5-tube superheterodyne radio kits- your experiences?
How many of you out there have built 5-tube superhet radio kits sometime in your past? If so, what were your experiences like? Did you find the instructions easy to follow? How well did the radio work after you have built it? If there were any problems, what were they? I'd be intersted in knowing.
I have built one from Antique Electronics Supply about 15 years ago. It worked halfway decent, but it seemed that no matter how well I tried to do the alignment, it would tune well and accurate at the high end of the dial, but the low end of the dial was not so great. Stations more toward the lower end of the dial (and the middle, but less-so), would tune in "too high" on the dial scale (in other words, a station on 660 KHz would tune at about 750 on the dial scale, a station on 880 would tune at about 920, but yet a station at 1240 might tune in at about 1260 on the dial- closer, and 1600 would be dead-on).
Then, if you tried to tune too low on the dial, you would get a strong and increasing motorboating sound, increasing to a near howl, the lower you would turn the dial.
Interestingly, I think it seemed like they had either a different oscillator coil or a different variable tuner capacitor than what seemed to be the "original design" parts.
Now that I know more about radios than back then (though, not that much more), let me know if my theory of what was happening sounds correct.
I think the oscillator coil, not matching the design of the tuner oscillator section correctly, had too much inductance. This caused the dial to have to be set higher on the scale (and hence the oscillator tuned circuit was requiring less capacitance to compensate for too much coil inductance) to get the proper osc. frequency to convert the station frequency to the IF. The lower you tuned on the dial, the lower the sensitivity the radio had, due to poorer and poorer tracking between the oscillator and antenna sections of the ganged tuner. The high end of the dial worked quite well, because the inductance part of the tuned oscillator circuit becomes less and less significant. The oscillator trimmer (adjustment on the tuner) was able to adjust for the high end of the dial (1600) .
The motorboating likely occurred at the low end of the dial because the "tuning point" of the radio approached the IF frequency of 455 KHz. Since 540 KHz was coming in considerably higher on the actual dial, one can imagine that the radio, under this condition, might "tune to" the IF frequncy, 455 KHz, and cause all kinds of feedback between the IF circuits radiating back into the antenna.
Let me know if this theory sounds right?
Darn, if I only had that kit now, I would try replacing the osc. coil with a "universal" type with adjustable inductance slug, and try to adust the slug for best performance between dial accuracy/sensitivity at 600KHz.
Tell us how/why your old AA5 tube radio kit was fantastic, or else maybe turned out poorly!
Better tracking at the low end of the dial can often be achieved by bending the outer plates of the oscillator section of the tuning cap. (This goes for low-end RF tracking as well.) This is why you will see that some caps have their outer plates "sectionalized" so different amounts of bend can be done depending on the dial position. It is also important to have the IF's aligned to the exact frequency.
I think your motorboating theory is correct and is due mainly to inadequate power supply decoupling or shielding between the mixer and IF stage.
I built a Knightkit "Star Roamer" shortwave kit back in the late '60's that had all the problems you described. This was a great learning experience. The set finally worked well after much tinkering.
Back in high school everyone in our radio/tv repair course had to build a 5-tube superhet as part of our classwork. I don't think there was one working radio out of the bunch.
"AC volts and DC volts and little lamps and lities..."
Back in the mid 60s I built the AA5 kit Allied sold with the octal tubes... Was a long time ago, but it seemed to work OK... Unfortunatly I loaned it to a freind a couple of years later, and his dad sold it at their antique shop... Otherwise I'd probably still have it...
I built a Knight kit AA-5 back when I was in high school, and it worked fine, without tracking or motorboating problems. Two thoughts: 1) Lead dress and placement are important to prevent unwanted coupling and to minimize hum. Unshielded signal carrying wires should be kept as short as possible, not run parallel to other signal carrying components, and, if possible, kept close to the chassis. 2) Station tracking is directly related to IF frequency. If the IF is aligned to too high or low a frequency, the radio may play well, but tracking will be off. I aligned mine with a signal generator, and tracking was perfect.
There was one on ebay recently. .
I put in the first bid, and went as high as $35, although I knew it was likely I wouldn't win at that price. I simply wasn't willing to pay more than that for it, although I figured somebody would. Sure enough it went for over $120.
Anyone ever do business with radioantiques.com?
In my neck of the woods, kit radios were fairly common, they were built by people trying to make an extra buck and some of them were professionals radio builders. I have a bunch of schematics showing the construction details and all of them include a "recommended chassis layout" in order to avoid unwanted effects. The connections to the BCB/SW switch were also shown and it was recommended to do them with stiff wire.
These kits were basically the "coil set" (BCB oscillator, SW oscillator, antenna coil and the two IF transformers) and then you could purchase the chassis and cabinet or your choice separately.
Over the past year or so I have picked up 3 kit radios that were (more or less) completed, they are all AA5's similar to the S5C shown in Rider's 17-1, they all restored satisfactorily.
I had good luck building an EICO FM tuner kit. FM 90 I think. Had a lighted exclaimation point for tuning indicator and pointer.
It's not an AA5, but AA5's were sort of passe the time I was ready to start learning electronics. When I was about fourteen or fifteen, my Dad bought me a Heath FM tuner/amp for Christmas. It worked quite well right off the bench, but Heath kits were pretty easy to build. I still have it as well as the manual for it. That same year I built a pair of speakers out of the "Philips" "How to build speakers" book and the setup won me a prize in the high school talent fair. Somewhere (if I can find my box of pictures) I have a photo of me with my setup and my prize ribbon. I don't have the speakers anymore
Built a knight Ocaen Hopper 740 regenerative in my jr high years. It used 3 of the AA5 tubes and worked well, easy to build. It came with 5 factory coils. One BC the rest short wave.