What’s this?

Whilst rummaging through drawers in my shack trying to find a power supply for an unused PC, I came across this TX. I have no recollection of making it, but it looks like an OXO QRP CW transmitter. It must date from the 1990s or early 2000s. 

AA7EE

Dave is a real wizard at home construction.  The photo shows his version of GM3OXX's OXO QRP rig. 

It goes to show how neat "one off" constructions can be, even without a PCB. It is my view that PCBs are not needed for one off designs. 

Clearly this is different where many are to be made, such as for a club project. What is always a good idea is a solid ground plane with all RF constructions. Dave is a great fan of me squares. Stuck on a solid piece of PCB material a very neat construction is possible as Dave shows.

See https://aa7ee.wordpress.com/2024/03/22/the-gm3oxx-oxo-transmitter/.

See https://www.qrpme.com/?p=product&id=MES .

OXO QRP TX

This circuit was originated by GM3OXX, hence the name. It has become a classic circuit. It produces about 1W CW. It has been built by many people in many forms and is very easy to replicate. You just need to add a low pass filter for your chosen band. I have used it to 10m.   This schematic was on the G3PTO website many years ago.

OXO on 14MHz

An OXO transmitter on 14MHz

This afternoon I decided to build a 14MHz OXO transmitter. As is often the case, I like to re-use old boxes and project parts, so the FETer box was photographed for posterity, then gutting and used for the OXO transmitter. In this version I did not go for full break-in and instead used a simple changeover switch, but added a push button net function to allow me to net the VXO without the PA being keyed. A small toroid in series with the crystal allows about 15kHz VXO range around the 14.060MHz QRP frequency. I used a 2N3904 for the oscillator and 2N3906 for the keying transistor and a 2N3866 for the PA, but may change this for 4  x 2N3904s in parallel for lower cost. My output is a little low at around 500mW.

It works fine with reverse beacon reports from Iceland and Slovenia and a nice 2-way QRP QSO with IK2RGV who was running 5W.

The OXO really is a classic circuit: you just build it and it works.  Another version is on the QRPkits page.

.....and NOT a TS-990 - something better!

Just to put some perspective on things, this is a small CW transmitter that just about anyone could build for a few pounds only and use it to work stations all over the world with a modest dipole antenna or similar. The OXO transmitter was designed by GM3OXX and is a classic design.

http://www.qsl.net/g3pto/OXO.gif

Power from the OXO is around 800mW - 1.5W depending on the band and on the final transistor and its emitter resistor. I have used this design on HF and had hours of fun with it. Long after a rig like the TS-990 would be gathering dust, this little beauty will carry on giving pleasure with each and every contact.

The OXO QRP transmitter

OXO schematic on the G3PTO website

One of the most simple and popular HF transmitters is the OXO, originally design by GM3OXX. The circuit appeared in the GQRP club's SPRAT magazine about 30 years ago. It is essentially a 2 transistor QRP transmitter (plus another for keying) capable of working as a fundamental crystal controlled or VXO controlled transmitter on an HF band. I used this design as the TX part of my Pipit 800mW transceiver for 15m and later the Tenner transceiver for 10m. On the higher bands there is more chance of a little chirp, but perfectly usable. On the lower HF bands the OXO is capable of over 1W. It is a very easy transmitter to build, is almost guaranteed to work first time, and is great fun to use.

As it is some time since I've built one, I might just knock one up this afternoon and see how I get on, perhaps on 80m or 40m CW.