Commercial air line interference

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Ebenge

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I have a couple of older scanners, one is a radio shack pro46 and the other is a uniden BCT-7. While scanning fire band frequencies I pick up commercial air traffic, usually pilots, at a low level but clear enough to understand them. Does anyone have any ideas of the cause or a fix. It is only on those two scanners, my others dont have that problem. One frequency that it does it on is 154.4000, which is also my fire dept frerq, my pager or handheld fire radio (GP300 Motorola) does have this problem, but I know the bandwidth is a lot narrower on the those two.
 

SCANdal

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Hmmm...

Ebenge said:
One frequency that it does it on is 154.4000, which is also my fire dept frerq, my pager or handheld fire radio (GP300 Motorola) does have this problem, but I know the bandwidth is a lot narrower on the those two.
Your pager and portable suffer from this problem as well?
 

fineshot1

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There were two versions of this scanner - The latest was a "Triple Conversion Superheterodyne" and the earlier was a "Double Conversion Superheterodyne" receivers.
You probably have the earlier version and it sounds like an IF freq conflict but I am not able to find the BCT-7 Specs IF frequency to do the math calculation but thats probably what it is so your probably gonna have to live with it unless you want to filter out the aircraft band.
 

DickH

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fineshot1 said:
... You probably have the earlier version and it sounds like an IF freq conflict but I am not able to find the BCT-7 Specs IF frequency to do the math calculation but thats probably what it is so your probably gonna have to live with it unless you want to filter out the aircraft band.

That's the likely answer. 2 or 3 times the I.F. crystal freq. mixed with the 154.4 comes out right in the 108-138? air band. The signal from an overhead aircraft might be strong enough to cause your problem.
Radio Shack has (or had) a tunable filter for FM interference that might work. I don't recall the number, but you may find it on line. It was only around $8 8 or 10 years ago. The only problem is it uses TV type connectors, but it worked OK.
Another solution would be to change the I.F. crystal. If it is now 10.245, change it to 11.155
or vice versa. That will not reduce the interference, but it will no longer hit right on 154.4.
 

N1508J

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IF crystal?

Ebenge said:
I have a couple of older scanners, one is a radio shack pro46 and the other is a uniden BCT-7. While scanning fire band frequencies I pick up commercial air traffic, usually pilots, at a low level but clear enough to understand them. Does anyone have any ideas of the cause or a fix. It is only on those two scanners, my others dont have that problem. One frequency that it does it on is 154.4000, which is also my fire dept frerq, my pager or handheld fire radio (GP300 Motorola) does have this problem, but I know the bandwidth is a lot narrower on the those two.

Pop open your scanner and look for a soldered-in crystal all by its lonesome. It should have it's frequency stamped on the top or on the side. If your rig has a 10.7 or 10.8 MHz crystal for the local oscillator, try changing it to the other frequency. If it has the 10.7, change it to a 10.8...if it has a 10.8, then install a 10.7MHz. I betcha your problem will vanish!:wink:
 

SCANdal

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A bumble on my part

fineshot1 said:
Thats a Band Pass filter for 112-140mhz. Thats the opposite of what he needs to do. The aircraft band spectrum needs to be suppressed.
I stand corrected. A notch, not a band pass, filter is what's needed.
 

DickH

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N1508J said:
Pop open your scanner and look for a soldered-in crystal all by its lonesome. It should have it's frequency stamped on the top or on the side. If your rig has a 10.7 or 10.8 MHz crystal for the local oscillator, try changing it to the other frequency. If it has the 10.7, change it to a 10.8...if it has a 10.8, then install a 10.7MHz. I betcha your problem will vanish!:wink:

The I.F. crystal is NOT the I.F. frequency, commonly 10.7 or 10.8. The I.F. crystal is either .455 above or below the I.F. frequency. You can not change the I.F. frequency.
For 10.7 I.F. the crystal may be 10.245 or 11.155. These can be interchanged.
For 10.8 I.F. the crystal may be 10.345 or 11.255. These can be interchanged.
 

N1508J

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Ur Rit!

DickH said:
The I.F. crystal is NOT the I.F. frequency, commonly 10.7 or 10.8. The I.F. crystal is either .455 above or below the I.F. frequency. You can not change the I.F. frequency.
For 10.7 I.F. the crystal may be 10.245 or 11.155. These can be interchanged.
For 10.8 I.F. the crystal may be 10.345 or 11.255. These can be interchanged.

Ur rit!!!!:D Thanks for the correction! The point is to change either the 10.245 or 10.345 MHz crystal in the attempt to move the apparent image off his recieve frequency by 0.1Mhz.
 

w0fg

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132.8 is a common ATC freq (at least in this area), and the image shows up on 154.400 (and vice versa) on the older double-conversion scanners.
 
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