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Jun 09, 2011, 02:25 AM
Will fly for food
ilektron's Avatar

Turnigy 9X 'resistor fix'

Update - You can also apply the resistor fix the wireless module, see post #11, for a video tutorial, see post #17

I haven't found any good documentation of the resistor fix on rcgroups, so I figure I'd throw one up. I followed the pdf document produced here. I recommend checking out the PDF as well

For a full write up check my blog rcshenanigans

The 'resistor fix' solves the problem of needing to unplug the wireless module in order to use the trainer port.

It involves cutting a trace on a PCB inside the transmitter, and soldering a resistor (usually a 1kOhm resistor) in series by bridging the cut trace.

Why do you need to unplug the wireless module in the first place?

A Little More Theory

The trainer port on the 9X contains a switch (that triggers when a plug is inserted into the trainer jack) that powers off the wireless module to ensure the 9X no longer transmits its signal. Normally, when wireless module is powered on, the radio sends a data signal to the wireless module that is between 0V and +3.3V.

Unfortunately, when the 2.4GHz wireless module loses power, the data signal coming from the radio processor is essentially shorted to ground through a high voltage protection diode on the signal pin of the wireless module. The data signal that goes to the wireless module happens to be the same signal routed to the trainer port. Adding a resistor in series with the signal going to the wireless module breaks the short to ground and allows the full voltage signal to reach the trainer port.

Notice, the short to ground only occurs when the 2.4GHz module loses power. When the wireless module receiver power, the data signal behaves like a proper input and maintains high impedance, probably in the 10kOhm and above range. By selecting a resistor much lower in resistance than the input impedance of the wireless module, say 1k, the resistance added by the fix is negligible and does not affect normal operation of the radio.

The Actual Fix

Items needed
  • Philips Screw Driver
  • Xact-O Knife or sharp blade
  • Resistor with value around 1k Ohm
  • Soldering iron

  1. Start by taking off the back of the 9X by removing the 6 screws on the back
  2. Carefully open radio. Manipulating the sides should allow you to be able to lay both sides flat without disconnecting any cables.

  3. Locate the trace for the data signal.

  4. Using the Xact-O knife, cut a small notch in trace and scrape the solder mask (green enamel) off of the trace far enough apart so the resistor bridges the notch and both leads lay on bare copper.

  5. Solder both sides of the resistor down to either side of the trace where the solder mask has been scraped off.
  6. Carefully close the radio, redoing the screws in the back
  7. Enjoy not having to unplug the wireless module to use your radio with a sim or trainer!
Last edited by ilektron; Jul 08, 2011 at 06:04 PM.
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Jun 09, 2011, 05:13 PM
60 years of RC flying
Daedalus66's Avatar
Thanks very much for taking the trouble to explain in detail. I will now modify my 9x accordingly.
Jun 09, 2011, 06:26 PM
Team PAS
purcilas's Avatar
Question, if I switched the module from the FlySky to a FrSky module, do I still have to do this? From the PDF it doesn't seem like I need to.
Jun 09, 2011, 08:06 PM
Will fly for food
ilektron's Avatar
Originally Posted by purcilas
Question, if I switched the module from the FlySky to a FrSky module, do I still have to do this? From the PDF it doesn't seem like I need to.
You may not need to. It all depends what happens to the PPM signal line when the module is powered off.
Jun 09, 2011, 08:08 PM
Team PAS
purcilas's Avatar
Cool. Ill test it when I get home.
Jun 09, 2011, 08:27 PM
Team PAS
purcilas's Avatar
Now that i think about it, i shouldnt need to since the problem is the flysky module. Since i am not using it anymore it should work as long as the frsky doesnt do the same thing.

I guess i will only be able to use the frsky with the airplanes when doing buddy box oh well
Jun 15, 2011, 09:30 PM
Team PAS
purcilas's Avatar
Confirmed that with the FrSky module you don't need to do this.

However I am going to try to modify the 9x actual module instead of the radio's PCB board for this. Opening the module is easy just not sure if I'll have space to put the resistor in it.
Jun 15, 2011, 09:31 PM
Team PAS
purcilas's Avatar
Oh btw, I am doing this on a another 9x I have. Not the one with the FrSky
Jun 18, 2011, 09:41 AM
Will fly for food
ilektron's Avatar
Not a bad idea to mod the module. Hadn't thought of this
Jun 18, 2011, 10:52 AM
Team PAS
purcilas's Avatar
Yeah i just dont want to damage the pcb board heh. Ill let you know how it goes. I dont habe the resistor yet.
Jun 20, 2011, 05:55 PM
Team PAS
purcilas's Avatar
So I did the modification to the spare module I had from when I replaced it for FrSky on one of my 9x. I tested it and it seems to work.

The module still works and when I plugin the trainer cable without taking the module out I see movement on the other transmitter.

There's no need to test each function right? If movement works then it will work for everything as this is the trace that carries the PPM signal?

I basically broke the PCB trace on the module and put the 1k ohm resistor in between.
Jun 22, 2011, 12:43 AM
Will fly for food
ilektron's Avatar
Looks good! Much easier than taking apart the radio itself.
Jun 22, 2011, 03:11 AM
Registered User
Nice work gentlemen

ilektron - I suggest you put a link to post 11 in the first post so folks can see that there is another way to achieve the same goal. I like the idea of fixing the module rather than changing the Tx in a way which may or may not be OK for some other module in the future.
Jun 22, 2011, 08:18 AM
Team PAS
purcilas's Avatar
Yeah it is actually much easier to fix the module in my own opinion

Just have to make sure you completely remove the trace. When I removed the trace I powered on the radio and tried to use it to make sure it was completely gone. Obviously it did not work, then I put the resistor and it worked
Jul 08, 2011, 02:06 AM
Will fly for food
ilektron's Avatar
Alright, I did the hack to the wireless module and will be posting a video tutorial shortly. Very easy hack! I highly recommend it!

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