[AMRadio] Heathkit -VF-1 low band band spread

Donald Chester k4kyv at charter.net
Tue Jul 12 12:22:08 EDT 2016

I used a VF-1 for several years before replacing it  with my current VFO, a
highly modified T-368 master oscillator. Overall, it was more stable than
the homebrew VFO I had used previously.  I never had an alignment problem;
the dial calibration with both frequency ranges was surprisingly good across
the entire frequency range with both fundamental oscillator frequencies.
The proper combination of settings of the trimmer capacitors and slug-tuned
coils should yield the correct amount of bandspread in the tuning range.

My frequency jumping problem was caused by one of the (10 pf, IIRC) ceramic
temperature compensating capacitors. I found a suitable replacement and that
cured the problem.

One of the more annoying problems with mine was thermal instability and slow
warmup drift.  I  had noticed that despite the fact that the circuits are
identical, the VFO in the DX-100 was more stable.  Then at a hamfest I
picked up part of the innards of a gutted DX-100 and upon examination, I
figured out what caused the problem.  In the DX-100 the coils and tuning
slugs are mounted on a single sheet of copper plated steel, with the slug
bushings located between the mounting  screws of the ceramic coils.  In the
VF-1, the coils, tuning capacitor and other frequency-determining components
are mounted inside a little metal box made of the same material as the
chassis.  The problem is, the coils are fastened to one side of the steel
box, while the slug bushings located on the opposite wall.  Any expansion or
contraction of the metal box  with temperature causes the tuning slugs to
move slightly in or out of the coils, generating drift.  I partially
disassembled the VFO and drilled mounting holes on each side of the slug
bushings, just  like what's in the DX-100.  Then the ceramic coil forms were
turned around and mounted with the new  screw holes on the same side of the
box as the slug bushings, identically to the DX-100.  A complete
re-alignment was  required, but the end result was noticeably better
stability in the 160m fundamental range, but little or no improvement in the
40m fundamental.  I didn't further investigate the reason this modification
worked less well on the 40m range, since I worked 160/80/40m using he 160m
fundamental and used the 40m fundamental only for 20m.

Another modification that further improved stability was to replace the
ceramic trimmer capacitors with some tiny air variable trimmers I had in the
junk box. I also replaced the tuning knob with a planetary drive mechanism
to improve the tuning rate.  After I replaced the VF-1 with the T-368 m.o.
unit, it sat on my shelf for almost 20 years until I gave it to Derb, N3DRB
(SK) when he stopped by here for a visit a few years ago, and have no idea
of what ever became of it following his passing.

All VFOs similar to the VF-1, including the Knight VFO and the Johnson 122
(and this includes the VFOs in the Ranger and Valiant), suffer a common
problem;  running the VFO straight through on 40m tends to result in CW
chirp and sometimes sporadic frequency instability, probably due to unwanted
feedback caused by rf leakage from the output of the transmitter back into
the VFO. Mine produced much better stability on 40m using the 160m
fundamental.  The problem was never noticeable when  running the VFO
straight through with the 160m fundamental, but as I recall, numerous VFO
articles from the pre-WWII "ECO" days up to recently, have recommended never
running the VFO straight through on any band, but building an oscillator for
half the lowest operating frequency and doubling or multiplying up to the
transmit frequency.

Don k4kyv

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