[W1ZY]

Thank you, Chas, for thinking about this puzzle. Your response has pointed out to me something I did not consider. Let me tell you a little story, for I am an old man.

I thought about the resonance curve getting skewed by loading the circuit down with a low impedance prior to removing the IF can from the radio. So I swept the can before it--the 3rd IF can--with the radio off. It showed it was parked on 455 kHz and moved +/- by adjusting its 455 kHz trimmer. That told me my set up was OK. https://youtu.be/6YJew2bmjfU?t=206

I then did the same thing to the 4th IF can--the problematic one--and it showed it was parked at 328 kHz and exhibited no response to turning its 455 kHz trimmer, or any other trimmer. https://youtu.be/s78BkxtmIrE?t=79

Now, Chas, the thing you have pointed out to me through your post is the tell-tale fact that when I performed these tests I had to increase the excitation signal into the 4th IF can to get the same scope reading. I think it was 0.5 V for the bad can, versus 10 mV for the good can. That is a big clue, which your thoughts now make me realize. Something in the transformer is attenuating the excitation signal. Since the coil resistances check good, this leaves the other two trimmers. Perhaps one is shorted out...