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Evan,

That tp2 scope trace for 40m looks pretty bad.
It's ok to have harmonics present, but they shouldn't nearly overpower the fundamental like that.?

What does tp13 look like, is there possibly something coming in from the 45mhz IF amp?
Probably not the problem, your tp2 trace looks somewhat regular from cycle to cycle,
so is mostly 7mhz plus harmonics.? But worth looking at.

>? Could this be caused by a 3rd order mixing product?? 3rd harmonic - fundamental would give the second harmonic.? Why only on 40meters?

Mixers are weird, I haven't figured them out well enough to predict what awful thing they might do next.
Could well be something like what you suggest going on, and/or some inadvertent resonance.?

I don't think it's a solution here as we want to keep the rig simple,?
but will point out that the only port properly terminated at 50 ohms is the clk2 port.
The filter at L31,L32 does allow low frequency energy to be terminated by what I think is the 50 ohm output
impedance of that bidi amp, but blocks high frequency stuff above 45mhz which will reflect back into the mixer.
The filter at L1,2,3,4 does the same for stuff above 30mhz.? (Actual cutoff frequencies are considerably higher,
up by a factor of perhaps 1.5x).

So, not as a solution but as a way of understanding the problem I might try the following steps:
1)? Remove L31, terminate that mixer port with 50 ohms to ground.? (There's no signal coming from the IF amp in CW mode.)
2)? Add a 6dB pad (150,33,150 ohms like R101,102,103) between T2 and L4 (perhaps? remove L4,C204 and tack it in there, avoids cutting traces)
3)? Replace the mixer with something else, perhaps an ADE-1.? Could be a resonance inside that mixer?

Jerry, KE7ER