[Hallicrafters] Anyone added padding to an SX-24 or SX-25?

[Carl]

Have you looked at the SX-17 circuit? They have individual padder 
trimmers for each band and utilize what looks to be an identical 
bandswitch.

Changing turns on the antenna coil primary affects the input impedance 
and not tracking. If you do it to the secondary its a crap shoot without 
a padder.

I rebuilt my SX-25 about 2 years ago and dont remember any particular 
tracking problems but then again I havent used it since.

Carl
KM1H


----- Original Message ----- 
From: "SX-25" <telegrapher at hotmail.com>
To: "Carl" <km1h at jeremy.mv.com>; <hallicrafters at mailman.qth.net>
Sent: Friday, April 03, 2009 10:57 AM
Subject: Re: [Hallicrafters] Anyone added padding to an SX-24 or SX-25?


Hello Carl,

Thank you for the very informative reply to my posting. Those are all 
excellent suggestions you made for the SX-24/SX-25 and its designed 
"deafness" on Band 4. The replacement of the RF coil with a low loss 
toroid is one I had not thought of and am interested in experimenting 
with later on.

Although these great suggestions will, no doubt, help beef up the 
sensitivity on the high band, the problem remains that the receiver was 
designed to track IF/RF fairly well on the lower bands, but it is as if 
Hallicrafters ran out of interest when they got to band 4. The SX-25 
provides a very effective means of adjusting the IF tracking with pads 
but when it comes to band 4, none exist. The prevailing wisdom out there 
about the SX-25 is that the IF tracking is the culprit and, indeed, I 
can get an SX-25 to be somewhat sensitive over a small portion of band 4 
where the IF tracking crossover exists, but the tracking linearity falls 
off when moving away from the narrow portion of the spectrum (1 to 2 MC) 
in which the tracking is set. There has been some discussion on this 
reflector (and elsewhere) over the years about the adding of a turn of 
inductance to the front end coils to "tune/detune" to bring the tracking 
into rough linearity and I have experimented a lot with this and it does 
seem to help but the results are still  anticlimactic.

 I also am transported back to my early days as a ham in the mid-1960s 
when I knew a very active ham, W9RYM, who had amassed some notoriety and 
was known then as "Mr. 10 Meters." He operated almost exclusively on 10 
meters but also was quite a terror on 15 meters. His only receiver was 
an SX-25. Probably his SX-25 was not a stock version and I also concede 
that it was near 50 years younger than it would be today so its 
components were all young and virile. But still I think to myself, "how 
did Shorty do that?"

When reading any excellent volume written about multiband 
superheterodyne receivers (Radiotron, Alfred Ghirardi etc) the concept 
of padding and trimming on all bands is described as an imperative for 
tracking. Why Hallicrafters omitted this on band 4 I do not know? But 
the fix should be a fairly easy one. I've succeeded in expanding the 
frequency spectrum over which tracking remains linear but my padding 
capacitors are either miscalculated or installed in circuits with 
interaction with other components that throw off the value of the 
pad...or something else. I'm still scratching my head. Which is why I am 
hoping to locate someone else who has also been scratching their head 
and maybe the two of us will be able to fill in the missing links.

Your suggestions were excellent and I do want to try the toroid 
replacement idea later. However when you wrote "BUT, first make sure 
your tracking alignment is correct" you verbalized my original reason 
for the posting. Indeed, THAT is the crux of the problem. Once I have 
that under control I can "massage" the SX-25 with performance 
enhancements. (Although I have just finished filling and repainting a 
hole cut in one of my SX-25s drilled by a previous owner and have no 
desire to bore any more holes into the panel---it took me a full month 
of daily mixing/adjusting/drying to get the color match alone correct).

Thanks for writing.

My best, WA9VLK