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Old 02-15-2019, 09:07 PM   #1
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Default 2 servos on one channel with Y cable = smoke

I'm hoping someone can help me out. I have TRX-4 with lockers. I'm using a 5 channel radio and have had everything on separate channels; no problems. I'm finally getting around to setting up the TRX-4 with a Mamba X and need a channel for the Aux wire. As a result, to use the lockers I need to put both locker servos on one channel. So I go grab a Hitec Y cable from my LHS and get started. One of the servos needs to operate in reverse direction, so I figured I could simply reverse the polarity on one leg of the Y cable.

Well this turned out to be a bad idea, as it burnt up both locker servos. I had a couple spares, so tried them individually on the Y cable. Proper polarity leg, no problems. I disconnect the servo from the proper polarity leg and then connect it to the reversed polarity connector, and another smoke show.

What am I missing here? I take it now that the Traxxas sub micro servos are polarity sensitive. Is this correct? Everything with the Y cable appears fine.

Can I use a Y cable to run two servos from one channel? And how can I reverse the direction of one without buring it/them up? The Mamba X BEC is set at 6V, as is the default voltage for the stock XL5HV ESC. I'm at a loss. What's going on here?
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Old 02-15-2019, 09:29 PM   #2
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Default Re: 2 servos on one channel with Y cable = smoke

I dont have any insight on whats going on but a servo signal reverser comes to mind as a possible solution https://hobbyking.com/en_us/turnigyt...E&gclsrc=aw.ds

Though I have no first hand experience with these, I guess its possible they simply swap wires like you did but I have no idea.

Last edited by HumboldtEF; 02-15-2019 at 09:31 PM.
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Old 02-15-2019, 09:35 PM   #3
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Default Re: 2 servos on one channel with Y cable = smoke

I did a quick search for sub micros and have found a few that are reversed. Perhaps one of these will work along with a regular direction servo and a straight up Y cable.

Also in the process I discovered "servo reversers", which apparently have little circuit as part of the device. I was not aware of this little piece of hardware. So I'm beginning to conclude that simply reversing the polarity was definitely a bad idea, although I've yet to understand why.
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Old 02-15-2019, 09:36 PM   #4
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Default Re: 2 servos on one channel with Y cable = smoke

maybe the type of motor in the servo had something to do with it, brushed is no problem but many new servos are brushless and reversing the polarity is no bueno
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Old 02-15-2019, 09:44 PM   #5
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Default Re: 2 servos on one channel with Y cable = smoke

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Originally Posted by HumboldtEF View Post
maybe the type of motor in the servo had something to do with it, brushed is no problem but many new servos are brushless and reversing the polarity is no bueno
That would make sense. I found a little info on the Traxxas 2065, but motor type isn't given.

Specifications
Modulation: Analog
Torque: 6.0V: 32.00 oz-in (2.30 kg-cm)
Speed: 6.0V: 0.20 sec/60
Weight: 0.46 oz (13.0 g)
Dimensions: Length:0.89 in (22.6 mm)
Width:0.50 in (12.6 mm)
Height:0.89 in (22.5 mm)
Motor Type: (add)
Gear Type: Plastic
Rotation/Support: Bushing
Rotational Range: (add)
Pulse Cycle: (add)
Pulse Width: (add)
Connector Type: J
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Old 02-15-2019, 09:48 PM   #6
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Default Re: 2 servos on one channel with Y cable = smoke

I'm planning on doing the same mod very soon. My research has led me to believe that we'll need a Y harness with reverser. https://www.amainhobbies.com/expert-...exra320/p57634
If you try it before me I'd like to hear how it goes.

This harness will probably be sold out by the time I need it.
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Old 02-15-2019, 10:17 PM   #7
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Default Re: 2 servos on one channel with Y cable = smoke

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Originally Posted by blkbart00 View Post
I'm planning on doing the same mod very soon. My research has led me to believe that we'll need a Y harness with reverser. https://www.amainhobbies.com/expert-...exra320/p57634
If you try it before me I'd like to hear how it goes.

This harness will probably be sold out by the time I need it.
Yes, I'm beginning to come to the same conclusion. That one there looks pretty tidy. I've also found reversers by themselves that one would use in conjunction with the Y. https://hobbyking.com/en_us/turnigyt...___store=en_us Not sure which route I'll go. I need to learn more about it. I've had no experience with setups like this and underestimated what it takes.

