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OH, come on, its not that obvious. That pad that you see, on the front side of PCB, is not in use. Soldering pad is on the back and there are the connections. Do You see that this pad leads to nowhere?
My fundamental theory is that when you find the fault finally, it will be a rather mundane and obvious one. It will remain a challenge till then...
Does the RF amplifier (part of the circuit) needs lots of input when you increase the output? Lots of load on the input side (of the power amplifier) *can* cause loss of lock (detuned)...
Output is 2sc1971 and it gives 5-8 W. Input is about 0,3 to 1 W max. Driver stage is BFR96 is about 0,3W, so that is minimum of input power for output transistor...
Wow... You really expect your circuit to work with that much power with no shielding? I'll answer that question: "No, it's not going to work." I've looked and looked, but I cannot seem to find your schematic. What does the other side of your board look like? I can't find that, either. What are all those printed inductors for? Are they harmonic fiters? If so, that isn't a good idea because the Q is so low. Better to used wound air core inductors.
You are asking for help, but still haven't provided the needed information. You might have pulling issues, but I believe the majority of your issues are radiated interference from your output.
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