Hey folks, I was wondering if anyone better versed in analog
electronics could help me out with an idea. I've seen various
projects on the internet where people have repurposed old radio
equipment for other uses, like using a CB to receive amateur radio
frequencies. So stuff like that got me wondering. I have an old
crystal-based scanner that belonged to my dad. It has a range of
30-50 and 148-174mhz. While police/fire/rescue have all moved to
digital systems, I know that the city's public works still uses
frequencies in the latter range. I'd like to be able to receive
those, without buying several crystals. So I'm wondering if it's
possible to create some kind of oscillator (LC-based or whatever) to
simulate a crystal for this type of device. It wouldn't be nearly as
accurate/stable of course, but I'm curious if it's possible.

I mean, I'm sure if one went in and started trying to physically
modify it they could find where to directly inject an external
oscillator frequency. But I'd prefer to keep it in its original state
if possible. That would mean I would have to work with whatever
circuitry normally generates the oscillation with the crystal. That's
why for starters I'm mostly asking in a broad sense if it's possible.
I know anything more detailed than that might be hard to offer without
knowing what's actually in the scanner (even I don't have a
schematic), but any insight would be much appreciated!

In article , (FyberOptic) writes:

| Hey folks, I was wondering if anyone better versed in analog
| electronics could help me out with an idea. I've seen various
| projects on the internet where people have repurposed old radio
| equipment for other uses, like using a CB to receive amateur radio
| frequencies. So stuff like that got me wondering. I have an old
| crystal-based scanner that belonged to my dad. It has a range of
| 30-50 and 148-174mhz. While police/fire/rescue have all moved to
| digital systems, I know that the city's public works still uses
| frequencies in the latter range. I'd like to be able to receive
| those, without buying several crystals. So I'm wondering if it's
| possible to create some kind of oscillator (LC-based or whatever) to
| simulate a crystal for this type of device. It wouldn't be nearly as
| accurate/stable of course, but I'm curious if it's possible.

My father did exactly that with a 70's vintage Radio Shack scanner. He
put a knob on the side near the end crystal in the crystal compartment.
I probably still have it somewhere.

Dan Lanciani
ddl@danlan.*com

hey ole man:

been there done that.

Scanners that used crystals are pretty straight forward in todays
standards, no secrets there.
I made a living selling the crystals for those scanners.
Or you could do a PIC stamp based cpu that programmed from a regular
PC could do what you need to be done.
a.k.a. frequency synthesis, and scan the scanner with just the PIC
using the scanner as an IF

To beef it up would need sum front end work or, software to program
out the images.

Transverters do that all the time and is very common place in ham
radio.
















hhey


On Feb 11, 12:35*am, FyberOptic wrote:
Hey folks, I was wondering if anyone better versed in analog
electronics could help me out with an idea. *I've seen various
projects on the internet where people have repurposed old radio
equipment for other uses, like using a CB to receive amateur radio
frequencies. *So stuff like that got me wondering. *I have an old
crystal-based scanner that belonged to my dad. *It has a range of
30-50 and 148-174mhz. *While police/fire/rescue have all moved to
digital systems, I know that the city's public works still uses
frequencies in the latter range. *I'd like to be able to receive
those, without buying several crystals. *So I'm wondering if it's
possible to create some kind of oscillator (LC-based or whatever) to
simulate a crystal for this type of device. *It wouldn't be nearly as
accurate/stable of course, but I'm curious if it's possible.

I mean, I'm sure if one went in and started trying to physically
modify it they could find where to directly inject an external
oscillator frequency. *But I'd prefer to keep it in its original state
if possible. *That would mean I would have to work with whatever
circuitry normally generates the oscillation with the crystal. *That's
why for starters I'm mostly asking in a broad sense if it's possible.
I know anything more detailed than that might be hard to offer without
knowing what's actually in the scanner (even I don't have a
schematic), but any insight would be much appreciated!

| Hey folks, I was wondering if anyone better versed in analog
| electronics could help me out with an idea. I've seen various
| projects on the internet where people have repurposed old radio
| equipment for other uses, like using a CB to receive amateur radio
| frequencies. So stuff like that got me wondering. I have an old
| crystal-based scanner that belonged to my dad. It has a range of
| 30-50 and 148-174mhz. While police/fire/rescue have all moved to
| digital systems, I know that the city's public works still uses
| frequencies in the latter range. I'd like to be able to receive
| those, without buying several crystals. So I'm wondering if it's
| possible to create some kind of oscillator (LC-based or whatever) to
| simulate a crystal for this type of device. It wouldn't be nearly as
| accurate/stable of course, but I'm curious if it's possible.

