Philco Mystery Control Receiver

[ghjkl67]

That sounds like a good idea. Instead of a battery, I can use a regulated DC power supply and increase it gradually. That will tell me where the exact cut off point is.

Would you mind clarifying this part?:

"Use a scope to look for noise pickup in the amplifier. Use an
isolation transformer of course."

Where exactly should the scope leads be placed? Is an isolation transformer necessary for the scope primary or radio primary? I'm guessing it doesn't matter.