[Lowfer] XNS & MP & EAR & some Heath HD-1420 mods

[Douglas D. Williams]

Nice captures, Mike.

It's been interesting reading about your continuing improvements to your
Heathkit converter.

Should you ever decide to construct a different converter, there are two
high performance designs that I am aware of.

The first, and most recent, is the AMRAD design, which appeared in the
April, 2002 issue of QST. If you're a member of the ARRL, you can pull up
old articles for free on their web site.

The second design is a modern update of a Doug Demaw/Jay Rusgrove design
that appeared in the June 1977 issue of QST. The 1977 design, which I
constructed back in the late 1980s and is a very good performer, uses a
balanced mixer that you construct with toroid transformers and diodes. The
newer design, "A High-Performance Low Frequency Converter for 1996, by Tim
Brannon, KF5CQ", replaces the hombrew mixer with a Mini-Circuits SBL-3 DBM,
and changes the output frequency from the original 1.7 MHz up to 4 MHz,
which makes for easier frequency dial reading (just ignore the 4). I think
the LWCA web page hosts Tim Brannon's writeup.

Either the AMRAD or the Tim Brannon design, IMO, benefit from using an OCXO
in place of a standard crystal, if you have any interest in receiving QRSS
signals.

I am currently using the AMRAD design, built by Todd Roberts with an OCXO,
and have found it to be an exceptional performer.

73, Doug KB4OER

On Thu, Dec 6, 2012 at 11:27 PM, Michael Sapp <wa3tts at verizon.net> wrote:

> Jay & All: I was testing my xtal heater and other improvements on the
> Heath HD-1420 and XNS paid a visit.  Looks like about .3Hz drift when the
> 125000btu furnance cycles on at  about 5 feet away from the radio desk.  My
> FT-817 IF receiver has the TCXO-9 oscillator upgrade and the PC was running
> NTP for clock reference.
>
> http://i1296.photobucket.com/albums/ag19/wa3tts/XNS126.jpg
>
>  Also some EAR shots Thur afternoon without the HVAC modulator in
> operation.  Likely about as good as I can do without locking everything to
> a LPRO rubidium osc collecting dust on the shelf....
>
> http://i1296.photobucket.com/albums/ag19/wa3tts/EARFT1.jpg
>
> http://i1296.photobucket.com/albums/ag19/wa3tts/earft2.jpg
>
> I spent a few evenings reviewing the MC1496 data sheet, the HD-1490
> schematic, and Jack Smith's/Clifton Labs review of the Jackson Press
> SA602AN VLF converter.  I came to the realization the HD-1420 active mixer
> has no output matching and a high-impedance output, so I added a 10.25:1
> (6.5T:2T) binocular autotransformer on the 3.5~4.0 MHz IF output---still a
> far cry from the MC1496 output impedance but I was rewarded with another
> >+10dB of signal level.
>
> http://i1296.photobucket.com/albums/ag19/wa3tts/HVLFCMOD.jpg   I changed
> out the original input coupling and emitter bypass .1uf caps for 10uf caps
> made from 1uf chip caps on the 2n3904 preamp a few months ago....
>
> I use a similar 10.25:1 (6.5T:2T) binocular transformer externally on the
> HD-1420 LF input but with four end-to-end BN73-03 cores and 1/8" copper
> tubing as first primary turn and isolated windings for noise suppression.
> Works reasonably well down to 100kHz (>1dB insertion loss), about 2dB
> insertion loss at 75 kHz, 6dB at 25 kHz....I have some FT-75 toriod cores
> to play around with over the winter for input matching experiments....
>
> Good learning experience toying with a so-so converter design.  When I put
> together a high level mixer converter I will have some context for
> appreciating the performance upgrade.....
>
> 73  Mike wa3tts
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