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PCM Receiver Problem

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Old 03-31-2004, 09:41 PM
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WindsorJim
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Default PCM Receiver Problem

Hope that you guys can help. This one has me pulling my hair out. First of all I am using a Futaba 6XA radio set up for PCM modulation. Trying to put a Futaba 8 channel R138DP PCM receiver in my 1/4 scale Sig Su-31 to help eliminate RFI issues. New 1100 nicad 6V battery. I'm using a MPI Miracle-Y to split the ailerons plugged into channel 2 of course. Here's the deal.. Both ailerons are getting power, but NO movement occurs when the elevator stick is pulled back/forward. I plugged a single servo into slot 2 on the PCM rec and it worked so the PCM rec is good.. I then swapped out the PCM rec for a PPM rec, changed mod on the radio, plugged the Miracle Y and both servos worked normally, so the M-Y works. What am I doing wrong guys? What button didn't I push? HELP!!!! Dieing to fly this bird after fooling around with it all winter. Thanks.
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Old 03-31-2004, 10:03 PM
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mr.rc-cam
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Default RE: PCM Receiver Problem

Here's one more test. Install the Miracle-Y on the PCM Rx. Use a 4-cell (4.8V) pack instead of the 5-cell. Does it work now?
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Old 03-31-2004, 11:02 PM
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Default RE: PCM Receiver Problem

I had a miracle-y on dual elevator servo's on 6 volts, had all kinds of problems, both elevators jumping, crank engine, rev up the higher the rpm the further the elevators would move toward full up & still jumping, radio was futaba 6xas with 138dp reciever & radrc 1650 nimh 6 volt. got rid of the miracle-y & bought a 9cap to program split servo's all ok. I am running the miracle-y in a 40 size plane with an fm reciever on 4.8 volts with no problems.

Gil
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Old 03-31-2004, 11:13 PM
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Default RE: PCM Receiver Problem

Thanks guys.. Lemme give a 4.8v a shot.. for kicks.. Was using a 6v to bump the torque to the rudder and elevators. Really need to push some extra oz's for this bird. Interesting that the M-Y worked with the PPM Rx and the 6v... Weird stuff. I agree.. I need to break down and buy a 9C one of these days. In the meantime there's gotta be an answer to this problem.
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Old 03-31-2004, 11:21 PM
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Default RE: PCM Receiver Problem

Just tried a 4.8v and it works..... What's up with that??? Why does a M-Y work with a 6v and a PPM RX and not with a 6v and a PCM???
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Old 03-31-2004, 11:52 PM
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Default RE: PCM Receiver Problem

The Futaba PCM Rx has an internal V-Reg on the microcontroller (which in turn talks to the servos). One of the side affects of it is that it limits the Servo Pulse amplitude to less than 4V. This causes some grief with some servos and buffers that are operated at vastly higher voltages.
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Old 03-31-2004, 11:58 PM
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Default RE: PCM Receiver Problem

So what your saying is that I can't use this R138DP Rx with a 6v batt?
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Old 04-01-2004, 12:02 AM
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Default RE: PCM Receiver Problem

Why would there be no effect on the other servos (t,r,a)? Why did the single servo that I plugged into ch 2 work? What role does the M-Y play? I'm using Hitec digitals for the elevator servos..
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Old 04-01-2004, 01:25 AM
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Default RE: PCM Receiver Problem

I have not dissected a Magic-Y, so I can only ponder a bit on the issue. As mentioned, I suspect the Magic-Y does not like the high 5-cell voltage in conjunction with the Rx's low voltage servo pulse (a threshold issue). If the Miracle-Y is a passive (unbuffered) cable then read on.

It could also be the Hitec Digitals. There have been many reports of them not working on Y-cables in conjunction with some Rx's. Again, it is a servo pulse threshold issue. For example: http://www.rcuniverse.com/forum/m_20.../tm.htm#202189
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Old 04-02-2004, 01:57 PM
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Default RE: PCM Receiver Problem

Thanks... Lots of electronic mumbo-jumbo to look at.. I think that I'll bang in a 4.8 v and call it good. 61 oz vs. 76 oz of torque trade off...
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Old 04-02-2004, 02:56 PM
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Default RE: PCM Receiver Problem

Why not try a voltage regulator. They are available in 5.1V, 5.6V, 5.7V and 6.0V. Central Hobbies and Radio South carry them. Bet the 5.6V or 5.7V will be the ticket.
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Old 04-04-2004, 09:06 PM
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Default RE: PCM Receiver Problem

So you think by taking the voltage down below 6V I should be good to go????
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Old 04-04-2004, 09:47 PM
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Default RE: PCM Receiver Problem

I just read that MPI had an older version of the M-Y that was not compatible with a 6v.. Gonna give MAXX a call and see what they say as well. Has nayone else heard of this M-Y issue with MPI?
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Old 04-04-2004, 10:23 PM
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Default RE: PCM Receiver Problem

Why are you using a miracle Y on Hitec digitals? Just reverse one of them with an available programmer and you wont need anything special.
The digitals do have issues with some buffers.
I have been using the Hitec digitals 6v and the R138 for a while now.
Just curious????
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