[HBR] Looking for that three section variable

[Walt Hutchens]

> At the time of the HBR... one thing that set it above and beyond the
> other simpler homebrew receivers, was 3-section tracking aligned
> nicely on all the bands.
> 
> In today's homebrewing, such tracking is often not considered an
> important goal. But back in the 50's, it was a goal.

Just to add to Tim's excellent summary, the HBR series came along just
as the tunable local oscillator tracking the signal frequency sets
were breathing their last. With these sets -- say, 80M covered with an
LO tuning 3955-4455, 40 with 7455-7755 (and so on) it was possible to
do single knob tuning. In fact it wasn't even terribly difficult for
the experienced constructor.

The new wave of ham receivers, however -- say 1970 onward -- generally
used a tunable 1st IF with a fixed (crystal) first LO. (Effectively a
fixed tuned converter in front of a single band tunable receiver.) In
many of these sets the tunable 2nd LO tuned in the opposite direction
from the signal frequency on at least some bands. For example
3500-4000 might be covered using a 9.5 Mcs crystal 1st LO and a
tunable IF of 6000-5500 kcs. 7000-7500 used a 13 Mcs crystal, and so
on. In these designs, tracking the RF stage(s) to the tunable
oscillator required seriously elaborate design and even many
commercial sets didn't do it.

I forget the details, but the R-390 sets solved the single knob
problem. Anyone looked at the cams and gears in there lately?

The switch from separate receivers and transmitters to transceivers as
the ham configuration of choice occurred in parallel and added another
level of complexity to a single knob scheme.  At the same time,
NOBODY expected a transmitter to be single-knob.

The new design approach yielded such superior performance that we all
got over our obsession with single knob tuning. But when going back to
basics -- to the W6TC scheme of a simplified standard design,
buildable on a kitchen table with hand tools by almost any careful and
determined ham and delivering performance comparable to the best of
ham receivers in 1960+/- -- there's no reason you shouldn't get all
the convenience that design offered, including one-knob tuning.

Using series caps with a 140 mmf tuning cap to get down around 25 mmf
maximum will yield such a severely distorted tuning curve (with nearly
the entire tuning range squashed into a few degrees at the low
capacitance end of the range) as to be unsatisfactory.  Do the
calculations for the series cap needed to pad 140 to 25, then
calculate the new value at half mesh (70 mmf) to see how this works.

When using series caps is contemplated -- and this makes excellent
sense for values comparable to the tuning cap maximum -- separate caps
must be used for each section because otherwise so much coupling is
introduced between circuits as to make stable operation impossible.

Look for junk ham transceivers of the 1960s-70's period. A trashed SB
33 or 34, MANY others, will have the necessary three gang cap with
something like 25 mmf maximum. The Tempo ONE is another, I believe.
All the better FM receivers and tuners through this period also had
what you need. (The cheapest ones -- local reception only -- sometimes
had two-gang caps.) A few hamfests or weeks of watching eBay should
deliver the part you need at a reasonable cost -- maybe even with some
other useful parts!

Or -- you can prune plates from the 140 mmf cap. But it's probably
better to save that one for a day when you need that value.

Walt
KJ4KV