[HBR] Re: Miller iron Core IF's Series 612 -- and crystal filters

[Walt Hutchens]

Byron wrote:
> I spent some time trying to learn the particulars of their construction, 
> such as the difference in coil spacing between the input IF {612-C1} and 
> output IF { 612-C4}. These are the iron core units which brings me to the 
> heart of my question : The IF coils are wound on a form that has a cap glued 
> on the end. So, I assume there are two iron powder cores secured inside the 
> form?

This all sounds correct.  However, you can confirm it with one of 
those tiny (very strong) rare earth magnets which will be attracted 
to the coil form if there's an iron core of any kind in there.   Or a 
magnetic compass would also work -- the needle will swing if 
passed near the coil form.

Kees posted:
> QTY 22 NEW Crystal Filters. 1650 kHz center frequency. These are
> brand new and in great shape. Simple connection with IN, OUT and
> SHIELD. Labelled as NETCOM Center Frequency 1650 kHz MDL. 1435 

And certainly a bargain at $5 each.   However the bandwidth isn't 
stated.   

If you get one of these for an HBR project (and the bandwidth 
proves suited to your planned use) then one could build it as a 
single conversion receiver.   Just put the filter after the 1st mixer 
and use two 1650 kcs IF stages.  Ditch the second conversion and 
the 100/85 kcs IFTs and all that.

I believe the reason the W6TC HBRs were developed as dual 
conversion receivers is that a decent crystal filter would have cost 
you at least a week's pay in 1960 and the extra conversion and 
lower frequency IFTs were cheaper.   Today, the situation is 
reversed if you can find a bargain filter that will work.   

(Using a crystal filter requires just a bit more care in layout and 
wiring if you're going to get high ultimate rejection, compared to a 
low IF with multiple transformers.)

In fact, I'll bet you could do a decent filter using 1843 kcs 
microprocessor clock crystals at $1.20 each.   Mouser carries 
them both series and 13 mmf load parallel resonance: those might 
well be far enough apart to make a usable half lattice filter.  Or you 
might be able to use a bunch of either one in a ladder filter -- the 
issue with a ladder would be whether the coupling capacitor values 
would be reasonable with crystals at that frequency for 2-4 kcs of 
bandwidth. Would certainly work for a CW-only HBR, though.

So many projects, so little time ... I'm still struggling with the 
chassis layout for the GC-HBR project.   The main discovery so far 
is that a 10" x 12" chassis isn't have a lot of room for 14 tubes.  

I did succeed in sawing off the back 1/3 of a three gang AM/three 
gang FM tuning cap to make a dual 370/dual 30 VFO tuning cap -- 
the big sections tune the 1400 kcs VFO, the small sections will 
tune the 5.5 Mcs IF.   Three hours of "if you slip, it's trash."

I am having trouble finding enough time to work on the project with 
all the usual work and the spring stuff, too.

Walt 
KJ4KV