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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...
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April 1952 QST
We live in days of plenty of everything. People throw away and stash away items that our parents - and particularly grandparents - could only dream of having available. Even households that have never seen a penny of earned income in decades are overflowing with stuff. Shopping carts in Walmart, K-Mart, and Target are filled to overflowing when I am there with toys, shoes and clothes, electronic gadgets, sporting goods, automotive accessories, pet food (Target has reefers with fresh meat for dogs) and accessories, lawn and garden implements, hand tools, DVDs and Blu-rays, televisions, disposable diapers (lots of disposable diapers), snack cakes and crackers, soda, bottles of - get this - water (that costs as much as soda), ice cream, frozen pizzas and microwaveable dinners, energy bars and bags of candy. You get the picture. People have so much stuff that one of the largest areas of the store is the plastic storage bin section - reserve one for each giant, electric fan-inflated Christmas or Halloween lawn ornament. This mid-twentieth-century tongue-in-cheek anecdote is not so far from how many - maybe most - people lived at the time.
How to Make Use of Leftovers
By Robert G. Seymour, * W9WJS
In this little yarn, W9WJS presents a graphic picture of the problems of a ham who is low on cash and yet isn't sure just how far he can go in making substitutions from the junk box. After reading it, perhaps you'll find that you can build that new piece of gear after all.
A recent article1 in QST described in some detail the things to be taken into consideration when building a transmitter. While the information given was very useful, it did not treat what is often more of a problem to many of us; that is, how to stretch what we find in the junk box to cut the cost to a minimum.
Any transmitter I build is a modification of some original circuit taken from a handbook or magazine article. Let us consider the procedure after deciding to build the same transmitter which was described by Goodman in the original article. We have decided that this rig is the one that will most closely fit our needs and the one whose parts we most likely have on hand. We like the idea of including a crystal switch so a look through our stock of switches reveals that we have one that will do, but it has only three contacts. So, holding our breath we look in the tin can where we keep our dimes and quarters which we laughingly call "radio money." Having already looked in the catalogs we find that the money on hand is enough to cover the cost of a switch, providing no other new parts are required.
Laying aside the question of the switch for the moment, we go on to the other parts requirements. Small parts, such as resistors, are already on hand. Of course, they have been used in numerous other pieces of equipment and the pigtails are a little short, but that problem can be taken care of by soldering on extensions. The values may not be just what the circuit calls for, but with a little luck we can get by with a few substitutions if they are fairly close. For instance, if we do not have a 47,000-ohm resistor a 56,000-ohmer might do. The only way to find out is to try it in the circuit. If the circuit calls for a 0.005-μfd. capacitor and one cannot be found in our stock of parts, a 0.004 will do - we hope. At any rate we can forget about the small parts until the rig is built and tested, then, if it does not perform the way the article says it should, we can start to suspect our substitutions.
Now we compare the larger parts listed for the circuit with what we have in the junk box. This rig requires two variable capacitors, both 300 μμfd. After a little searching one is found, but it is only 150 μμfd. However, by winding the coils properly, this can be used on 80 and 40 meters, which are the two bands to be used, anyway. But we still need another one. Shall we use the money to get a variable capacitor and forget about switching crystals? Well, let's wait a while before deciding and go on to the other parts required.
What other parts do we need? Well, the circuit shows a 150-ma. meter. Now the only meter that does not already do constant duty is the one in the grid-dipper. Of course, that one has only a 1-ma. scale, but we could use it by winding a shunt for it. But now comes another problem. We do not have the coils specified in the diagram and will have to wind our own, and it will be much easier to wind them if the grid-dipper is in operation, especially since the capacitor on hand is not of the value specified. So we will dispense with the meter and use a pilot lamp instead. We know we are not going to exceed the FCC power limitation with a 6L6 and a pilot lamp will tell us when the circuit is tuned to resonance and tell us if the antenna is loading the circuit.
Now the only thing left is a couple of r.f. chokes, and, lo and behold, here is a brand-new one. Wonder where that came from - must have swapped somebody something for it. That leaves only one choke that we need. Now let's see - there were a couple of chokes in that portable rig that was built to take on vacation last summer. Won't be needing it again until next year and, anyway, if this rig works OK, we can take it on the trip. So now we get out the destruction tools, a pair of cutters and screwdriver, and go to work on the portable. What's this? Here is another variable capacitor, and just what we need. We do not know what the capacitance is but it tuned 80 in this rig so guess it will work OK in the new one.
We can now go back to the problem of the crystal switch. Everything else has been taken care of and the money is still in the tin can. Just how advantageous would it be to have that switch in the circuit? Well, with the switch we can QSY in a second or two unless we make a big change in frequency. If we do that we would have to retune the final and antenna tuner which would take - oh, maybe ten seconds. We could probably change crystals by hand in eight or ten seconds, and another ten seconds to retune if necessary. Of course, this is assuming that we can find the crystals, so we would have to make a point of keeping them handy. So, by using the crystal switch, we can save a maximum of ten seconds or so. Is it worth it? I don't think so. Besides, we will need a couple of beers while sweating out the actual construction, and the "radio money" would come in very handy for that purpose.
The next problem is how to fit the parts in and around the holes already on the chassis we found in the junk box. It would be nice to start out with a clean chassis, but we are already using parts that have been used and reused, so why change now?
The thing to do is to choose the chassis with the fewest holes, or the one with the holes most nearly in the proper places. Having done this, we begin. But wait - how about a panel? Let's look back at the article and see what has to be mounted on the panel. Just two tuning knobs, a meter and a key jack. Remember, we have decided not to use a crystal switch, so that will not have to be mounted. We are not using a meter, so the only things left are the two knobs and the jack. The jack and the pilot lamp can be mounted on the lip of the chassis, and the knobs can be mounted directly on the capacitor shafts. So we don't need a panel, and besides, we would have to do without a couple of beers if we wanted to buy one. No panel.
Now, using the available holes in the chassis, we start laying out the rig in the way it is to be constructed. Following the accepted practice of making all leads as short as possible, we can use most of the holes. If a hole does not line up where we want it, we can punch another one - one more won't make any difference in the appearance. But we try to use as many of the existing holes as possible because it saves a lot of work. We want to keep away from making any more large holes because we would have to borrow a socket punch, which, of course, we do not have.
Because we have used this system of utilizing existing holes, we do not have to worry much about the placement of the small parts. They will just have to go in the space available. One thing we must try to do, however, and that is to avoid placing the parts in more than one layer. The reason for this is that, if when testing the rig a change has to be made, the parts are much easier to get at if they are in only one or two layers.
Before turning on the power we check our wiring against the diagram to be sure we have made no mistakes. Remember that we have made some substitutions and that what may appear to be a mistake in wiring may not actually be so. Now we plug in the oscillator tube and crystal and turn on the power. Listen to the receiver on the frequency of the crystal. If we hear the signal from the oscillator, we plug in the 6L6 and the final tank coil. After a suitable warm-up we should get an indication on the pilot lamp. But the lamp does not glow. Well, let's try another 6L6 first. Oh, oh. We don't have another. Well, temporarily we can swipe the 6F6 out of the receiver. Now everything is fine, except the pilot lamp will not show a dip in plate current. So now we get out the grid-dipper and find that the frequency of the tank circuit is too low, so we remove a few turns from the coil and try it again. Now we get a dip in current and are ready to connect the antenna. The antenna seems to be loading up properly, so the only thing left is to check the quality of the signal in the receiver.
But now the receiver is inoperative because the output tube is being used in the transmitter. Let's see, the d.c. receiver upstairs has a pair of 6V6s in the audio, and we could probably borrow one without anyone knowing about it. So we get one of the 6V6s and put it in the transmitter, return the 6F6 to the receiver and check the signal. Everything seems to be working fine.
So now we have a new rig ready to try on the air and the beers that we bought with our "radio money" helped us get through the construction and testing without too much trouble.
1 Goodman, "How to Lay Out a Transmitter," QST, July, 1951.
Posted July 12, 2016