[HBR] Current day HBR

[Walt Hutchens]

Pete said:
> One thing I was thinking about is the availabiltiy of three section tuning
> caps for AM FM tuners. Folks often comment about the the lack of general
> coverage bandspread on Ted's designs. Using one of these AM FM caps opens
> some possibilities. The FM sections could be used to replace the original
> HBR three section tuning caps. Shortwave coils could be wound, with  short
> flying leads off the top,  that would be connected to pin jacks on the 365pF
> sections for general purpose SW coverage. That would give a bit more utility
> to the receiver, besides ham use.

The problem of coil forms is solveable at home if you take the base
from a metal octal tube and use something like JB Weld to secure it
inside a 3" length of 1" Schedule 20 PVC pipe. With an actual ID of a
hair over 1-1/8" this pipe is a perfect fit on that base. The top
flange can be sliced from 1" PVC pipe couplings and glued on with the
PVC pipe glue. Minor 'adjustment' with a Dremel on the inside is
needed to fit the APC caps in the top of the coil form; they can be
secured with the pipe glue.

A whole set of coil forms will take you a few hours and a bunch of
junk tubes but will hardly cost over $10.

The 3-section AM/FM cap mentioned above is a sterling idea for
building a receiver that does both ham-band-only and general coverage.
Instead of flying leads to make the switch, why not use coil forms
(like those just described) with enough pins to use a jumper between
two pins on the coil to connect the AM sections of the cap for general
coverage?  My gut feeling is that using the same coils for both h-b-o
and GC won't give very nice bands anyway, because of the much higher
minimum capacitance of the 365 mmf sections.

For example if you use a 40M coil that was pruned for just a bit over
7.0-7.3 Mcs using an FM cap section, I'd expect it to cover something
like 2.3-6.9 Mcs or so with the AM section.  Also you may want to
change other parts in the circuit to get a more linear dial scale when
using the AM section.

Unfortunately stray capacitances are so important in these questions
that you about have to build the receiver and wind the coils to be
sure.  At least I do.

I like this idea so much that I'd be thinking of that as the next
project if I were not already in the middle of this Five Year Plan.

Walt
KJ4KV