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This is how to modify a Hitec Flash 4 X RC transmitter.  We will be modifying it to add a 5th channel and unlock some additional features.

You will need:

Hitech Flash 4 X RC Transmitter
some wire (and the obligatory solder and soldering iron)
several switches:
  -1 on/off SPST
  -2 on/off/on SPDT (needs to have a center off position)
drill
screwdriver


Warning:

Opening your transmitter up and modifying it will void your warranty.  If you do not feel comfortable performing this procedure, do not attempt it.  I will not be responsible for any damaged transmitters or ones that fail to function after the mod.

This mod involves resetting the radio, meaning all data will be lost.  Write down anything you do not want to lose prior to starting this.

Credit for this goes to Bernard who outlines the whole procedure in his website http://www.medviking.com/hitec.

Step 1: Radio

The mod we are doing is essentially converting the Flash 4  X into its bigger brother, the Flash 5 X.  The Flash 5 X has the same circuit boards and software as the Flash 4 X, but on the Flash 4 the additional features have been disabled.

I realize that this radio is an older radio and I could probably upgrade to a better one with 2.4 Ghz and all the bells and whistles I could want, but for a 4 channel park flyer, this does a pretty good job.

Start by removing the battery and all 8 screws on your radio.  There are 2 small ones in the handle, and 6 larger ones on the case

Step 2: Case Mod

Once you have opened the radio up, remove the antenna by unscrewing it and unplug the trainer jack cable and battery cable. You can now set the back shell aside, we will not be needing it.

Look to the top of the radio where the trainer switch is. There are 4 holes drilled in the plastic, but on the Flash 4 only the trainer switch is populated. The remaining 3 are still there, but covered up by the aluminum faceplate. Take the drill and drill theses 3 holes out.  I used a small bit and a reamer. The aluminum is fairly thin so go slowly. Make the hole in the plate match the size of the hole in the plastic, but do not go any larger. If you have your switches handy you can test fit them.

Step 3: Switches

Now that all the holes are drilled, begin installing the switches.  Switch one is the trainer switches, #2 is a SPDT, #3 is a SPDT, and #4 is a SPST.  On the actual Flash 5 radio, Switch 1  and 2 are reversed, so #2 is the trainer.  I left it as is because the switch 1 position has "Trainer" written next to the switch.

Step 4: Wiring

This is where it become slightly complicated.  The switches need to be hooked up to the switch boards via several 4.7k ohm resistors.  These function as voltage dividers and pass the correct voltage to the micro controller to use certain features.  Use the schematic and accompanying pictures to wire the switches and resistors. 



Note: Credit for the schematic goes to Bernard at http://www.medviking.com/hitec/

Step 5: Board Mod

Once all the switches have been connected, we will unlock the features on the micro controller.  There are two resistors that need to be removed to unlock channel 5 and open up the additional features.   They are marked on the picture below.  Both are SMT resistors, so it might take a little effort to remove them.  Make sure there are no stray solder bridges

Step 6: Program

Before we close the radio up, we need to enable the additional features.  To do this plug the battery cable back in and short the 4th hidden switch while simultaneously turning the radio on.  The screen will do a test pattern, lighting up every single LCD character while beeping about once per second.  Next hit the up key until you reach a screen that says GLID.  It is in the off position so hit CUT/SAVE once so it changes to on.  This enables the GLID and GLIDACRO modes on the radio.  Next scroll through the available menus until you reach the RST EP screen and hit CUT/SAVE again to clear the radio.  While scrolling through the menus you might notice a lot of other settings.  I don't know what they do, so I suggest not messing with them.  Turn the radio off and back on again.  You should now have access to Dual Rates, Exponential Rates, and the 5th channel.  The GLID and GLIDACRO modes will also be available from the initial setup screen.  If you are using the radio in Mode 2 (throttle on the left) you will need to set that up again.  Also, this will erase your model presets, so set those up before attempting to fly.

Put the two halves back together and tighten all the screws.  Congratulations!  You have successfully converted your Flash 4X to a Flash 5X!
<p>So it appears that you had to change the orientation of your switches at some point. The wiring in the wide shot doesn't match the wiring in the closeups. Are the close ups of the switches the way the are wired in the final version?</p>

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