Comparison of SB303 vs. SB300/301??

[Steve Harrison]

At 11:27 AM 1/23/99 -0500, Edward Swynar VE3 CUI wrote:
>Steve...
>
>The SB-300 does, indeed, have provisions for a grand total of THREE
>filters, 2 of which were optional (AM & CW)...
>
>I was wondering: if you REALLY wanted a "...high fidelity" bandpass filter
>for AM, why not simply remove any existing AM filter, & replace it with a
>jumper wire---in other words, NO IF filter whatsoever...?

Because I still need opposite sideband rejection. I'll be leaving the BFO
running; my application isn't to receive AM, but merely wideband SSB, wider
than just a few kHz. SOME degradation in opposite sideband rejection will
be acceptable since the radio will be used as the IF receiver behind a set
of V/UHF receive converters for high-speed CW communications (>3300 wpm) on
uncrowded bands.

>Might be a tad TOO broad...still, I wonder if anyone has ever tried this on
>their own rig?

I remember doing that on both my old SB300 and a Galaxy V transceiver. You
hear everybody and his brother, sister, father and mother, and most of
their aunts, uncles and cousins, including both their sidebands, not to
mention pileups up and down the band for kilohertz! The Galaxy had audio
AGC; the wide-open IF just KILLED the receiver performance since ANY signal
within the bandpass of the IF transformers triggered the AGC. I can't
remember exactly what happened with the 300, but it wasn't real impressive,
I'm sure!

>Might be a cheap & easy alternative to trying to hunt down
>that ever-elusive 6-KHz wide AM filter (which I'm sure I, too, have read
>about / seen advertised SOMEWHERE!).

I found a couple of reference postings in the Tempe archives; someone was
looking for the 5 kHz Heath filter for the SB313 SW receiver to install in
their 303.

Were Heath not out of business, it might be possible to build such a filter
with a handful of crystals from the "Candy Store". Back in the '70s, the
rocks were something like $8.50 each, I think, not too bad. Wes Hayward and
others have published design info for crystal filter design and testing in
ARRL pubs for some years, and I've even tried several; the designs do work
and aren't difficult to make. The real problem is finding enough crystals
at about 3395 kHz! Otherwise, you'll have to make some sort of converter to
another IF frequency, which may be the easier way to do it anyway.

But being short of RoundTuIts, I've held off on such an ambitious
undertaking as long as possible. Maybe it's time to bite the bullet and
just get on with designing and building a whole new receiver. :o(

For those of you with another receiver that has excellent AM performance,
you might consider tapping the wideband 8.895 MHz IF output (such as the
panadapter output) from the Heath stuff and injecting that into the older
AM receiver. But you'd want to disable the Heath's AGC, I think, since the
following receiver would do the gain controlling. Remember when we used to
use R9er's following older Hammarlund and Hallicrafters receivers at 455
kHz to obtain the greater selectivity afforded by the 50 kHz IF in the R9ers?

73, Steve Ko0U/1

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