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some wild thoughts (without benefit of schematic): can you hook up the video probe at the demod grids and while looking there also hook the direct probe used for IF alignment to TP301? If adding the the direct probe (which showed the correct IF response) causes the correct video response at the demods, then maybe you have an open resistor somewhere that is supposed to be de-Qing the peaking coil or some other coil/transformer in the circuit - and the direct probe is doing the de-Qing/detuning instead when you have it attached.
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