|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.