I didn't think the three switched automaticly,  I thought there was a
switch.  I have never had any hum on mine, but it has one strange thing,
and I've seen this in several radios.  If you're running on batteries, and
the temperature gets around 40 dgs or colder, the volume gets distorted
the more you increase it.  As the temperature drops, the volume has to be
turned low and lower to remove the distortion. If you plug the radio in a
wall outlet, the thing works fine.  I've seen this on several of these and
triede batteries, so I've concluded it is a strange problem with the
voltage converters in the radio.
73s
Butch Bussen
wa0vjr