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PCM-Servo problem

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Old 07-20-2007, 10:11 PM
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vertical grimmace
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Default PCM-Servo problem

I have 2 Hitec Servos in my wing and everything worked fine with a Hitec PPM reciever. I just purchased a Futaba PCM reciever and now 1 of my aileron servos will not work with this reciever.
When I switch it back to FM it works fine again. This single servo will not function in any channel port of this new reciever. Other servos I try work fine but this one servo will be difficult to replace. Any thoughts? I am sorry but I cannot remember if it is digital or not. I will check. If so does the servo need to be made to be compatible to PCM? Also the servo is getting power as it wants to stay where it is. When you put resistance on it by hand it buzzez and does not move, but it does not respond to stick inputs.
The TX is a Futaba 9C. Maybe there is a programming issue with the TX?
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Old 07-20-2007, 10:28 PM
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Default RE: PCM-Servo problem

Are you using hitec digital servos? If so there is a compatibility problem with some of the Futaba PCM receivers. You can use a amplified Y harness to try and get around the problem.
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Old 07-20-2007, 10:31 PM
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Default RE: PCM-Servo problem

I need to check but I think they are digital. I might have one of thhose amplifiers as well. Do you know of any threads discussing this issue? I have never heard of this before.
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Old 07-21-2007, 04:06 AM
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Default RE: PCM-Servo problem

G'day Mate,
The servos don't care if it is PPM, PCM, or 2.4Ghz, they will still work, IF the TX is programmed properly, & the servos are plugged into the right slots in the RX, to match said programming.
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Old 07-21-2007, 06:07 AM
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Default RE: PCM-Servo problem


ORIGINAL: alan0899

G'day Mate,
The servos don't care if it is PPM, PCM, or 2.4Ghz, they will still work, IF the TX is programmed properly, & the servos are plugged into the right slots in the RX, to match said programming.
Except that my servo works with the FM reciever and not the PCM reciever. Other servos I try do work in the PCM reciever. I wonder if adding the filter-boost in the extention will help?
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Old 07-21-2007, 07:35 AM
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Default RE: PCM-Servo problem

G'day Mate,
Are you sure that the slot you have the servo in question plugged into in the Futaba RX, is the right one for that RX, it may not be the same configuration as the Hitec RX, & of course you do have the TX set for PCM modulation.
Check your Futaba manual.
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Old 07-21-2007, 08:49 AM
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Default RE: PCM-Servo problem


ORIGINAL: vertical grimmace

I need to check but I think they are digital. I might have one of thhose amplifiers as well. Do you know of any threads discussing this issue? I have never heard of this before.
You can do a search for references to this problem.

It surfaced with some of the Hitec digital servos which required a higher sinking current than what the Futaba R149DP rx's could put out. It only occured with multiple servos on one channel, i.e. a Y-harness and was fixed by an update from Hitec.

If they are older digital Hitec servos, you're using a Y-harness or something like it, and you're trying to get an R149DP to play, it isn't going to happen.

IIRC, the fix is to update the servo's to the ver. 1.04 programming which can be done with the Hitec programmer or by sending them to Hitec.

Other than that one problem limited to this particular rx and servo combination, there should be no difference to the servos between PPM and PCM rx's.
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Old 07-21-2007, 09:57 AM
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Default RE: PCM-Servo problem

I do not have it on a y harness but my lead is very long (48"). It is the 149 PCM reciever so even though I do not have it on a y harness I think I could have signal loss without boosting to the servo. I will probably just swap out the servo. It is just interesting that it works fine other than with the PCM reciever.
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Old 07-21-2007, 10:37 AM
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Default RE: PCM-Servo problem

It is something to do with the amount of current the futaba receiver can generate in the signal wire. It isn't enough to run the older hitec digitals. So all receivers are not created equal and some of them will not work and it doesn't have anything to do with the signal source it is other parts of the receiver. Usually this showed up on a y'ed setup, but with the long servo leads could cause the signal amplitude to be reduced. I'm not sure but check out maxxprod.com and look at their long servo leads with the choke. I think this might help, but no guarantee.
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Old 07-21-2007, 01:47 PM
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Default RE: PCM-Servo problem

OK, I solved it. Good thing about these threads is they will no doubt help somebody. After removing the servo from the wing, it was a 5645 or something, I forgot Digital Hitec. Anyway I had a 48" Expert aileron extention with a filter. The filter is not PCM compatible! They do not say this. I could not get that extention to work with anything. I originally had it in hopeing it would help with RF noise from my Gas engine. Well I swapped out the leads with normal extentions and now all is right with the world.
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Old 07-21-2007, 04:44 PM
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Default RE: PCM-Servo problem

Well it's a good thing you found it as the long lead could very well have been the problem. I'm not sure what you mean by a filter, but it no doubt has some additional impedence in the circuit. If it was indeed an older Hitec ver. 1.03 then that plus the extension and filter would contribute to the total resistance in the circuit much like a Y-harness would.

FWIY, I would definitely get a hold of some of the MPI extensions like lrb75 mentioned because a 48 inch extension on top of the existing servo lead is a bunch. They are the only outfit I've found that produces a 20g. wire servo extension and I've gone to them on all my gassers with digital servos. Standard leads are 24g and everyone else's HD leads are still only 22g.

http://www.maxxprod.com/

Lots of people who are converting to the Spektrum stuff are now in the process of upgrading all their power systems as a voltage drop to 3.5 causes the Spektrum rx's to shut down and takes longer than the rx you're using to reaquire the signals. Your rx will also shut down with low voltage but comes back up quicker. I suspect that is the cause for what a lot of folks have written off in the past to "getting hit" or glitching.

Last thought is that if you're not running a 6 volt system, I'd definitely consider it for it's additional servo torque and available voltage before the rx gets shut down due to servo current (amps) draw...[8D]
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Old 07-23-2007, 01:17 PM
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Default RE: PCM-Servo problem

I think this is a seperate issue from the low signal wire voltage issue experienced with the Y harness. I do have the new generation Digital servos, but it did not matter what type of servo ( I tried several including analog) I plugged into that extention cord with the filter on it, none of them worked. Whatever that filter does, it certainly prevents your PCM reciever to work with your servos. These should be avoided. They are produced by Expert. I was of the impression that the Filter suppresed RF noise on the long lead. That is why I chose to use them. Now everything is working fine after swapping out the cord.
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