Hi Ologierd,
I also replaced C33 (1nF -> 150pF) some time ago because I had the same issue. I did not see any oscillations since that.
It seems that many people had the same problem. Most of the dead Q6 might be caused by these oscillations because current was over 1A with no power output.
I wonder if the oscillations are specific to the 20m version (as mine and yours) or if all bands suffer from that issue.
Unfortunately one of the controller pins died during one of these "events". It is one of the keying inputs so the QCX is keying now all the time. At least I found a workaround while waiting for the replacement controller. I simply set initial keying speed to 0 and can use the QCX as receiver
73
Karsten- DL3HRT
