Hi Richard:

On analog sets, the tuning capacitor is never disconected from the antenna
coil, so the L-C circuit that makes the antenna resonate is always
completed. With a PLL set, a variable capacitance diode (varicap) is used
to dial up the right frequency, but the diode is only "on" when the radio
itself is on, so powering the radio down means you've got a ferrite loop
with no capacitor to tune it.

Hope this helps - Kevin S
Bainbridge Island, WA