[Mondial]

Perhaps a better suggestion would have been to use a signal generator to inject a known level signal into the RF stage and mixer grids through a small coupling cap while leaving the original grid cap wire in place. I just noticed that the mixer grid is RC coupled, with a 2 Meg resistor to ground. This would make it a very high impedance circuit which would be susceptible to hum pickup, especially from a long wire antenna.

Does the antenna resonant circuit fail to tune on all bands or just AM broadcast? I could see it possibly tuning somewhat broadly, but the trimmer having no effect at all is a bit suspect.