The transmitter was working fine on 40, but was horribly unstable on 20. In the past, this kind of thing would really drive me nuts, but experience has made me more patient. I know that "taming the beast" is part of the homebrew process.
I knew that layout was part of the problem: I had significantly less room on the board with this rig than I'd had with the BITX17: the additional bandpass filter and low pass filter, and the associated relays, used up a lot of copper clad real estate. So by the time I built the PA chain, the inputs were too close to the outputs.
The fact that the rig was stable on 40 but not on 20 led me to believe that this was not a problem of insufficient decoupling. Instead, I thought that I was getting additional inductive feedback at the higher frequency.
I noticed that the instability disappeared when I put the 1X scope probe on the input to the first amp in the RF chain (Q14). That was an important clue. Looking closely at the circuit, I realized that the base of Q14 had a long lead (several inches) up to the low pass filter. I had experienced problems with this lead on the BITX 17 project and had cured it with a relay at the low pass filter -- this relay took one end of the lead out of the circuit on transmit, preventing it from becoming a little radiator. I used that mod in this rig, and figured that that cured the trouble. Wrong. The other end of that lead was still connected to the input to the RF power chain. It was picking up enough RF to send the PA chain into oscillation.
I put a SECOND relay at the other end of the line. That took it completely out of the circuit. And the instability disappeared. I fired up the rig and worked California on 20. Very satisfying.
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