I've been cracking at my 400C for a while trying to get rid of the hum, and it always creeps back in somewhere, somehow. I followed an earlier post about rerouting and cleaning things up, lifting the heaters and isolating them from the chassis. There is always a hum that creeps through and I think I finally found where its coming from. However, I'm dreading what I suspect it is. Connecting my scope from chassis to the heater, there is 280v AC floating between the two. Tried putting a 10uF capacitor between the heaters and the chassis, verified through the multimeter that there was no current going through it, and grounded it. Hum got quieter, but it's still really annoyingly there. Removed the heater lift circuit since the heaters had no DC return path to ground ground from the cathode (which should overcome H-K differential) and its still there, just ever so faintly quieter but still ever present. Am I seeing a failing power transformer?