[EdHolland]

Well, the plan worked.

I unsoldered the wires to the primary of the third 455kHz IF transformer. A small toroid was made up as a 1:1 rf isolating transformer so that I could feed 455kHz from my generator, and apply HT to the IF primary without damaging the generator. This way guaranteed the test focused on the specific IFT. HT for the 3rd IF tube was also disconnected.

With just the generator connected, the modulated 455kHz signal came through clear and undistorted. Connecting 95V from the IF tube HT rail immediately brought on the familiar crashes and crackles which plague AM reception.

With that test concluded, it seems we "have our man", and I'm now about to descend into the dark underworld of IF can repairs...