On 2/18/2015 2:43 PM, Jon Elson wrote:
micky wrote:

I have several simple table radios, all transistor, made between the
70's and 80's, and 2 or 3 of them hum, even when they are turned off.
The humming is coming out of the speaker. How can that be?

In each case, the on/off switch must be working, because when I turn it
on, a radio program comes out. When I t turn it off, only the hum. In
one case, I put a separate switch on the wire to the speaker, and now
the radio makes no noise when it's off.


A number of these radios will have 60 Hz (mains-powered) transformers
that are always on. The power switch switches low-voltage (either AC
or DC) power to turn the radio on or off. probably there is magnetic
leakage from the transformer that is getting into the speaker or wiring
somehow. If the audio output stage has an audio transformer to drive
the speaker, the power transformer might be coupling magnetically
to the audio output transformer. Possibly, altering the way the
power transformer is mounted might reduce this coupling.

Jon


That's probably the best answer, although in the 70s, I'm not sure how
many radios were always on. That throws in a little wrinkle.
"A number of these radios will have 60 Hz (mains-powered) transformers
that are always on."

Mikek