[ARC5] 75 To LF Converter - Improved

[Glen Zook]

Do you have the "value" of the coils available from Tanner?  Or, did you just go through the parts bins?
 
Glen, K9STH


Website:  http://k9sth.com


________________________________
 From: David Stinson <arc5 at ix.netcom.com>
To: milsurplus at mailman.qth.net; ARC-5 List <arc5 at mailman.qth.net> 
Sent: Monday, December 31, 2012 11:08 PM
Subject: [ARC5] 75 To LF Converter - Improved
 
I was tinkering and added another tuned circuit and sorta-loose coupling between them.  This made the images
undetectable on my test receiver.

http://home.netcom.com/~arc5/Imprvconverter.jpg

Here's a crummy photo of the prototyped converter with the second coil just tacked onto the back:  
http://home.netcom.com/~arc5/convtest.jpg

Of course, if the coils were shielded from each other or even installed in opposite directions the image rejection would be even better.  But I didn't plan for the
second coil so it's just "cobbled in."   Will clean it up later.
Could probably lower the 12 pFd coupling cap to 5-7 pFd.
But even a sloppy as this, it works.

The new coils are pre-made except for the 10-turn link
and available at a local surplus outlet (Tanners) in quantity for like 50 cents each.
Put a 100 pFd across the coil and tune with the coil slug.
The NE612 chips are available on Ebay for less than
$2 each shipped.   A full set of the remaining parts including perf board is available at Tanners for 6 or 7 bucks.

Be aware that if you use a crystal above 75 meters, like the 4.2 MC in my test unit, tuning will be inverted- 4000 KC will appear at 200 KC and  3800 KC will be at 400.
Also, if you're using a receiver with a product detector as
the IF, the LSB signals will be detected as USB.
This does not happen with a color burst xtal as shown.

73 Dave AB5S

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