[HBR] Looking for some parts, leads welcome..

[Peter Bertini]

I've been looking at the Star SR-700A receiver schematic, which looks
to be a loosely based copy of the Drake 2B receiver. The receiver uses
a second IF at 1600kc, and a final IF at 55kc.I like the method they
to use to achieve variable
selectivity at 55kHz; which is basically a top coupled four pole LC
filter with switch selected top coupling caps. One of the switched
poles also places loading resistors on the input LC circuit to
presumably equalize gain at different selectivity settings. Pretty
cool.  They also use a tunable notch filter right after the variable
BW 55 filter.

According to the data in the manual, the upper slope of the
selectivity curve is the one that moves up or down as selectivity is
varied; the lower frequency slope remains fairly constant.  That sort
of means that the BFO is best set for low side injection, and they do
the sideband reversal is done switching the conversion oscillator
crystals for either up or down conversion to the final IF, which
reverses the sidebands in the slope BW.   I kind of like how they used
double tuned IFTs at 1600kc, and how they used cascaded LC circuits
before the 1st IFA stage to set the final selectivity.  I'm thinking
about adopting some of the ideas into my HBR. Just mulling it over...
The two conversion xtals I have would work with a 97.5 kc IF, and a
second IF of 1612 kc.  Just some random thoughts.  Where could I find
some adjustable inductors that would be high Q to make a 97kc tuned
circuit???

Pete

On Sun, Sep 21, 2008 at 9:28 PM, Peter Bertini
<radioconnection at gmail.com> wrote:
> On Sun, Sep 21, 2008 at 3:45 PM, Walt Hutchens <waltah at earthlink.net> wrote:
>>>
>> Four 85 kcs IFTs from R-23/ARC-5 in two pairs (single 2nd IF stage) gives about the
>> maximum selectivity that you can use for SSB with all the rods pulled
>> up; it's a bit tight (but useable) for AM if you tune to one sideband
>> or the other.  With rods pushed down it'll be fine for AM.  You
>> cannot, however, move the rods up and down frequently -- the mechanism
>> isn't designed for that.
>>
>
>> Walt
>> KJ4KV
>>
>
> Walt
>
> I would replace the fiber rods with brass welding rod and use a rack
> and pinion mechanism
> to move the coupling up and down. It isn't rocket science, and like I mentioned
> Silvertone used link driven IF transformers for variable selectivity
> settings in home
> receivers back in the 1930s. Whether it is worth the effort, or
> whether the idea is even
> worth pursuing is probably open for discussion; not that it can or
> can't be done in a
> manner that works mechanically...
>
> The only thing I'd question is how the selectivity curves on the
> different IFTs would track
> each other..  Esp. with cascaded IFs at the front of the strip and
> single IFTs following
> at the rear. The mechanical part would be easy, whether the results would be as
> expected I am not so sure. It would be nice to have a good shape factor for AM
> bandwidth, and a relative sharp peaked response for CW. Maybe I should
> just settle
> for the Q-Multiplier and leave it at that.
>
> Matched crystals in the 1600 to 1700 kc range might allow for some
> simple ladder filters
> for CW and SSB reception, I'm aware of the limitations on lattice
> crystal filters and there
> are other options, such as the pair used in the HR-10 and other Heath
> receivers. There must
> be junkers out there.
>
>  None of this is etched in stone, but I am mentally running
> through all of the options. I'm almost tempted to build the RX in sub
> assemblies, to permit
> playing with different ideas for the IF (and sweeping them) to see the effects
> before committing to chop up an expensive chassis.
>
> Pete k1zjh
>