Interchanging Crystals

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steelbeak

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I have a Bearcat III H/U which uses 10.8 IF crystals.

I have a crystal marked "460.425MHz MRU-1" which I interpret as being a 10.7 IF crystal used to receive on a frequency of 460.425MHz.

Can I put this crystal into the Bearcat III and if I can what frequency should I receive in the Bearcat?

My initial assumption was that if I subtract 10.7 from the frequency, then divide by 9 I would get the crystal's real frequency. And then armed with this value I could find out what its UHF frequency in the Bearcat would be (460.525MHz).

Does that sound right? Or does the fact that it's a 10.7 IF crystal change something in this equation that I'm missing?

And to answer the first half of my first question I tried putting the crystal into the Bearcat and, by some arcane magick, is currently picking up traffic off a local repeater. However the repeater (as far as I'm aware) is a high band frequency and not UHF, so I shouldn't be picking it up in the first place.

And I have check at least a dozen times that the crystal is plugged into the UHF module and not the high band module.
 

w8jjr

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Oct 18, 2005
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Rochester Hills MI
Yup

460.425MHz >> 460.525MHz

Use to interchange them all the time.

10.7 IF crystal cut for 155.010 will
resonate at 155.110 in a Bearcat. Close enough for the 155.115 PS Freq.

subtract 0.10 for 10.8 IF crystal
155.010 will resonate at 155.000 in a 10.7 xcvr

I have a Bearcat III H/U which uses 10.8 IF crystals.

I have a crystal marked "460.425MHz MRU-1" which I interpret as being a 10.7 IF crystal used to receive on a frequency of 460.425MHz.

Can I put this crystal into the Bearcat III and if I can what frequency should I receive in the Bearcat?

My initial assumption was that if I subtract 10.7 from the frequency, then divide by 9 I would get the crystal's real frequency. And then armed with this value I could find out what its UHF frequency in the Bearcat would be (460.525MHz).

Does that sound right? Or does the fact that it's a 10.7 IF crystal change something in this equation that I'm missing?

And to answer the first half of my first question I tried putting the crystal into the Bearcat and, by some arcane magick, is currently picking up traffic off a local repeater. However the repeater (as far as I'm aware) is a high band frequency and not UHF, so I shouldn't be picking it up in the first place.

And I have check at least a dozen times that the crystal is plugged into the UHF module and not the high band module.
 

gmclam

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I'm still running some crystal controlled scanners. Fortunately I also have a frequency counter. I routinely check the actual frequency my crystals are oscillating at. Over time they all seem to slip. Then plug the frequency into a formula to get the frequency that crystal will make the receiver receive.

My UHF scanners are triple conversion, so the formula is quite different than the dual conversion Bearcats. But you do seem to have the right formula for your situation. Now if you had UHF crystals designed for my triple conversion scanners (they use a 44 MHz IF), it would be much more different than using 10.7 MHz IF crystals in a 10.8 MHz IF scanner.
 
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