John Sandin wrote:
I removed the lowpass filter and loosened the wiring, and for a while
it seemed to help. The transistor didn't get as hot. But the output
stayed the same (less than 1 watt). But then I noticed, as I've
noticed before, that the voltage measured at the dummy load doesn't
always drop when the key is released. And the transistor stays hot
even after the key activity has stopped.

This seems to me to be a sure sign the amplifier transistor is
oscillating on its own. In this circuit the code key doesn't interrupt
the amplifier - if it's oscillating on its own, it will do so whether
the key is closed or not.

What *frequency* is it oscillating at? Good question. (if I were you
I'd try to get my hands on a 51-ohm or 47-ohm 2-watt resistor and use it
in place of the antenna until you get the oscillation in the amplifier
fixed. Unfortunately, simply replacing the antenna with the resistor
*might* fix the oscillation! The antenna is essentially part of the
circuit..)

I have my suspicion that the fact the leads for L1/L2 (the coils between
the two transistors) and L3/L4 (the coils between the amplifier and the
antenna) are long and close together may be your problem. You really
need wires in RF circuits to be as short as possible. Maybe remove the
connectors from the back of the box, put the coil sockets there, and put
the *connectors* on top? I suspect the RF on the leads for L3 is being
coupled into the leads for L2.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com