[Milsurplus] 75 To LF Converter - Improved

[David Stinson]

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