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

[Michael Sapp]

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