[Lowfer] Heath HD-1420 mods

[Michael Sapp]

Doug, Dex, John & All: Thanks for the comments on the HD-1420 and ARGO caputres. 

I have the AMRAD converter information but have not looked at the W1VD converter design. Basically I have a couple of +17dBm DBMs here and a 10MHz LPRO which should work well with a single or push-pull 2N5109 preamp and suitable RF and LO filters---all in good time. I had a Heath VLF converter back in the 1980s for awhile, so it was a trip down memory lane with some added ham experience to re-visit the converter and really understand it's design.  Once I got the RF BCB levels under control with an input LPF and figured out the I/O matching issues the unit works OK.  

Dex, the FT-817 works OK in the 495~505 range but the sensitivity starts to fall off at 470~475 kHz. The HD-1420 is opposite, it had less sensitivity at 495 but outperformed the FT-817 at 470~475 kHz.  To remedy that situation I pulled the 680pf caps on the LPF internal to the HD-1420 and replaced them with 560pf caps. A quick check on the AM band showed a bit more signal strength on AM stations than I wanted at the bottom of the band, so I tacked in an additional 47pf for about 610pf and that seemed to be a reasonable compromise. 

 Some online reading on the HD-1420 others have suggested rewiring the filter in front of the preamp---that would seem a logical option---but I built up the VE3BPO 533 kHz LPF design that W7ZOI had a hand in and that works well in front of the HD-1420.  I did have to add a 1020kHz shunt trap 'cuz the KDKA 50KW station is 4.5 miles to the east.....With the 1020kHz shunt trap my intermod went away, although I could likely use one more shunt trap around 700 kHz or so to lower the total rx RF input power to the converter. I ended up setting it at about 1060 to have some nulling on a strong 1080 station as well.

 On my last visit to a local TV engineer friend we put the VE7BPO filter with the 1020kHz shunt trap on his network analyzer and the results are shown below....

http://i1296.photobucket.com/albums/ag19/wa3tts/SCRN13.jpg

This one is the rolloff point at 1dB per div...

http://i1296.photobucket.com/albums/ag19/wa3tts/SCRN14.jpg


 I built up one of those W1VD 505~515 preselector filters for the 475~495 range and that was more than what I needed for handling the local RF environment here---so it's living at WF2XXQs QTH to help him with the Miami BC market overload.  That was a fun build, tho. Really nice design.  I ended up with an extra two turns on each toriod (149uh vs 139uh)  to pull Jay's preselector design down into the 475~495 range.  It's a bit tight at 475 tho. If I build another one I'll likely add another turn on each toriod.....

http://i1296.photobucket.com/albums/ag19/wa3tts/IMG_2590.jpg

http://i1296.photobucket.com/albums/ag19/wa3tts/IMG_2565.jpg

And a few plots....

http://i1296.photobucket.com/albums/ag19/wa3tts/475.jpg

http://i1296.photobucket.com/albums/ag19/wa3tts/495.jpg

http://i1296.photobucket.com/albums/ag19/wa3tts/505.jpg

Well, I best get myself off to work....Thanks all for listening.....it will be interesting to see if the HD-1420 hears better at 495 kHz this weekend with the internal filter adjustment.

73  Mike wa3tts