[Boatanchors] VF-1 modification for improved stability

[Donald Chester]

I used a VF-1 years ago, although I eventually replaced it with a modified T-368 master oscillator unit.

I had noticed that DX-100s tended to be more stable than my VF-1 even though I thought the VFOs were identical, until I looked at the innards of a partially disassembled DX-100.  Although the VFO circuit is the same in both units, there is a major difference in the mechanical construction.  In the DX-100, the oscillator coils and tuning slugs are all mounted on the same chassis panel in the VFO unit.  In the VF-1, the coils are mounted on one side of a small metal sub-chassis that forms a rectangular box, but the slugs are mounted on the opposite side of the box.  Changes in dimensions of the metal sub-chassis cause the the slugs to physically move in and out of the coils as the chassis metal expands and contracts with temperature.

I drilled holes for the coil mounting screws in the side of the box that has the slug bushings, and re-mounted the coils on the same wall of the sub-chassis as the tuning slugs as in the DX-100 configuration, so that there is no relative shift in position between coils and slugs as the temperature rises from a cold start.  This noticeably improved warm-up stability of both frequency ranges, although I was surprised to see more improvement in the 160m range than in the 40m range.

A common problem with all these similar dual-range VFOs like the Heathkit, Johnson 122, Knightkit, etc. is chirp and instability on 40m when the 40m range is used to work straight through.  This doesn't seem to be a problem when working straight through on 160m, but for 40m, stability is much better when multiplying up from the 160m range.  The VF-1 and DX-100 have a 40m dial calibration for both oscillator ranges, making it easy to use either one, but the 40m range is more satisfactory when used only for multiplying up to 20, 15 and 10m.

Don k4kyv