Dusting off the crystal scanner

[KE0SBX]

Not sure if this is the right place to put this but I want to share this in case you're in a situation like me.

Do you have an old crystal scanner that doesn't pick up anything anymore? Wanna give this radio a new purpose?

This isn't a new concept but it was to me originally when I learned about and experimented with it last year. For years I wondered, "what is the best way to repurpose an old crystal scanner?" The answer - down- or upconversion!

All it takes is a single low-voltage diode, germanium or Schottky. I used a single 1N5711 diode with two RF inputs, one for an RF antenna and one for a local oscillator. It works like this.

Let's say I have a crystal scanner with a frequency at 156.210MHz. I want to receive the local NOAA Weather Radio service at 162.425MHz. I COULD have a custom-made crystal but those are kinda hard to find nowadays...

Why not "convert" 162MHz down to 156MHz? We can actually, and it's really simple too!

I've attached the schematic/circuit I've used in this post. R1 and R2 are 100 ohm resistors, D1 is a 1N5711, and R3 just represents the scanner's impedance (50 or 75 ohms). After D1 (before R3/OUT), I'd recommend placing a 1-10nF capacitor here before feeding it into the scanner's antenna input. These aren't static values, they just worked for me. So don't be afraid to experiment! The mixer works best when the 5351 is set to the highest drive level (again, experiment with this).

For S1, just use your standard external scanner antenna. For S2, we need a local oscillator. I used an Si5351 + Arduino Nano basic oscillator sketch - works just fine. You could build your own Colpitts or any other RF oscillator too.

If you want the math and theory behind it, check out my blog post on this: Dusting off the crystal scanner.

73 de KE0SBX

 

[RFI-EMI-GUY]

Good scheme for out of band reception.

However could you not also program your Si5351 + Arduino Nano to generate the native LO frequency (at the crystal fundamental) ?

Also you could program the Arduino to store a bank of frequencies and wire the scanner lights to the Arduino I/Os to address a specific frequency to generate. Just 1) Inject the LO after the crystal switching circuit and you may need to slow the scanner dwell time a bit because crystals are the speed of light while an Arduino operates at a more leisurely pace. Maybe you could make a product for old school scanner buffs. I find the old scanners with crystals fascinating.

 

[Omega-TI]

The thing I like the most about my crystal scanner... its full quieting!
On every single synthesized scanner I own, from the cheapest to the most expensive, on analog frequencies, I get a slight hum that does not exist on the crystal radio.

 

[RFI-EMI-GUY]

The thing I like the most about my crystal scanner... its full quieting!
On every single synthesized scanner I own, from the cheapest to the most expensive, on analog frequencies, I get a slight hum that does not exist on the crystal radio.

My Bearcat 210 was my first synthesized scanner. I can still hear that buzz. They have gotten better, but crystals were great. I would love to find a crystal scanner and all the crystals for GMRS,

 

[GlobalNorth]

The sequential lights rolling across the front of many crystal scanners can be mesmerizing.

 

[KE0SBX]

Good scheme for out of band reception.

However could you not also program your Si5351 + Arduino Nano to generate the native LO frequency (at the crystal fundamental) ?

Also you could program the Arduino to store a bank of frequencies and wire the scanner lights to the Arduino I/Os to address a specific frequency to generate. Just 1) Inject the LO after the crystal switching circuit and you may need to slow the scanner dwell time a bit because crystals are the speed of light while an Arduino operates at a more leisurely pace. Maybe you could make a product for old school scanner buffs. I find the old scanners with crystals fascinating.

Me too. There's something about an old crystal scanner that a top-of-the-line synthesized scanner can't even top.

I originally thought about natively using the 5351 but never got around to doing it. Wouldn't be a bad idea though if I ever get a parts scanner!

 

[kc2asb]

Thanks for posting this! I have a number of crystal scanners that I collected over the years. My favorite is the Bearcat III - I used it to monitor a nearby PD until they went trunked. The audio was rich and clear - none of the synthesized scanners have the same audio quality.

Yep, the original Bearcat 210 did have a healthy amount of synthesizer noise. Most of the old Electra units did.

 

[jeepsandradios]

Still have a couple xstal scanners. One is loaded with all the WX channels. So glad I got xstals long ago. Not even sure anyone is making them anymore.

 

[ladn]

The sequential lights rolling across the front of many crystal scanners can be mesmerizing.

Another thing I loved on the BC-101!

 

[KE0SBX]

Thanks for posting this! I have a number of crystal scanners that I collected over the years. My favorite is the Bearcat III - I used it to monitor a nearby PD until they went trunked. The audio was rich and clear - none of the synthesized scanners have the same audio quality.

Yep, the original Bearcat 210 did have a healthy amount of synthesizer noise. Most of the old Electra units did.

No kidding, this is my next scanner to do this to!!! Love the Bearcat IIIs.

My first-ever crystal scanner was actually my dad's. It's a Montgomery Ward GEN856A. They do have a much different and rich sound than the digitally-programmed ones.

 

[kc2asb]

Another thing I loved on the BC-101!

Agreed! The Realistic COMP-100 too. The "rolling 0's" on the Bearcat 210 were not the same. The PRO-2001, Radio Shack's first keyboard programmable scanner, had both the frequency display and an LED for each channel on the front panel.

 

[K9KLC]

My favorite is the Bearcat III

I had III in VHF Lo-HI, in the late 70's. GREAT scanner.

 

[KE0SBX]

Hi all, I'm back again

Good scheme for out of band reception.

However could you not also program your Si5351 + Arduino Nano to generate the native LO frequency (at the crystal fundamental) ?

Also you could program the Arduino to store a bank of frequencies and wire the scanner lights to the Arduino I/Os to address a specific frequency to generate. Just 1) Inject the LO after the crystal switching circuit and you may need to slow the scanner dwell time a bit because crystals are the speed of light while an Arduino operates at a more leisurely pace. Maybe you could make a product for old school scanner buffs. I find the old scanners with crystals fascinating.

I'd like to try my hand at this and see how it goes. First things first, I need to find an injection point from the Si5351 to a point on the scanner. I have attached the schematic of the 856 scanner. I have a feeling my best bet would be the centermost pin of SO1 through SO4 in the schematic, but I could be wrong. What are your thoughts? My goal is to make this completely reversible, so I'd rather not solder the Si5351 and/or the microcontroller anywhere within the scanner if I don't have to.

EDIT: I keep trying to insert the photo but it's very low quality. Here's the PDF of the schematic instead.

 

[RFI-EMI-GUY]

First things first, I need to find an injection point from the Si5351 to a point on the scanner. I have attached the schematic of the 856 scanner.

I think that you are correct that the center pin of (SO1-SO4) would be the injection point of your synthesizer.

I also think that the physical pins of the crystal being placed in the socket operate the UHF Switching (Q7) and VHF Switching (Q8) transistors. This can be verified if the socket has 5 rather than 3 leads soldered into the board. You will have to place a dummy pin into the upper (UHF) or lower (VHF) sockets, or use an I/O from the controller to simulate that function. If you are just using for VHF scanning, then just stick a dummy pin in each of the lower sockets. If you are mixing VHF and UHF, then use the appropriate upper and lower sockets. If you want to expand the channels, use I/Os to simulate U or V logic.

If receiving VHF, the synthesizer would need to be programmed as follows, assuming low side injection. 162.425 MHz - 10.7 = 151.725/3 = 50.5750 MHz (Synthesized "crystal")

If receiving UHF the synthesizer would need to be programmed as follows, assuming low side injection. 462.600 MHz - 10.7 = 451.900/9 = 50.2111 MHz (Synthesized "crystal").

Q17 and Q18 and D20 form an AFC that "pulls" Q6 the local oscillator to correct for frequency drift. You are using Q6 as an amplifier. That AFC will continue to attempt to pull Q6 but will not have any affect on frequency control. It may cause noise or AM modulation of the LO, or may do nothing at all.

You will need the scanning logic to steer your synthesizer frequency. I suggest using I/Os to sense voltage (going low) at the junction of the LED's D5-D8 and their companion switches SW1-SW4. SW1-SW4 can continue to control the channels being scanned.

Having said all this, I should note that the synthesizer will be noisier and slower than the crystals these receivers were designed for. There are a few scanner enthusiasts about who have stockpiled the most common scanner crystals. Prices are still reasonable from these sellers.

 

[RFI-EMI-GUY]

The thing I like the most about my crystal scanner... its full quieting!
On every single synthesized scanner I own, from the cheapest to the most expensive, on analog frequencies, I get a slight hum that does not exist on the crystal radio.

The ever annoying Bearcat 210 buzzzzzzzzzzz... was the worst. I was so glad to get rid of it...

 

[KE0SBX]

I also think that the physical pins of the crystal being placed in the socket operate the UHF Switching (Q7) and VHF Switching (Q8) transistors. This can be verified if the socket has 5 rather than 3 leads soldered into the board.

@RFI-EMI-GUY I took a look at the PCB page in the Sams manual for this scanner and I think you may be right - it does look like those outer pins are indeed 2 separate pins, which activates the scanner's switching logic!

Haven't tried it quite yet (life is stupid busy despite it being summer) but once I get some free time, I'm definitely trying this out.