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Author Topic: Info request- Swan VFO stabilization  (Read 3520 times)
KC8HXO
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« on: July 14, 2004, 11:10:33 PM »

Please go to Ebay, and spend $25.00 on the latest Swan Compendium. There are several answers to this question, and MANY others related to all of the Swan gear in this book. Great reading, too!
73- Greg, KC8HXO
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WD4DUG
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« Reply #1 on: July 02, 2004, 03:39:30 AM »

The other evening I was on 75m chewing the rag with a group of hams, using my old Swan 350.  One of the guys asked about the VFO stability, and I replied, "Well, I've been warming it up since April, so it'll be stable for Field Day."  Well, this might be an exaggeration, but anyone who owned a Swan knows that it is not by much.

So this fellow told me that a ham had designed a varactor-based stabilizer for the Swan VFO, based on the principles of a PLL, that would actively adjust the Swan VFO to remain on frequency.  It employs a varactor across the tuning cap, an XO, and standard PLL technology to constantly tweak the capacitance in the Colpitts VFO oscillator tank circuit of the Swan, keeping the VFO rock-stable, or so I was told.

I was intrigued, and wanted to know more, but he was unable to provide more details.  I have asked around, and searched the Web, to no avail.  

This idea seems plausable, and if this mod exists, I want to use it.  Does anyone have any more info, or contacts, that they could share regarding this Swan VFO mod?

I have the later-model Swan VFO, the five-band one with the green scale on the dial for 40 and 15m.

TNX and 73,
Mike
WD4DUG
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WB2WIK
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« Reply #2 on: July 02, 2004, 06:53:10 PM »

There's more to it than that, and there's a whole lot more to it when you're trying to phase lock a low frequency and fairly drifty tunable oscillator...it's likely to break lock fairly often and require resetting.  Also, any noise or pulses on the varactor bias will frequency modulate the transmitter -- not a good thing.

Far easier to do what I did with my old Swans, even back in the 60's when they were new:

-Use a separate power supply to let the VFO run 24/7, even with the rest of the rig powered off.  This only costs about $15 or $20, is quite simple, easy to implement, and adds maybe $1 per year to your electric bill.

-Using scientific trial-and-error, replace caps in the VFO with TC ones that will compensate your particular VFO for improved stability.  What I found was that no two were really the same, so there was no universal cap cure...but in my old '350, I ended up replacing the FB and CC caps with a mix of ceramic TC caps that pretty much took all the drift out of it, at least when operated over a normal "house temperature" range.

These options are far less costly and less complex than trying to update the rig to a VCO/PLL combination that's likely to add transmitted spurs and other undesirables unless done very well.

WB2WIK/6
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G3RZP
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« Reply #3 on: July 06, 2004, 07:41:25 AM »

What was probably meant is what is popularly known as a 'Huff and Puff' stabiliser, which PA0KSB did a lot of work on getting on for 30 years ago. See 'Technical Topics' in the latest issue of RSGB's Radio Communications magazine, or older editions of RSGB's 'Technical Topics for the radio amateur'.

73

Peter G3RZP
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KB4EN
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« Reply #4 on: July 31, 2004, 06:34:03 PM »

Had Swan 350 20 plus years ago.. solved most of drift by replacing zenier  voltage rectifier diode..there is reason i remember this..that is (DO NOT LOSE OR FORGET TO INSTALL THE LITTLE MICA INSULATOR) it is cheap, easy to install and makes you feel all warm and fuzzy if it actually helps,,,
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K4DPK
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« Reply #5 on: August 25, 2004, 03:40:41 AM »

Drop me an e-mail or check my web page for the info you need.  Address is available via QRZ.com.

73
K4DPK
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