[HBR] Relay switched coils?

[Walt Hutchens]

Pete:
> T'would be nice if someone could engineer a duplicaticable plug-in coil
> system, as used in the National HRO receivers. <SNIP>

AMEN!

> Didn't someone come up with scheme using the RF decks from old ARC-5/Command
> surplus RXs for this purpose?

I won't claim to have come up with it, but I did do this. I mounted
the ARC-5 receiver plugs as a unit behind the front panel of the
receiver, perpendicular to the chassis and put the ARC-5 coil units in
through the receiver panel. This worked -- I think I made four coil
sets and the receiver performs quite adequately -- but:

Doing it as I did, It's A Bad Idea.

There are three problems:

1. It's mechanically challenging because everything has to be precise,
the layout isn't simple, and the coil plug bracket setup must be very
sturdy. This is just 'the breaks of the game'; presumably anyone who
would try this sort of design would know what he was getting into.

2. The Q of the command set coils wasn't high enough for the rest of
my design so I had to replace them with toroids on some bands.  A lot
of potential simplicity went out the window with that.

3. And the killer: There's not enough room in the coil cans to put
trimmers inside.  So each coil must be padded with one or more tiny
FIXED caps which must be adjusted by trial and error.  (Remove the
coil set, remove the screws holding the coil in the can, change the
cap, and reverse.)  This is not quite as bad as it sounds -- you can
use the trimmers on the command set tuning cap to estimate changes to
the fixed cap -- but it isn't great, either.

I made it work, but it's a ship in a bottle exercise: Once it's done
you have proved you can do it.  Now you have a ship in a bottle.

I was not able to think of a way to adapt or design around the
limitations of that tiny can with only one fixed mounting area -- the
mica base. You might be able to do it using command set IF cans but
that would be a way to build a ship on a bottle to put on the mantel
of the captain's cabin in your ship in a bottle.

The road is open for someone else to revisit this idea and get past
the issues I ran into.  But that someone probably won't be me.  In a
weeks time, these days, I'm lucky to get an hour for radio.

Tim commented:
> What makes an HBR, be an HBR, is not just the electrical topology,
> but also the construction and especially the bandswitching.

Electrically these are dirt-conventional receivers: It was the
construction and bandswitching that made them unique.

> For the typical (e.g. doesn't own a machine shop) homebrewer a
> design that allows construction by a normal guy with
> hardware-store-class tools (possible exception: chassis hole punch)
> defined the HBR's construction.

AND will deliver excellent ham band performance, absent only the
various gizmos, bells and whistles made possible by digital signal
processing. None of the well-known 1950's-70's products from National,
Heathkit, Hammarlund, (etc.) will equal a well-built HBR's
performance.  Collins, perhaps, but those sets cost what would have
been most of a year's pay for an average ham, back when they were new.
Your dentist might have owned one, but that was as out of reach for
most hams as his annual new Cadillac and country club membership.

> I don't think it's impossible to build a homebrew,
> front-panel-bandswitched multiband radio. In fact two-band
> mirror-image receivers (esp 80M/20M) are practical and appear in
> many handbook receiver projects.

I've done three or four 'band imaging' receivers.  There are many
excellent performers in this class, especially if you want something
even simpler to build than an HBR but still highly useable.

> And there were occasional
> every-band-under-the-sun-incredibly-mechanically-complicated-bandswitching
> homebrew radios in QST and handbooks too.But those aren't HBR's.

These were mostly good for ideas.  Only a guy with the same machine
shop and shop skills as the original builder could duplicate them.

The G2DAF receiver (especially the second version) is probably an
exception. It would have been a monumental project but requires only
conventional tooling. A push-pull front end based on triodes is going
to be a fine performer.

Today, however, it would be impossible because you'd never find all
those coils and would have to re-design around modern products.

Maybe also that single conversion receiver by W1 ... the USAF Colonel,
that used a beam deflection tube as a mixer and premixed the various
crystal frequencies for the bandswitching.  That also could be built
by anyone with some receiver building experience and a full drawer of
conventional tools.

I'm currently resurrecting the band imaging receiver that I built with
a push-pull 12AT7 mixer and 12AU7 oscillator. When a few other things
are out of the way (a year ...) maybe I'll build a multiband receiver
based on that. Basically just a bandswitching converter ahead of what
I've already done. It shouldn't be that hard ... he said.

> I say all the above having done some non-HBR-homebrewing. I think
> the beauty of the HBR can only be realized, in fact, if you attempt
> and complete a non-HBR style bandswitched radio.

Yep. Doing some of the others -- and solving all the problems that
come up -- is what it takes to appreciate what W6TC accomplished with
a design that could be built successfully just by following the
directions.

Walt
KJ4KV