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

[Peter Bertini]

Blah! Some typos crept into that missive!!  I wish I could edit posted text :)
Anyway, I've come to the conclusion that I'd like to try some of these
ideas at a 97.5 kHz last IF.  The two crystal I have on hand would
give me a first IF of 1612 kHz (gets me off the local 1600 kHz
daytimer), and I would have to move my last IF up to 97.5 kHz. That
sort of precludes using the R-23 IF transformers, which have at best a
few kHz tuning range.  Variable L seems to be the way to go for IFTs
in that range.  I don't mind rewinding one or two of them to 97.5kc
for the following stages, but I suspect I'll be going to a variable L
"LC" filter for the variable BW filter. Any ideas on what's available
for high-Q tuned coils these days?

Pete

On Sun, Sep 28, 2008 at 2:28 PM, Peter Bertini
<radioconnection at gmail.com> wrote:
> 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
>>
>