that horizon recommended.

You can find heyok in the vendors section.

[Hydrocarbon92]

I'm not a fan of Spektrum, but it seems like you're experiencing brown-outs from excessive draw from the servo. A glitch buster cap may help, but that's a band-aid. The long wire bulletin is likely because Spektrum has a very weak PWM signal (not RF signal), which is weakened with long wires.

When you tried the BEC, did it power both the servo and receiver? I find it best to have the ESC power the receiver and the BEC only power the servo. If the servo pulls too much, it just loses power from voltage drop instead of resetting the receiver.

[HumboldtEF]

Originally Posted by Hydrocarbon92

I'm not a fan of Spektrum, but it seems like you're experiencing brown-outs from excessive draw from the servo. A glitch buster cap may help, but that's a band-aid. The long wire bulletin is likely because Spektrum has a very weak PWM signal (not RF signal), which is weakened with long wires.

When you tried the BEC, did it power both the servo and receiver? I find it best to have the ESC power the receiver and the BEC only power the servo. If the servo pulls too much, it just loses power from voltage drop instead of resetting the receiver.

It was more of a glitch than a brown out I think, and using the BEC (hooked to ESC) eliminated the initial glitch but it would get stuck at 30 degrees for a solid second before centering again.
I also tried 3s which they mention in the bulletin would not be affected, but it is the same with 3S too.

I just ended up swapping that servo into my TRX-4 which is still using the stock Traxxas receiver, and its working great.

[gottorque]

Is the red wire on the ESCs RX plug pulled out? When you run an external BEC you need to disable the internal ESC BEC, otherwise you can over volt the RX. It wont immediately burn out, but will cause the servo to stall out and act funny...similarly to what you've described. I run all Spektrum DSMR RXes in my rigs and have no such issues.


I run my BEC at 7v, which powers my 7950 nicely. If you run 2s, set the BEC to 6v.

[HumboldtEF]

Originally Posted by gottorque

Is the red wire on the ESCs RX plug pulled out? When you run an external BEC you need to disable the internal ESC BEC, otherwise you can over volt the RX. It wont immediately burn out, but will cause the servo to stall out and act funny...similarly to what you've described. I run all Spektrum DSMR RXes in my rigs and have no such issues.


I run my BEC at 7v, which powers my 7950 nicely. If you run 2s, set the BEC to 6v.

Yeah I pulled the red wire when I swapped over.

I ended up using this servo in a different truck (TRX-4) with a traxxas receiver and its working flawlessly.

Its definitely something to do with signal strength being too low when using these 2 brands together.

[Hydrocarbon92]

Signal voltage was one thing I wasn't able to find much data on when I looked. Most radios seem to use a 1.7-3.3v signal and servos are generally 1.7-6v tolerant (signal wire, not middle wire); you'd need an oscilloscope to test signal voltage. If HiTec gets flaky below 3.3v and Spektrum is attempting 1.7v, then this incompatibility would make more sense. Spektrum could also have high-impedance from an abnormally high-val resistor (for protection).

The problem with fixing it is how companies try to enforce a standard by simply not budging from their idea of "good".

[HumboldtEF]

So I revisited this issue last week, I wanted to put a Hitec servo in my Ascender which also runs on a spectrum receiver. I picked up the Spectrum signal booster and it completely fixed the issue.
Its like $10 and fairly small, I needed a short servo extension on the Ascender anyway so this worked out good for me.

[Hydrocarbon92]

Glad that worked, too bad Spektrum can't just run their receivers at a higher TTL voltage.