[HBR] HBR - BFO progress

[w0ep at w0ep.us]

I think I'm finally off the ground with the HBR project, at least a
little, tiny bit.


I took one of these coil-cans that I have in my parts box and took it
apart.
It wasn't going to do the job as it was, so I stripped out the coil and
the
other parts.


Using various web pages for reference, I figured out that I needed
somewhere
around 8000-9000 microHenries of inductance in order to have a 50 pF
variable
capacitor swing the BFO between 83 and 86 kHz  (I'm using the 85 kHz IF
cans).


I wrapped 10 turns on the coil form and figured what I thought was the
right AL number by measuring the inductance with this little digital
meter
I've got.  And that led me to conclude that I needed about 800 turns on
my coil.
Also, the web again provided the info that I wanted my coil tap to
be at around 25% up from the grounded end of the BFO coil for a
Hartley oscillator.


With all of that info, I wound 200 turns then put a loop,then 400 turns
and another loop and then the final 200 turns.
The coil form has four "legs" where connections solder in
and have the two end and the two loops soldered in.


I did a mock-up of the HBR-13 vfo using a 7-pin socket, some PC board
and a 6BH6.  I used junkbox parts for the capacitors and resistors,
tried
to be close to the schematic values.  I terminated the output (pin 5)
with a small trimmer cap followed by a 26 kOhm resistor  (I didn't
have a 22k).


I had a small tragedy when I found that my regular O-scope was
acting up and won't start.  More work to do there!  But I found an
old scope in the garage that sort-of works.  


I have a Heathkit IP-17 power supply which worked for filament
and B+ with alligator clip leads to my circuit board.


And, I had a waveform!  The frequency counter helped me trim
the inductor core to achieve 85 kHz at center setting of my air-variable
50 pF BFO control capacitor.


It turned out that my scramble-wound coil was not able to get
to 8k microHenries, more like 6k.  That meant adding more
capacitance to get things at the frequency I want, and the 50 pF
only gives me a swing of a bit more than 1 kHz both sides of 85.
I may make another run at that if you all think I need more swing than
that.


The only other objectionable thing was that the negative peaks
on my waveform are a bit truncated.  It looks like I am hitting a
voltage
supply rail in that direction, or something.  I may fool around with
that
a bit.  Does the BFO need a nice waveform?


I'm really tickled that I was able to make this work at all.  I'm going
to
watch at our hamfest later this month for a better 50pF trimmer cap, and
I have my
tube list.  I need to make a more complete parts lists, that's a fact. 
There
are probably some potentiometers that I could pick up.




Chris Howard
w0ep
Columbus, MS