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use hemostats or aligator clips close to the body of the germainium diode (same for germainium transistors) when ever soldering, esp with solder guns.
leave the clip on until the residual heat is gone.
Modern DMM type Cap testers check ESR and Value fine, but they dont check for leakage at rated voltage, which is the most common failure on old paper caps.
Hum could be a heater cathode leak in the audio. If you have a scope you could look at the audio from the detector and trace.
Look at the b+ for ripple with the scope. Is it hum or is it buzz?
I had a tough dog hum in a radio once, turned out the radio had the power tranny and the audio out tranny right next to each other another model of the same set did the same thing. It would hum from the instant it as turned on, even with tubes pulled, magnetic field from the power was coupling to the audio. I could min by putting a heavy steel pipe around either tranny shielding it.
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