[G6Tanuki]

I acquired this for a pittance at a car-boot-sale today; it's a 2.5-Watt-input, 30-Watt-output linear using a Motorola MRF240A transistor, with RF-sensing for the TX/RX switching, and a grounded-gate preamp using a U310 FET.

The PA side looks to have been inspired to a good extent by Motorola Application Note AN-0791 from 1979.

These little amplifiers were popular 40 years ago as an add-on for the likes of the Yaesu FT290 and Icom IC202, converting your portable into a relatively-respectably-powered mobile station.

This one worked - but then there was a burning smell! There's a RF choke between the output of the transmitter low-pass-filter and ground. I guess this is intended as a static-discharge path. The choke is about 15 turns of fine wire wound on a phenolic/bakelite former - the whole thing being about the size of a modern 1/4-watt resistor. The former had crumbled, the enamel on the winding had failed and it was acting as a dummy-load across the output. Given that my antenna is DC-short-to-ground I was happy to excise this little choke - and the smoke stopped.

Now with 2.5W input from my Yaesu-Vertex VX150 handie-talkie I get 30W output - most of the time! Occasionally it only produces about 10W but a bit of vigorous keying/unkeying the PTT on the Yaesu gets it going again. I'm thinking tarnished TX/.RX changeover-relays....

Circuit showing the offending choke below.... it's also intriguing for the way the output-filter uses two resonant coax-lengths as both capacitors to tune the Pi-filter and 'stubs' to suck-out harmonics.

That's cunning!