Tom, if I recall it was not fastened to the chassis… at least in the same way. ?If the power transformer is near by the choke, it’s flux can/might? use a bit of the chassis and the choke could pick it up from the flux in the chassis. For that I would try removing steel fastening on one hold down point or both and grounding the core for safety at one point only or separately.? You could Put a little space under the choke as a test?

?

But it is also possible that wires from the on/of switch run too close to the BASE ?switch.

?

If you want more stable scope traces sync to the “line” or whatever it might be called now.

?