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Author Topic: P94 mixer  (Read 674 times)

Paragon

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P94 mixer
« on: July 08, 2018, 09:45:15 PM »

Hi all . Hope someone can help .
I am using a p94 action mixer for the first time.
I have it wired  through a P92 power distribution board with two 540 brushed motors running on two 7.2 volt batteries wired in parallel.
I have it wired in fine and running well , mixing as it should.
The issue I am having is the steering servo seems to be doing a lot of twitching when I engage the throttle.
I have the receiver using its own power supply and Iím using a 2.4ghz Hitec optic set .
Anyone any idea?


Thanks


John
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malcolmfrary

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Re: P94 mixer
« Reply #1 on: July 09, 2018, 09:15:16 AM »

Just how separate is the RX supply?  The P92 has a BEC to supply the RX, and, through the RX, the servo and the input side of the P94.
Servos work by comparing the position that they think they are at with what their incoming instruction tells them it should be, and driving the motor to eliminate the difference.  The reference is derived from the supply voltage of the servo.  If that fluctuates due to changing load, the servo tries to keep pace, often changing the load.  The radio could also have it reference voltage messed up by changing supply voltage.
The P92 and P94, between them, should have a good solid ground line connection, with power only going via the heavy wiring, but that is worth re-checking.
Less likely with modern radios is motor generated interference, but a frequent offender on AM systems.  This usually had the most effect on Channel 1, almost invariably the rudder channel.
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roycv

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Re: P94 mixer
« Reply #2 on: July 09, 2018, 09:24:42 AM »

Hi Malcolm I recently bought some (cheap) 9 grm servos and they all chattered in use on 27 and 40 Mhz.  I wondered if as electric motors do not apparently interfere with 2.4ghz equipment there is not the same noise reduction capacitors on the servo motors that are currently on sale.  There are implications for using replacement servos on old equipment.
Any ideas?
regards Roy
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Colin Bishop

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Re: P94 mixer
« Reply #3 on: July 09, 2018, 10:02:24 AM »

From comments made in the past by Dave Milbourn it may be a signal timing issue. The current R/C sets have a different rate to the older ones and some servos might not be able to accommodate this.
Colin
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Paragon

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Re: P94 mixer
« Reply #4 on: July 09, 2018, 11:55:00 AM »

Thanks for the replies
Malcolm The Rx is using its own power supply as suggested in the instructions for the P94 . No power coming from either the p92 or p94 . Both motors are suppressed but I donít think this should matter running 2.4ghz system . The twitching hasnít got great travel just quite quick and erratic but does only happen when the throttle is engaged.
Roy I am running 2.4ghz system the motors should not be causing the issue . As I have gather from various other threads .
Colin do you suggest trying a different possibly an older servo ?


Thanks
John
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Colin Bishop

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Re: P94 mixer
« Reply #5 on: July 09, 2018, 12:03:26 PM »

If you have a servo which is contemporary with the R/C gear then it's certainly worth checking. These problems are usually a process of elimination to resolve.

Colin
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john44

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Re: P94 mixer
« Reply #6 on: July 10, 2018, 09:12:51 AM »

Hi John, have you tried plugging in a spare servo to see if that glitches?


John
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malcolmfrary

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Re: P94 mixer
« Reply #7 on: July 10, 2018, 09:59:04 AM »

Hi John, have you tried plugging in a spare servo to see if that glitches?


John
A different servo is well worth trying.  A friend recently bought a pair of servos which exhibited twitching, they also did it when plugged into a servo tester.  He filmed it on his phone, went back to the shop and got a refund.  His was the steering servo on a yacht, so not really any motor issues.  Servo testers are cheap enough that everybody should have one, they do eliminate a lot of guesswork.


If the radio is being powered by its own battery, and the BEC on the P92 is not being used, it it powering not just the radio, but the servo and the input side of the P94.  If the battery (and its connections, and the wire) are not 100% OK, a changing load could change the supply voltage into the servo, thus changing the reference voltage, causing it to seek a new position.
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Paragon

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Re: P94 mixer
« Reply #8 on: July 10, 2018, 05:56:22 PM »

Thanks for the replies .
John To be honest I haven't tried another servo , but I will do . I have the running gear out at the moment as I wasn't happy with motor alignment.
Malcolm the Rx is running on its own battery pack and it should be fine as the radio gear wouldn't be a year old yet , saying that I did order a new battery pack at the weekend and I expect it tomorrow. So I will be trying both ideas out went I build her up again .
I will be making sure everything is well charged when I try it again . Hope it is something you both have suggested.


