[Boatanchors] Hallicrafters SX-71 Raspy CW note?

[Philip B Atchley]

Hi All,
A quick question to the group.  I've been using the SX-71 for a lot of
Longwave Beacon work (with an up-converter) and I've noticed something
"odd".    As these beacons are typically MCW and are often quite weak the
usual mode of listening to them is in normal CW, with as sharp an IF
filter as you can muster to separate out the desired "offset" (Sideband
CW note) from the carrier and other extraneous carriers, beacons etc on
the same/adjacent frequencies.

Like I said, I've noticed something "Odd" in using the SX-71 for this
listening, something I've not experienced with other receivers.  I
"usually" have the "Reception" switch in sharp Crystal, BFO on, Audio
gain around "2" (it drives an outboard audio filter and any higher
overdrives the filter) and the RF gain control for suitable
sensitivity/volume, usually about 4-5.  Of course with the BFO on, the
AGC is OFF.  This works fine MOST of the time, no RF/IF stage overload,
good CW detection etc.

However!  Many times, especially if listening to a beacon near a strong
carrier or other signal the CW note is extremely raspy and rough
sounding, like there is severe clipping or overload in the IF strip, or
that the signal is overpowering the BFO.  HOWEVER, I can turn down the RF
gain control and turn the AF gain up further and there is NO improvement
like there would be if it were overdriving the IF strip/detector.  It
just stays "dirty".

Now, if I widen the Filter by going to "Broad Xtal" the signal will clean
up somewhat and the note sounds much cleaner, and switching the Xtal
filter out completely is even better.  This doesn't add up as you'd think
that more nearby strong carriers etc would hit the detector just that
much "harder".

It almost seems like IMD (Intermodulation) or clipping is taking place in
the Xtal itself.  No, I'm not talking about "filter ringing".  With all
the beacon work I've done I'm most certainly familiar with that sound! 
This is just plain "fuzziness".  

Any ideas?  Or is this typical when you use a single Xtal filter set for
it's sharpest selectivity?
73 from the "Beaconeers Lair".
Phil, KO6BB

Where DX begins at the noise floor!
Merced, Central California, 37.3N  120.48W  CM97sh

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