On 03/18/2014 11:00 AM, Michael Black wrote:
On Tue, 18 Mar 2014, Geoffrey S. Mendelson wrote:

dave wrote:

Now you get to experience the joy of building a crude preselector. A 20
dB pad might work, try that first.

A friend of mine made one with a coax stub, and a T connecter. I'll
ask him
the length and type of coax.

Wouldn't that have been a notch filter, to get rid of a particularly
strong and pesky specific signal?

If it's just one station, then it is a solution, set and forget. But if
there are multiple stations causing problems, then more is needed.

I was going to say in the old days endless preselectors and tuners and
preamps were built (and even sold as commercial products) in an attempt
to boost the performance of a lot of those low end receivers. I
remember one mod for the Hallicrafters S38 that was just one tube, no
tuned circuits, the performance boosted because the tube meant the
existing tuned circuit wasn't loaded down.

There'd be low pass filters to get rid of FM and TV stations, high pass
filters to get rid of AM broadcast stations, and just things to peak up
specific frequencies to get rid of images.

I don't think any of that would have helped my Hallicrafters S-120A (the
transistor model), that thing didn't need an antenna to receive endless
local broadcast stations. It would have needed a lot of work with
shielding to make a filter between the antenna and receiver useful, it
wsa just picking up the signals directly.

Sometimes it is a too strong local signal, sometimes it is a badly
designed receiver that will never be fixable.

Michael


The MFJ-956 passive pre-selector is in my tool kit. They also make
several actives. They all cover HF, which includes 24 MHz.