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Attention: This modification applies to boards Rev1 and Rev2 only! If you happen to own a QDX Rev3 or newer you don't need to retrofit, since your QDX already offers PTT output.
After many happy QSOs with my QDX Rev1, I decided that a wanted some more power on TX and add my DIY PA for another 10 dB. For doing so, I had to retrofit my QDX with a PTT out signal. One of the most challenging tasks in this process was finding some area on the backplate to mount the PTT connector. There is absolutely no room (on the inside) for a speaker jack, so I went with a tiny 2 pole JST-XH connector, which at the same time holds the small additional PCB. I designed an alternative 3D printed backplate to mount this connector. After producing the PCB and soldering the 3 components (2 resistors and 1 transistors in SMT according to Hans' design), 2 connections to the QDX have to be soldered: GND and PTT-signal on pin 37 of the processor STM32F401RBT6. The latter is not for the fainthearted! Above average soldering tools and some soldering experience are required. If in doubt, you will be better off by buying and building a newer version QDX.
The following is a quick rundown on the retrofit, please check the attached pictures while reading through it:
1.) 3D print a new backplate with the opening for the JST-XH connector (picture). I attached some self adhesive copper foil to the inside, but this is not a must. You can download the 3D file here:
2.) Glue the JST-XH connector onto the new backplate, pins pointing north.
3.) Produce the PCB. Dimensions are 12 mm x 7 mm, single layer. I used the toner transfer method (picture), but you could use any other method as well.
4.) Solder the two resistors (470 ohm and 10 kohm) and the transistor (BSS123) to the PCB (figure schematics) and solder the PCB (copper side up) to the JST-XH connector.
5.) Put a thin, isolated wire through the hole in the QDX which is between the letters "R", "P" and "b" of the "QRP Labs" logo (picture).
6.) Solder the wire from point 5 above to pin 37 of the microprocessor (picture). The attached picture will help you to identify the correct pin, but be aware that this is a very error prone process. After soldering, I glued the wire to the PCB with a drop of epoxy resin. Disclaimer: There is a very high risk of killing your processors if done wrongly and I will take no responsibility ;)!
7.) Solder another wire to GND (I used the grounded leg of the "extra DC supply capacitor")
8.) Connect the wires from points 5 and 7 to the PTT PCB
Note: The above deals with the active GND version of a PTT as required by many amplifiers. In case your amplifier (like the QRP Labs 50 W PA) needs active high, an almost identical retrofit can be applied, with minor modifications: Pin 38 instead of 37, VCC in addition to GND connection and a new PCB layout as well as a different MOSFET. Consult Hans' manual for the Rev3 version.
Have fun and happy soldering,
Chris, OE1CGS
<Solder Pin 37.jpg>
<QDX top side.jpg>
<QDX PTT schematic.JPG>
<PCB Layout.png>
<PCB and connections.jpg>
<New backside.jpg>
