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Author Topic: The Role of the Crystal Filter in a Cobra 138  (Read 7981 times)
KD6LZV
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Posts: 19




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« on: May 25, 2010, 03:51:34 AM »

I have changed the IF crystal in a Cobra 138 from 7.8 to 8.8 MHz as a way to get 23 new channels in the 10 M band. The radio receives in the 10 M now. The radio does not transmit.

There is a component called a crystal filter which is labeled as 7.8 MHz. Clearly, the crystal filter frequency is matched to the old IF. Is there no an incompatibility between my new IF crystal and the old crystal filter ? If so, what performance changes would I expect as a result? Would this fit the receive but no transmit scenario?
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WA3SKN
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« Reply #1 on: May 25, 2010, 04:18:52 AM »

I am not familiar with the Cobra, however... the role of the synthisizer crystal adjusts the frequency (local oscillator, might be used both transmit and receive) while the crystal filter (4-8 crystals in a ladder lattice) is for receiver selectivity.  Are you sure you changed the right ones?

-Mike.
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TANAKASAN
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Posts: 933




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« Reply #2 on: May 25, 2010, 04:20:21 AM »

I'm guessing here as I have never seen a schematic of this rig, I've also never seen an SSB filter working at 7.8 MHz although it would be possible. To generate SSB you use a carrier oscillator which is modulated and a narrow crystal filter, the oscillator and filter are normally matched. To generate upper sideband the carrier oscillator is just below the filter frequency and to generate lower sideband the carrier oscillator is just above the filter frequency. As an example a rig with a 9 MHz filter will use 8998.5 and 9001.5 MHz crystals.

If your rig has a 7.8 MHz filter then your new crystal is way WAY outside the range and you won't transmit a thing.

Tanakasan
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KD6LZV
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Posts: 19




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« Reply #3 on: May 25, 2010, 08:52:44 AM »

Thank you both for your comments.

Assuming that the 7.8 MHz crystal filter is part of both RX and TX circuitry, would you expect the radio to receive but not transmit? I know that the crystal filter enhances the selectivity of the rig but do not understand the details.If I could find a 8.8 MHz crystal to match my new IF frequency and did the right tuning it might work (assuming I have not overlooked yet another component that can't get along with the new IF frequency.)
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WD8T
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« Reply #4 on: May 25, 2010, 10:06:16 AM »

There is a CB to Ham conversion group on Yahoo which may be a great resource for you to complete this project.  Linky below.

http://groups.yahoo.com/group/CB2HAM/
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KD6LZV
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« Reply #5 on: May 25, 2010, 12:11:20 PM »

WD8T

Bulleseye !

73

Kurt KD6LZV
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N4NYY
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Posts: 4816




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« Reply #6 on: May 25, 2010, 02:29:22 PM »

I still do CB rag chew on an old restored rig, but I don't know why any ham would want to use a CB on 10M, as it's restrictive. I go on 10M quite often, and you need a VFO, not channels, plus some filtering. Especially if you are DXing South America. At least those old Uniden 2510 had a VFO. But CBs are utterly useless on 10M, and I am a one who still does local rag chew on them.
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