|Why do I get poor performance on the low end of the broadcast band? || |
John G. answers...
This problem is caused by people “tuning up” IF transformers and tuning gang trimmers while listening at the high end, or sometimes “to spec” ; there is no adjustment of low end oscillator or antenna tuning on most inexpensive radios, although one or two RCA from 50’s Golden Throat era had an adjustable oscillator coil, and some rather excellent performing Crosleys of that period show an ability to move the last turn of the loop antenna around to optimize low end loop tuning. What happens when you adjust osc gang trimmer by ear, the trimmer only impacts the top end, and that, by itself, may make things only a little worse at bottom; but main thing is IF’s. If IF’s are set up at high end, by ear, after peaking the osc gang trimmer, they may be at 450 kc or something (off either way) which is what the difference frequency really is after you have messed with osc trimmer , so high end signal gets louder and stronger , However at bottom end, difference may be 457 , you have just lost the bottom end, as IF is wrong freq at bottom.
The right way, if you do it without equipment, is to maximize IF’s at about 700 khz, then adjust osc trimmer at 1400, but dial readings must be good. . Some “experts ‘ will tell you set it all up perfectly to 455khz, ,however you may still be off at bottom end, with a perfect 455, if it isn’t 455 the radio is giving you ! A cynic might say forget about 455, set IF up at 700 for best you can get then match whatever it is at 1400..by osc trimmer. At least you can set the top end afterwards! Related and yet independent of this , antenna loop has to resonate at the same time, at same physical angle of tuning cap as osc + IF wants, as there is no adjustment for that at low end, and antenna may now be off resonance when osc and IF are on. The way to fix that is to increase or decrease that L of loop by moving turns, or even adding or subtracting a turn. It is more critical than you think. But the need to do that to any degree may indicate other problems with osc and IF... Just moving the last turn on the Crosley was enough “trim” . Note that frequency of broadcast station itself is constant all through this, and that is ALL that determines that particular resonance, along with tuning cap angle (Why dial pointer has to be PERFECT!—that is job #1 ) a signal generator can help with finding out about that. ; it is either right or not right on the antenna gang tracking at 700. So adjust antenna L at 700 kHz, either turns on loop, or set slug in oscillator to match whatever antenna gang resonance at cap angle is (if you have osc slug), or set tuning gang angle to dial and real antenna resonance, and then set up IF, then adjust high end trimmers. I have seen literally 30 + AA5 radios set up by “the pros “ that work great on 455 signal generator, have lots of stations over 1200, pick up nothing at 700khz...so have you! Yet they insist they know what they are doing. Why is the radio dead at 700 then, after an expert alignment ?? , when untouched, new, they all worked fine? “they are all like that “ is the typical answer!
I have used the big 7” Amidon rod to replace missing loops, about 55 turns of #20 , and you can fine tune at 700 , by spreading turns ,or stretching like a spring. It is REALLY critical, you get a big strong peak when right . Then epoxy in place. Doing that, I picked up a Toronto station about 600khz(? ) in Boston at noon, something I had never seen before on any radio, including a comm receiver. It was a Belmont push button style art deco, with an RF stage. That also proves loops are quieter than long wires, another topic.
Some radios have bendable wings on the tuning caps, and having worked on a few where those were bent all over the place, straightening them out and resetting as described usually works. That bending is for small errors in tracking, not curing misadjustment ; they are rarely bent at the factory
For radios with an adjustable osc coil, you CAN use 455 and get it perfect; this whole thing may be why you see 456 sometimes etc.. without an adjustable oscillator coil. It works better at the bottom end!
Kind of a lesson here; you stand a good chance of making the radio worse by aligning the IF to a number, as it may have been set up slightly off at the factory to help bottom end . I do not personally believe IF’s change at all, except for junk Zenith and Automatic Electric IF”s with those open flat silver and mica sheets in the bottom ; if you “see it get better” by tuning IF, and IF transformer is physically OK, you are probably losing tracking somewhere else.. So mark it or make a sketch of screws before you move it, at least, so you can go back. And it in fact it may HAVE to be tweaked to get it to 455, but consider it was not on 455 for a REASON , especially if both are off to the same side. .. realize that! Imagine if factory setting was 449 for tracking reasons?
Very few people have “the 10,000 foot view” to see all this, they think they are improving things by tweaking screws beyond where factory left it, while pondering lots of fancy dials and readouts, to get it at 455.000 kc . . Kind of like tuning the timing on a car “by ear,” and swearing by it.. smile!
The only way 455 will show up out of the mixer at 700 khz tuned in on the dial, at full design strength, is if tuning gang tracking , osc coil , antenna coil and antenna cap are all perfect, no error, right on. The trimmers have almost no impact at the low end, so parts have to be precisely made. I don’t think that always happens ; the only variable you have then is IF center frequency, as none of the above are adjustable in general , and which one is wrong is not that easy to figure out. But the next easiest one to change is antenna L, like the rod.
One last thing, many slug tuned IF’s will crack or strip when you try to tune them, especially Zenith; now you are really in trouble, as they are stuck in wax. And now wrong, due to the now missing piece of core. They were probably just fine, per above, if left alone .
One man’s opinon..and my radios do work at 700, even Philco 20’s (TRF) another long story like this one..