Thanks
John
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steamboat66

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Re: P94 mixer
« Reply #9 on: July 15, 2018, 11:04:43 AM »

do not rely on 2.4GHz radio to ignore motor interference. NO radio is completely immune to it. that said, i'd start with the servo. 
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Paragon

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Re: P94 mixer
« Reply #10 on: July 15, 2018, 12:11:41 PM »

Hi all .
I tried 2 different servos and I'm still getting the same twitching on the steering servo.
But I have noticed that it reduces the more throttle I add , to the extent that at full throttle there is no twitching.
This makes me think that it is interference from the motors at low revs.
I did Use the two motors that came with the model kit as I don't intend on keeping the model  and I'm sure they aren't great quality .
But the fact the twitching stops / lessons the more throttle I give does make me thinks its motors .
Any ideas ?


John
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KitS

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Re: P94 mixer
« Reply #11 on: July 15, 2018, 02:25:32 PM »

How well are the motors suppressed? Going 'full house' with three caps arranged in a 'T' seems to be much more effective than just a single one.
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Regards
Kit

Paragon

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Re: P94 mixer
« Reply #12 on: July 15, 2018, 03:07:48 PM »

They both have 3 capacitors fitted , but the motors are at least 20 years old . They came with the kit , unopened wouldnít think they are top quality.
The kits a Midwest models US Coastguard rescue boat , the wooden kit hasnít been made since the 90s .


John
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KitS

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Re: P94 mixer
« Reply #13 on: July 15, 2018, 03:24:37 PM »

So much for that idea then.  :((
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Regards
Kit

MartinL

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Re: P94 mixer
« Reply #14 on: July 15, 2018, 07:55:06 PM »

I may be clutching at straws but I had a similar problem with a very cheap Chinese mixer, it sent the servos haywire. What eventually cured the problem was to put one of those iron rings on one speed controller lead. I think the electronic wiz kids call them a choke but not sure. Whatever they are called, it worked.
Got to be worth a try.
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Stavros

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Re: P94 mixer
« Reply #15 on: July 15, 2018, 09:30:08 PM »

I personally think the answer to this is simply to Phone Component shop




Dave
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tsenecal

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Re: P94 mixer
« Reply #16 on: July 15, 2018, 09:38:56 PM »

something like this keeps my submarine's servos happy.

wrap the servo wire from the servo (or ESC) to the receiver a couple times through the ring

https://www.ebay.com/itm/12pcs-Ferrite-Ring-Toroid-Core-330nH-T-7mm-ID-13mm-OD-5mm-H/131844529483?hash=item1eb28bdd4b%3Ag%3AEWIAAOSwbYZXW0wy&_sop=15&_sacat=0&_nkw=12mm+OD+ferrite+ring&_from=R40&rt=nc&LH_TitleDesc=0%7C0
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Bob K

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Re: P94 mixer
« Reply #17 on: July 15, 2018, 10:32:33 PM »

I reckon Stavros has stated the obvious route to a solution.

Also, it is always good practice to keep the DC wiring loomed separately and away from the sensitive control wiring.  tie-wrap them all together may look neat, but . . .
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malcolmfrary

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Re: P94 mixer
« Reply #18 on: July 16, 2018, 09:34:40 AM »

The more information, the nearer the answer. 
Motor generated noise, if not suppressed at the motor will be reflected all the way back to the power source through the ESC, and radiated from the wiring.  While the radio might not pick this up and try to use it as a signal, the servo lead could be affected and cause the servo to respond. especially in a "tidy" layout with the servo lead strapped to the power wires.
Another one of those cases where judicious use of a servo tester would cough up useful clues.
A couple of turns of the servo lead through a ferrite ring to form a toroidal choke as near as possible to the servo might help.  Such windings work best if they are firmly taped down.  Feeding a length of sticky tape through the hole with the wire can be tricky - better to take the tape round the "rim".
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