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Author Topic: 12mc trap what does it do ?  (Read 604 times)
AC0FA
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Posts: 298




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« on: April 19, 2007, 11:26:53 AM »

I have an adjustable inductor coil L601 in my old.

Swan 700cx That is on the grid of the reciever tube V6. The manual calls it a 12mc trap.

It is near the big TX/RX relay with the 3 terminals. The adjustment procedure is to turn the dial to 14mc and inject 11.9 MC carier into the antenna jack and adjust for lowest ammount of noise.

I need to know what it does. I am checking adjustments for the 20 meter band. I have 500w on all bands but 20. Output dropps off sharply below 14.300 I thought this might have something to do with it.  

     
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WB2WIK
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« Reply #1 on: April 19, 2007, 02:18:41 PM »

Nope, that trap would have nothing to do with the transmitter at all.

It's between the antenna relay and the grid of the receiver RF amplifier stage and is there to reduce mixing products (images) that can be created by any 12 MHz (or so) signals coming from the antenna; the receiver is susceptible to this problem due to its heterodyne mixing design.

It wouldn't matter if that trap is even in the rig or not; you can bypass it with a piece of wire if it makes you feel better, but there's no way this trap would influence the transmitter.

WB2WIK/6
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AC0FA
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« Reply #2 on: April 20, 2007, 07:10:59 PM »

Thanks Steve;
One less thing I need to worry about.

I have looked real close at the 20m driver coil and I dont see any obvious damage.

It has been suggested to me my elmer.
If I were to add 20% additional winding to the 20m driver coil would it lower the power peak on 20m? I took his word for it that kind of math makes my head hurt.

Will the additional 28 ga wire need to be soldered to the wire already there?
Would it need to be super glued to the form some how?
Would the rig then need an alignment?

Should I just leave it alone and use my 350 for 20m?

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W5HTW
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« Reply #3 on: April 21, 2007, 11:02:49 AM »

A simple alignment will probably solve that problem.  Sounds as though the radio was 'peaked' for the upper range of 20.  But that sort of depends upon how much power drop off you see.  If it is truly substantial, like from 500 watts to 20 watts, or something equally as radically noticeable, it may be a misaligned hetrodyne oscillator (again, it is still alignment.)  I would not suggest modifying the radio unless, of course, there have been other mods and you want to remove them.  

Do an alignment.  The manual should tell you where the VFO should be for proper alignment procedures.  This will likely solve your problem.  Pay particular attention to oscillator alignment.  

If the radio drops off only slightly, from 500 to 450 watts, still the alignment may help, but really that is not a significant problem.  In that case, I'd leave it alone.

Ed
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WA1RNE
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« Reply #4 on: April 21, 2007, 12:23:26 PM »


 The manual is actually a little more specific and instructs you to inject a strong 11.9 mc signal at the antenna input - and tune the receiver around 14.225 for a signal, then tune L601 for minimum output.( on the S meter.)

 I would leave the trap installed in the radio as Swan probably saw some images on 20 meters - around 14.225 - produced by strong shortwave broadcast signals. If you don't have a signal generator don't adjust it, just leave it as is.


 As others have said, this has nothing to do with the transmitter. The transmit mixer and/or driver output stages for 20 meters are probably out of alignment.

 
...WA1RNE
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