Well, maybe I've actually managed to do something different... I've been adding a 1 pf chip cap (0603) between the oval shaped pad in the filter, and the edge of the filter sectiont. One cap for each section. This lowers the frequency down below 900, and then I use a dremmel with a sharp bit to walk the filter back up in frequency. This is all done while watching a sweep of the filter on a network analyzer.
I've not really attempted to narrow the BW of the filter at all, just shift it down some. As to the VCO, that's where the conductive ink comes into play. Just dab some in through the slotted window, no need to remove the cover. The microstrip resonator needs to be about a 1/4" longer. The conductive ink pens work just fine for that. I have a web page up on converting the 406-433 spectras to 440 that has some useful pictures on how I go about retuning the filters. Just add the 1 pf cap to the process, and it's otherwise the same. http://mysite.verizon.net/res8teuc/Range1spectra/Range1Spectraver1.1.htm Maybe that, and my comments above, will prove helpful until I can put up a similar article on the moving the 800 and 900 Spectras down. My next project? See if I can get a VHF Spectra to receive on 220. Mel - WA6JBD > Well, you have me curious also, maybe > I took the dark path for tuning the front end filters. > I had to resort to silver paint (not cheap), a grinder, and > a little blood and guts to tune those puppies. What's > your story...... > . > bill > w4oo