[HBR] pondering an HBR

[Walt Hutchens]

Chris posted:

> About a month ago, at the Jackson, MS hamfest, I picked up a homebrew
> receiver from a table full of interesting stuff.  It was my only
> purchase  that day, $5.  I just bought it because I hate to
> go home empty handed.
> 
> After getting the thing home, asked some questions on the eham
> boatanchors forum, and was steered toward the HBR as a possible
> design model.

Welcome to the world of HBRs.  There's a lot of fun and interesting work
here and some darn effective radios, too!

> The thing fires up, and I can get signals on 40 meters.  But
> I don't think it really is put together very well.  I think 40 is
> the only completed signal path, maybe 80 once worked too.

It sounds like this is a bandswitching set -- or possibly a band imaging
one, although that would require double conversion since you need either an
IF around 1700 kcs and an oscillator tuning around 5700 kcs or the reverse.
In the latter case you'd have no bandswitch but an antenna and mixer coils
covering both bands with one sweep of something like a double gang BC tuning
cap.   Then a mixer plate coil tuned to one or the other of the frequencies
above and a second mixer to bring it to 85 kcs.

Band imaging sets work quite well -- I've built a couple or three.   The
front end (effectively the bandswitch) will tune quite sharply and unless
there's a reduction drive or you tune with VERY great care, you may miss it,
in case the set will be pretty much dead.

80M will be much sharper than 40M.   You'll have to retune as you move
around the band when receiving on 80.  Look for the low end of 80 very near
the fully meshed position on the front end cap.
 
> So, now I'm thinking about using it as the basis for an HBR project.
> Had no idea about HBRs up until this came along.

Except for a very few sets built by engineers -- the G2DAF Mk II and W1OMX
designs would be the main examples -- the W6TC series of receivers are the
best ham designs of the late 50's to 60's.   AND unlike the 'engineer'
designs, they're buildable by anyone with determination and modest
construction skills, even today.

When thinking about building an HBR set the first consideration is the dial
mechanism followed closely by the tuning capacitor.   This is the heart of
the radio: If the dial doesn't have a slow enough tuning rate then the set
will be usable only on AM signals.   The Eddystone 898 (frequently
advertised on eBay) is the benchmark: With a 100:1 ratio it covers 80 in
over 40 revolutions.   A really good string drive would be second and about
equal to a two-speed (typcially 36:1 and 6:1) planetary drive.

There are various other (less common) choices:  One of the 'command set'
tuning caps/dials is sometimes used -- that's the military receiver from
which your IF/BFO cans were obtained, though it would likely be from another
set in the series because the one with the 85kcs IF has too big a cap.

What kind of dial mechanism do you have?
> 
> This thing uses the BFO and IF coils from the military radio,
> 85 kHz.  The BFO actually puts out a pretty good signal. But there
> is no shielding around it.

Shielding isn't necessary with careful layout.

In receivers of 1940's design the BFO was often injected into the 2nd IF.
Since this took over the AGC, AGC had to be turned off to use the BFO but
with SSB rare when these set were made that was of little consequence.

Hams who regularly receive SSB signals, however, want AGC (and a working
S-meter, usually driven from the AGC!) so as SSB sigs became common on the
ham bands, receiver designs had to change.   In particular the BFO had to be
kept out of the IF until AFTER the AGC was taken off so both could be used
at the same time.    Generally that meant completely isolating the BFO and
injecting its signal right in to the SIGNAL detector, often a 'product
detector.'    The easy way to isolate the BFO is to put it in its own box
and run the signal from inside the box to the detector with a shielded
cable: This approach is generally taken in HBR designs and frequently
others.

It isn't however, essential.   With careful layout and just the usual sorts
of bypassing the BFO can be quite adequately isolated just by keeping it
away from the IF string and AGC detector.   This requires less mechanical
work.   
 
> I guess I'm poking my toe in the water before deciding whether
> to jump in or not.

There's quite a bit of HBR experience on this list -- I'm working on two
offshoot designs and there are at least a couple of other ongoing
construction projects; nearly everyone either has one of these sets, has
used one, or has plans/hopes for the future.

My two are very nearly done:  One (a 'transformerless' design) needs only
the most minor of tweeks and a whole flock more coils; the other (based on a
Heathkit HR-10) needs a bit larger tweek (or three) and about the same
number of coils.  

Both also need cabinets and I haven't quite figured out how to do that yet.

These projects have been stalled for several weeks due to pressure of other
work and some health issues, all now (hopefully) winding down.

Walt Hutchens
KJ4KV