Hi folks. I was wondering if someone who knows a bit more about this
can help me out. I have an old Realistic PRO-77a scanner which
belonged to my grandfather, which was passed along to my dad when he
died, who then used it for years until the city here switched to an
800mhz trunk system. His brother bought him a new scanner, so he put
the Realistic in storage for a while, until he finally gave it to me
and said to sell it or whatever if I wanted since it wouldn't do him
any good anymore, and it was just piled up in the way now, etc.

In the meantime, I found that some of the city's public works channels
are still separate from the trunk system and in the 150-160mhz range.
My dad used to work for public works, so he likes to listen in when
the weather is bad and such. But the scanner he uses isn't like some
of the newer ones, and can't track single frequencies and a trunk at
the same time, so he has to manually switch in and out of trunk mode
and only listen to one at a time.

Well, I'm not only a bit of a pack rat, but I guess the sentimental
side of me couldn't part with it. So the Realistic is still laying
here. And as I've taught myself more about electronics/radio/etc,
I've started wondering about how I might make this thing usable for
something again. I mean sure, I could order crystals at all the
different frequencies, assuming I could find them all. But it seems
like a good learning experience for me to do it a different way. Not
to mention, surprise my dad with the results!

What I had in mind was to build some kind of variable oscillator to
connect to one of the crystal sockets. The PRO-77a is capable of
30-50mhz low and 148-174mhz high. So the high range is fine for the
public works frequencies I want to receive. How to build such a thing
is the question, though.

I doubt I could build a totally external oscillator and feed it in
there, since the scanner would have components to make the crystal
start oscillating and interfere with that. Right?

I also doubt I could use anything crystal-based since they're fairly
fixed in frequency as far as I know, so I assume some kind of LC
oscillator might work? I don't have any service manual or anything so
I have no idea how the internals of the scanner work, so whatever I
built would have to pretty much simulate a crystal.

I don't know any formulas to determine the values of inductors/
capacitors to do anything like that, so I'm hoping someone can point
me in the right direction. Assuming it's possible, of course. I
think I heard about people doing it before though, so it seems like a
fun project to try!