[HBR] Another HBR Project -- Chapter 5

[hbr@mailman.qth.net]

The hope of doing the "1 week HBR" in only two weeks turned out to 
be no more than that.   The project's going well, but there's an awful 
lot of work in one of these and six hours a day max isn't cutting it.   
Probably another whole week.

I haven't really hit any major problems yet -- just lots of the usual 
kinds of details.

The 2nd mixer and 2nd Oscillator (1750 kcs crystal) worked without 
a hitch.   However that brought one more 85 kcs IFT into play which 
forced a return to alignment issues.   Basically those transformers 
have very little tuning range and a fair amount of built-in variation -- at 
least when working strictly from junkbox stock, with what are surely 
transformers from four receivers built years apart.  The fixed caps, 
nominally 180 mmf, range 180-192 or so, the trimmers have a 17 
mmf delta-C and adding the normal variation of the coils to the 
somewhat different tube capacitances put some trimmers at the 
extreme minimum and others all the way at the max. I collected a 
dozen or so spare 180 mmf caps from the archives and measured 
them; as time permits I'm going back and substituting 'max' caps 
where the trimmer is maxed and 'min' ones for the other extreme.   It 
will all work out, but six hours a day disappears pretty quickly when 
you do this kind of thing.   

Store bought parts would have been more expensive when these sets 
were first built but they'd have saved the builder a pile of time as 
those transformers come stock with a pretty good tuning range.

The last three days have gone to mechanical work.   Marking and 
finishing up the panel and bezel, making a knob and dial pointer, 
picking out knobs and finding setscrews for them, and doing the 
panel and front end assembly.    From here on it's all wiring (1st 
mixer, 1st oscillator, and RF stage) and coilmaking.   

Other than the front end, I just have the S-meter, the calibrator (I'm 
going 100 kcs) and the 'standby' function to do.

I found a website that would be very handy for anyone who designs 
superhets:

http://members.wiavic.org.au/vk3bhr/superhet.html

That's a calculator for trimmer, padder, and inductance values in the 
traditional front end design with the oscillator above the working 
frequency; with 'trickery' it will also do the other cases that occur in 
the HBR designs.   It will sometimes (but not always) get you a nice 
three-point fit; the other results look like pretty good starting points 
but not optimum by any measure I can imagine.   Sharyn and I are 
going to take a look at it -- it's probably all in the HTML and if so, we 
should be able to fix the algorithm.   The web is WONDERFUL!   

Mr. Crosby's approach -- tapping the tuning cap down on the coil 
instead of using a padder cap -- was unarguably easier and cheaper 
for the ham without a good supply of small capacitors.   Moreover, it 
means fewer parts in the RF circuits which translates to a simpler 
drift pattern and greater ease of adjusting compensation if one wants 
to go that far.   If duplicating an original design, that's the way to go.  
Everything has already been worked out -- you just wind the coils to 
specs.   

However starting with a blank sheet of paper front-end wise, I had a 
hard time facing trial-and-error tapping.  Plus I want to use toroids for 
some coils; because of the extremely tight self-coupling toroids don't 
do fractional turns well.  

Back to work.   Pictures -- which will look like the finished product -- 
later today.

Walt Hutchens
KJ4KV