Futaba 7C 2.4 not sim compatable beware! - Page 2 - RC Groups
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Dec 30, 2012, 01:44 PM
I measured resistance on the HK USB-dongle and found it to be in the 2-5 Mohm range. This is a sure way to make the S/N ratio way to low for safe noise free operation. Think they just feed the PPM signal direct to the IC without using an input resistor (in the range of 10k+/-) between input-signal and ground.. This is _in my opinion_ a design error from the dongle manufacturer, and Futaba is not to blame.
With this "knowledge" it is easy to understand why it often works just fine.

It is for sure a old thread, but I really hope this posts will help some lost soul in the future :-)
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Dec 30, 2012, 02:20 PM
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Wintr's Avatar
That could explain it - I have the GWS USB cable.
Apr 27, 2013, 02:09 PM
Registered User
I have a very similar problem with a 72mhz Futaba transmitter (T4VF) and a GWS USB cord. It sometimes connected/sometimes didn't, and when it did, the control inputs bounced every which way.

I checked out the board the DIN mount is on (I have a big round Futaba plug, I think it's a DIN-6), and found two potential signal pins, one was a large square wave that looked like PPM signal, and the other was a small (~1V) wave form that moved with the square wave. When I tried nanofarad-range capacitors, it killed the signals, so I put a 7pF cap across what looks like a resistor in series with the ground pin. This flattened the background transmitter noise from all the signals, but didn't decrease what looked like actual signal.

Subjectively, using js_demo (from FlightGear) and FSM, this looked like it reduced the jumpiness slightly, but didn't kill it. Then I noticed that anytime I had something connected to the ground (like one end of my multimeter), the jumpiness on the computer side dropped almost completely. I ended up wrapping a stripped wire around the slightly-exposed base of the DIN plug, and holding it under my fingers. This much grounding seemed to make the computer happy.

But it's not fixed. Things are still _incredibly_ squirrely, and at the bottom end of the ranges, they flip-flop to max. So a little stick back causes a little nose up, but a lot stick back causes full nose-down elevator, and this is the same across the baord on all the channels, it's not some odd aerodynamic effect of a stalled elevator.

Needless to say, this is not very useful. Any ideas?

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