[TenTec] [] Running a Century 21 on external 12V power

[Chris Howard w0ep]

I'm not an electronics whiz.  But to me, if you're getting
the noise in headphones but not in the speaker, it sounds
like the headphone cord is picking it up.

If you have cracked capacitors in the power supply,
I wonder if you've got a 120 Hz spark gap going there.
The headphone cord is acting as an antenna and picking it up.

How to test that theory, turn down the audio on the Century 21.
Use the same headphones on some other piece of electronic
equipment in close proximity to the C-21 and see if you get the noise.