I did lots of experiments with VFO/LO circuits when building the homebrew receiver. I found the most important thing in my circuits was the coil. The best setup I came up with was large B+W coil stock.
Any slug tuned system I tried sucked, as did coils wound on any type of form. I tried ceramic forms, slug tuned or not, plastic, bakelite, I tried using various npo caps to offset things, etc. I tried various types and sizes of coil wire, close wound, space wound, more or less inductance, hot wire wound on cold forms, glue to hold the turns, etc... The B+W coil LO seems to be REAL stable after a 2 minute warm up, less then 500 Hz drift, no regulated filament voltage, although the B+ has a VR tube to regulate it. I also found using octal tubes as VFO/LO tubes is tough, they are large and change a lot as they heat up. There is a wide range between octal tubes as far as how they drift, and how much. Brett N2DTS > Recently, I have seen VF-1's go for big bux amongst the > collectors. I > wouldn't throw one away nowadays. > > One problem I discovered with my VF-1 was thermal drift. I > took it apart > and noticed the construction; the slug tuned coils are > mounted on one wall > of the oscillator subchassis, and the slugs and their > bushings are mounted > on the opposite wall. So as the temperature varies, the > whole compartment > expands and contracts, and moves the slugs in and out of the > coils. No > wonder the things drift! Then I noticed the insides of a > DX-100, which has > a nearly identical VFO. The coils and slugs are mounted on > the same piece > of chassis panel. > > So I remounted the coils in mine to the same side of the > subassembly. It > was easy - just drill a couple of mounting holes on either > side of the > original coil slug bushings (with the slugs removed during > the process). I > had to slightly re-route some of the wiring, but it was easy to hook > everything back up, and it worked FB once reassembled. It > didn't stop the > drift entirely, but reduced it by at least 90%. It was nice > and stable on > 160/80, but was still drifty on the 40m. range. > > I ran the VF-1 off a CVT. It too, drifted with slight > changes in fil. > voltage. I think both the VF-1 and Collins PTO's use a 6AU6 > as oscillator > tube. My "silent" Sola beats fooling around swapping tubes. > I suspect even > a good tube would eventually become drifty with age. > > Don k4kyv > > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb >
