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

[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