OK, I'm back and spent most of the day on the 9x, flashing, finishing the mods, reading the manual, programming a couple of models I previously had on my Graupner radio, and... flying
So, let's start from the beginning. As I wanted to use an FrSky module with telemetry, I had to do the popular mod to free the UART pins of the AVR. I started there as there is some fine soldering involved, and it's much more convenient to tackle that before the bunch of flashing wires is in the way.
Next up is the actual telemetry wiring. My goal was a cosmetically clean solution with a removeable module, and of course as simple as possible. After reading up what others had done and seeing in this
document that there were 2 unused module pins, I decided to use them instead of drilling the case to route wires to the FrSky module data pins from outside like some do, ending with a 2nd connector and an ugly case. I didn't keep much more from that file than this and the use of the servo connector between the mainboard and module board to still allow separating the 2 halves conveniently after the mod, because I wanted to skip the RS232 level converter for simplicity.
So here is the back PCB, with the RX/TX wires connected without level converter. Note the PPM resistor mod
done as well while I was there.
One of the unused pins was intended for the MHz antenna connection, so nothing was required and I left the large pad connected just in case I needed it. The second, while not connected in the FrSky module, was grounded in the TX and its stock module. It was needed to cut the traces, 1 on the "solder" side and 2 on the other side, which the black paint doesn't make so easy.
I also removed the charging diode and replaced it by a simple bridge to allow connecting a Li-Po charger to the original charge jack without it being confused by the voltage drop.
Next comes the matching mod on the FrSky module. The PDF linked earlier connects the 2 unused module pins to the rear RS232 connector pins, but that's for use with the RS232 converter. As I omitted that one, I had to find the TTL level tracks like Thomas did on my different module version, and cut the processor-side line from the converter. Here they are:
The cut track is the one where the blue wire is soldered, just right of the solder point.
The whole module with the connector:
Next up was installing a backlight, I got myself the white version from HK. That's very simple, just remove the mainboard, add the backlight on top of the LCD, stick it with some tape as it just won't hold in place without, and put the mainboard back in place. I didn't wire software backlight control, as we'll see later the power draw of that radio is so ridiculously low there's just no point in bothering. Full-time it is. I didn't bother removing the wire harness and soldering the backlight wires to the board, that way it still stays easily removeable and saves me some time
Then, the actual flashing wires. I used a 2x3pin header that matches my AVRISP mkII programmer. That was of course before I noticed I had lost it.
Final view of the inside:
To power the radio, I had an HK transmitter-type 2500mAh 3s Li-Po. It just tightly fits in the battery compartment, but as Thomas said the 3rd cell serves no purpose apart from heating up the case and making the TX heavier. So I simply dismantled the pack, removed a cell, and repacked, removing the unused wires while I was there.
Final ready to use FrSky TX:
The flashing connector is accessible from the battery compartment, as is the balancing cable to use once in a while: