Sure, this receiver is not "state of the art." But that's the whole point. I wanted to finish the receiver project that I couldn't finish back in 1976.
I tried to stick as close as possible to the original design and parts. NORCAL came up with an updated schematic in 1998 with parts that are more readily available. But Designer Jay Rusgrove was shooting for something that could be built with all the parts coming from Radio Shack. I think that is probably one of the factors that attracted me to the project way back when. That's why Jay went with varactor tuning (no hard-to-get variable caps!). And that's why he used coils that were wound on Radio Shack 10uH RF chokes (no need for hard-to-find toroidal cores). In this sense there is some common ground between the BITX rigs and the Herring Aid 5.
I stuck with the RF-choke as a coil idea for the VFO, but went with the NORCAL-prescribed toroids for the front end and mixer coils. (I may go back and try to use chokes in these circuits, but I'm not sure my junk-box will yield the kind of RF chokes that Jay used).
I wish I had known a few things when I was building this back in 1976: More knowledge about how to wind the coils would have been a big help. I wish I had realized that I could use a SW receiver to get the oscillator on the right frequency. I guess this was in the days before Ugly and Manhattan building techniques, but it would have been nice to know that there was no need to actually etch a board for this project (I did!).
The coils really are a bit tricky. Jay didn't use any trimmer caps, so I guess you had to just hope that the front end coil and cap resonated somewhere near 40 meters. As for tuning the oscillator, Jay recommended scrunching and un-scrunching the turns on the RF choke. Yikes! Give me some trimmer caps!
I also found that you have to watch the level of the RF going from the oscillator to the mixer. Too much, and the receiver is deaf. Too little, same result. You need to experiment a bit with the number of turns on the pick-up coil from the oscillator.
The warnings about the pitfalls of that single BJT mixer were right on the mark: Lots of AM SW breakthrough. But I kind of like the background music. Strong RFI from local FM broadcast stations was another story (WMZQ is a country music station!). I reached into my junkbox and found a low-pass filter from a Heathkit DX-60. I just put that between the antenna and the receiver and the country music was GONE!
I really love this little receiver. I have it playing 40 meter CW as I type. It sounds great. I feel the urge to built a Tuna Tin 2 and put both of them on 40.
In the original Tuna Tin 2 article, Doug DeMaw notes that Jay Rusgrove was thinking of doing a companion receiver and says that he was thinking of calling it the "Clam Can 5" ! There were jokes about receivers for hams with "tin ears" and about there being "something fishy" about these rigs.
Thanks to Doug DeMaw and Jay Rusgrove and QST for bringing us these little circuits.
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I wasn't saying you shouldn't have built it, just giving historical context. There were endless articles back in the early seventies about improving direct conversion receivers, and nothing seemed to be a significant change, until later, when it seemed like the solutions were all in the wrong direction.
ReplyDeleteIt's not unlike superregenerative receivers. Whatever was known in the early days was consolidated into a brief paragraph in the handbook by the sixties. A black box, the description didn't really say much. Yet in the sixties, lots of articles appeared about making superregens (and sometimes specificall7y the Heathkit Lunchboxes) narrower. And it was only by degree that any change happened. Then Charles Kitchin went back, looked at early material, converted them to solid state, and realized that by playing with the shape and size of the quenching that they could be narrowed up. Not really a surprise if you see a schematic where the quenching is done with a separate oscillator; then it looks like a regen modulated by another oscillator. Anyone knows that if you feed a square wave into an AM transmitter you get lots of sidebands, and that's some of what happened with superregens. Look at it later from a different angle, and one can apply a new solution that actually does give different results.
Michael VE2BVW
ON a different matter, "Ugly" wasn't formally described till later, certainly didn't have that name.
ReplyDeleteBut it was out there. Even before QST turned to the larger size and started building in tuna cans, I was definitely using circuit board as breadboard. I have no idea if I thought of it, or someone suggested it. It was easy, I had lots of 1/2 resistors with leads that I wouldn't use for "proper" construction, things could be changed endlessly. I was making circuit boards, but I was also using copper circuit board as breadboard.
There was precedence. Circuit board had been shown in articles as a useful thing for tubes. Instead of building a converter on a sheet of metal (and using a chassis as the base) use a piece of circuit board. You'd still punch the holes for the tube sockets (though it was easier) but then all the grounds could be soldered directly the copper. No need to drill holes or find some existing hole for a solder lug. And if you needed to change things, it was easy to unsolder the old, and solder in the new.
I remember an article in QST about that in the late fifties (well I saw it as a back issue in the early seventies). But Frank Jones was definitely using it for his nuvistor then transistor converters in the sixties.
Bill Hoisington, K1CLL, was in "73" almost every issue, and virtually all of his stuff was built on copper circuit board, even a power amplifier. His was generally a hybrid, the board making for easy ground, but often he'd use other bits of circuit board to mount pieces in a more formal style.
The local surplus place had endless circuit board scraps, some even drilled, that provided more than enough copper circuit board for breadboarding, and really cheap.
Just as solid state meant you could add more stages without much cost or additional size, it meant you no longer needed a heavy chassis to hold up the big and sometimes heavy parts. Might as well build easy, and then cover it up with an external chassis that looks swell, than do everything perfectly from the ground up.
Michael VE2BVW