I will see what my lhs have in stock tomorrow and hopefully have the rig up and going some time this weekend. I'll keep the thread alive with what I end up doing and how it turns out.
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Old 02-16-2019, 06:32 AM   #8
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Default Re: 2 servos on one channel with Y cable = smoke

Reversing polarity on electronics is usually not a good idea. Servos aren't just motors, there is also what's basically a mini ESC inside the case.

I've never seen a TRX4 so this suggestion might be useless.

Is there enough room to flip the horn and use a longer turnbuckle?
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Old 02-16-2019, 07:16 AM   #9
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Default Re: 2 servos on one channel with Y cable = smoke

As was mentioned, the servo doesn't want to see its polarity reversed.
You can get a circuit that will reverse a servo signal, or you can go inside a servo and move four connections to make it work in the opposite direction. Somewhere I have posted about this before but I can't seem to find it now.
Want to do it yourself, without an extra circuit?
This isn't a micro servo, but the concept is the same.
Here are the steps:
Take the case off the servo and flip the two wires on the motor and the two outside wires on the servo position pot.
That's it.
The reversed motor wires on the servo motor make it spin the opposite direction. The swapped wires on the position sensor pot make the pot see the motion going in the direction it expects.

I don't have pictures of this procedure, so I stole pictures from another thread to help explain it



The red wires are feeding the motor. Swap them so that the one that was on the bottom terminal now goes to the top terminal and the opposite for the other red wire.



The pot in this servo is soldered to the circuit board. My suggestion would be to desolder the pot from the board and add wires. The center terminal of the pot goes to the same location it came from but the two outer terminals get swapped.

Hope that all makes sense.
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Old 02-16-2019, 09:20 AM   #10
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Default Re: 2 servos on one channel with Y cable = smoke

Quote:
Originally Posted by Xx Losungen xX View Post
Reversing polarity on electronics is usually not a good idea. Servos aren't just motors, there is also what's basically a mini ESC inside the case.

I've never seen a TRX4 so this suggestion might be useless.

Is there enough room to flip the horn and use a longer turnbuckle?
Yes, I should have known better, but lesson learned. The servos connect to a cable that actuate the lockers. The location of the support for the cable wouldn't allow flipping the horn. I actually did investigate the possibility, but I don't think it would be possible without kinking the shift cable.


Quote:
Originally Posted by heyok View Post
As was mentioned, the servo doesn't want to see its polarity reversed.
You can get a circuit that will reverse a servo signal, or you can go inside a servo and move four connections to make it work in the opposite direction. Somewhere I have posted about this before but I can't seem to find it now.
Want to do it yourself, without an extra circuit?
This isn't a micro servo, but the concept is the same.
Here are the steps:
Take the case off the servo and flip the two wires on the motor and the two outside wires on the servo position pot.
That's it.
The reversed motor wires on the servo motor make it spin the opposite direction. The swapped wires on the position sensor pot make the pot see the motion going in the direction it expects.
I've had these servos open before, and again last night while taking gears out of the burnt servo to replace in a working servo with stripped gears. Being sub micro size, the components are very small inside. I love the idea, but don't think it's worth the effort for me on these servos.

A LHS has a Y harness with reverser for $12, so that's the route I'm going to take today. Really should have looked into all of this in first place, but fortunately it was a rather inexpensive lesson learned. I also greatly appreciate the help and contributions I've received on this thread.
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Old 02-16-2019, 09:25 AM   #11
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Default Re: 2 servos on one channel with Y cable = smoke

I don't blame you at all for wanting to use the external circuit. It's so tight inside those servos!
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Old 02-16-2019, 05:21 PM   #12
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Default Re: 2 servos on one channel with Y cable = smoke

So in the end I went with a small inline servo reverser and a separate Y harness. I already have a ton of wires in there so this minimized the space required with a minimum amount of wire added. Works like a charm now. Often times experience is the best teacher. Thanks again for the input and pointers.
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Old 02-17-2019, 12:13 AM   #13
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Default Re: 2 servos on one channel with Y cable = smoke

I know a lot of people put a Y cable on ch3 so that the Aux cable changes function based on whether the trans is in low or high gear.

Last edited by Inspector86; 02-17-2019 at 12:17 AM.
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