(previously posted to rec.antiques.radio-phono)

My dad recently gave me a National NC-109 bought at a surplus auction.
It works but has a problem. There is a "dead space" at the bottom of
each band. It's totally silent down there, no scratching, no
whistles, nothing. For example, I can't tune regular AM broadcast
below about 650. As the radio warms up the problem becomes worse,
after an hour I can't tune below about 750. The problem is similar in
all the bands, but less severe in the higher ones.

Otherwise the radio works fine, and stations come in where they should
on the dial when I can get them. Thanks to the folks at
rec.antiques.radio-phono I've got a schematic and a manual now, as
well as some pointers to look at the local oscillator. I've not done
anything with tube electronics for decades (since I was a kid), but
I'm looking on this as a project to maybe get back in. Would the
simple, dumb (and expensive) step of replacing all the tubes be at all
useful? Unlike when I was a kid at least I have money now... ;-)

Thanks in advance for any advice about how to go at fixing this
problem,

--arne