Almost certainly, yes. The details would depend on just how the
circuitry is designed.

You would probably want to completely bypass the existing crystal-
oscillator circuit, and inject the appropriate local-oscillator
signal (created by your new tunable oscillator) near the point where
the crystal-oscillator signal is fed to the "mixer".

As to the type of oscillator you could use... you could build
an LC circuit, of course. If you want something a bit more
off-the-shelf, you could buy a commercial VCO (voltage controlled
oscillator) module from a company such as Mini-Circuits... feed
this a very-well-regulated control voltage, through a multi-turn
potentiometer, and feed the VCO's output to the scanner's mixer.

The scanner documentation probably tells you what crystal
frequencies correspond to which over-the-air frequencies
(i.e. what the "offset" is). If it's a single-conversion
receiver, the offset would probably be equal to the operaring
frequency of the radio's IF strip and filters. You would need
to build (or buy) a variable oscillator circuit, which would
generate frequencies covering the correct offset range.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In article
,
FyberOptic wrote:

So I'm wondering if it's
possible to create some kind of oscillator (LC-based or whatever) to
simulate a crystal for this type of device. It wouldn't be nearly as
accurate/stable of course, but I'm curious if it's possible.

FO-

Yes it is possible, as others have stated.

The frequency of the oscillator is related to the Intermediate Frequency
of the radio. One common IF is 10.7 MHz. As an example, a crystal
frequency of 39.3 MHz would receive either 50.0 MHz or 28.6 MHz
depending on how the radio's Radio Frequency stage is tuned.

Scanners commonly use 3rd overtone crystals in the 25 to 50 MHz range.
Higher frequency crystals are possible, but maybe not practical.
Therefore, the radio will usually multiply the oscillator frequency to
receive the higher bands such as 160 MHz or 470 MHz. What this means to
you, is that a Variable Frequency Oscillator (VFO) that covers 25 to 50
MHz (or whatever it takes, you work out the math) will work for both low
and high bands.

To avoid modifying the radio, you may be able to connect the output of
an external VFO to one of the crystal sockets. The original oscillator
would then act as a buffer amplifier. That is if you are lucky. Bad
luck would be if the original oscillator oscillates at some weird
frequency!

Fred
K4DII

First of all, thanks for the responses so far, guys. The info is
certainly helpful. I'm not completely opposed to modifying the
scanner, but it's good to hear that I might not have to. I'm much
more optimistic now on attempting the project either way.



Scanners commonly use 3rd overtone crystals in the 25 to 50 MHz range. *
Higher frequency crystals are possible, but maybe not practical. *
Therefore, the radio will usually multiply the oscillator frequency to
receive the higher bands such as 160 MHz or 470 MHz. *What this means to
you, is that a Variable Frequency Oscillator (VFO) that covers 25 to 50
MHz (or whatever it takes, you work out the math) will work for both low
and high bands.


That may very well be the case. If I recall correctly there are high/
low ranges. The "low" range is 30-50mhz, with the "high" at
148-174mhz. Back when my dad used it we'd always assumed that you
bought a crystal for the particular frequency you wanted to hear, but
that may not have been the case!

It's a Radio Shack PRO-77a, by the way, which I saw someone comment
elsewhere that Radio Shack tends to use 10.7 for the IF on that
brand. A shame I can't find any kind of service manual or something
for it.

FyberOptic wrote:

First of all, thanks for the responses so far, guys. The info is
certainly helpful. I'm not completely opposed to modifying the
scanner, but it's good to hear that I might not have to. I'm much
more optimistic now on attempting the project either way.

While it may be technically interesting, I am amazed that one would
consider embarking on such a project...

At least here, scanners have become mostly useless because so many
services have switched to digital, and uncountable unused "computer
scanners" (= those that already have a frequency synthesizer) are
floating around. A search on a secondhand marketplace site returns
tens of "for sale" scanners sold for maybe $10. Just get one, and
key in the frequency you are interested in.

(remember that solutions with a VFO will mean you lose the scanning
capability so you can monitor only a single frequency at a time, and
unless you add some digital readout you won't even know to what frequency
you are listening)