Mac's Service Shop: A Little Dog and SSB Tuning |
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Gimmick capacitor (winding) Do you know what a "gimmick winding" (aka "gimmick capacitor) is? It's been a long time since I have seen or heard the term that describes a twisted pair of insulated wires used to create a very small value of capacitance (~1 pf/inch). They were often found in vintage radio and television sets for use in fine tuning a filter response, interstage coupling, impedance matching, etc. Some service shops and hobbyists would solder in ad hoc gimmick capacitors into circuits to optimize factory sets or maybe to compensate for fixed value components that had drifted in value over time. After achieving the proper value, a dab of wax or cement of some sort is added to keep everything in place. I doubt that gimmick capacitors are acceptable in a high reliability, critical application like spacecraft and military equipment (does anyone know?). Back in my design engineering days I sometime used them in prototypes, along with wire stubs to fine tune amplifier impedance and/or phase matching. Such "gimmicks" are handy because you can easily tack solder them in place just about anywhere to test the effect of a little extra capacitance, without disturbing the primary circuit components. Mac's Service Shop: A Little Dog and SSB Tuning Ad
Barney was muttering to himself and Mac, his employer, knew from experience that this meant the youth was having trouble with his work. Finally the older man laid down his solder gun and strolled over to where the youth was poking aimlessly around the wiring of a little a.c.-d.c. receiver with the noise probe of the signal tracer. "Want to tell Daddy?" Mac asked soothingly. "I want to tell somebody!" Barney exploded. "This cotton-picking set about has me ready to blow my stack. For a while it plays fine; then it starts getting noisy; next it goes dead on all but local stations; finally it will play OK again for a half hour or so. And it's not tubes, for I've changed 'em all," he concluded. Mac cocked a practiced ear at the set that was now in the noisy part of its cycle. "Bad i.f. transformer?" he hazarded. "Nope," Barney denied with conviction. "I tacked in new ones to make sure." "I see it has an r.f. stage. How about the coupling capacitor between it and the mixer?" "Ain't no such animal. This little gem uses a regular tuned transformer for coupling." "Maybe one of the transformer windings is bad?" Barney shook his head. "I was just checking both windings with the noise probe of the signal tracer. When you pass a current through a winding from this probe, if there is the least sign of a break in the winding you get a heck of a racket from the tracer speaker." "Oscillator coil?" Mac suggested as he picked up the diagram. "I checked out the two main windings on that, too," Barney said triumphantly. "Hm-m-m-m,' Mac said as he studied the diagram. "This oscillator coil is a little unusual: it actually has three windings. One is the tuned, frequency-determining winding that has its bot-tom connected to the a.v.c. bus. Then there is this feedback winding between the cathode of the 12BE6 mixer and ground. Finally there is this little gimmick winding that has one end going through a 1000-ohm resistor to the oscillator grid. The other end is free. Apparently they just use it for capacity coupling to the tuned circuit. Have you checked for leakage between windings?" Barney shook his head in a crest-fallen manner and picked up the noise probe. When its leads were connected between the gimmick winding and the tuned circuit winding, a scratching crackling sound came from the signal tracer speaker. "Guess that's the trouble," Mac said as he continued to study the diagram. "When that gimmick winding intermittently short-circuits to the tuned winding, it places the oscillator grid voltage on the a.v.c. bus and biases the tubes so high only the local stations can get through." "I'll buy that, but what do we do about it? This set's an orphan, and we can't 'get an exact replacement; but on that crowded, shallow chassis, an exact replacement is about the only thing that will fit. I hate to try re-vamping the oscillator circuit." "Maybe you won't have to. Try connecting a small mica capacitor, say about a 0.001 μfd., between that gimmick coil terminal and the 1000-ohm resistor. That should furnish a path for the r.f. but block off the oscillator d.c. voltage. Then the gimmick coil can short or not, just as it pleases." Barney carried out this suggestion and the receiver acted perfectly normal. Oscillator tracking was not disturbed and there was no sign of noise. "Why didn't I think of that?" Barney growled. "This must be one of my stupid days." "You would have thought of it," Mac said soothingly. "You were on the trail. I just used my experience to beat you to it. I've had quite a bit of trouble with that same general condition with two-winding oscillator coils in which a mica coupling capacitor between the tuned winding and the oscillator grid becomes leaky and produces the same symptoms. But let's talk about something else. How are you making out with that new selectable-sideband ham receiver of yours?" "Fine, fine," Barney said; "and you know something? I'm finding out that quite a few hams who own this general type of receiver do not know how to tune them correctly." "Yeah," Mac said skeptically. "I know it sounds wacky, but it's true. It's on tuning AM stations they fall down. On SSB, either you tune the thing right or you get nothing but gobbledygook; but you can get some reception of AM stations even though you mistune the set." "Spell it out for me," Mac suggested. "Well, the main point is that you're supposed to receive just one sideband of an AM station at a time with these receivers. That's all you need since the two sidebands ordinarily carry the same information. Now in order to receive just one sideband, the carrier has to be placed pretty exactly on the selectivity curve of the 50 kc. i.f. passband. You don't place the carrier in the center of that passband the way you do with an ordinary receiver; you put it on the low-frequency skirt." "Why?" "Well, suppose this is the 50 kc. selectivity curve," Barney said as he sketched a flat-topped hairpin on a piece of paper. "Now suppose we have the passband adjusted for 3 kc. bandwidth. If we put the carrier in the center, that leaves only 1.5 kc. on either side, which means our high-frequency response will be restricted to 1500 cycles. But if the carrier is put over here about half-way down on the low-frequency skirt of the curve, the sideband can use the full 3 kc. bandwidth, giving us frequency response up to 3000 cycles. There will be some attenuation of the frequencies immediately adjacent to the carrier, but these are very low frequencies that would never be passed by the receiver's audio system anyway." "Isn't it pretty hard to know when you have the carrier in just the right place?" "Not really. There's a simple way of doing it. First, you set the b.f.o. to exactly 50 kc. This is done by tuning in a steady carrier and adjusting the b.f.o. and receiver tuning as you keep flipping back and forth between upper and lower sidebands. When the b.f.o. is set right on the nose, the receiver will stay at zero-beat with the incoming signal as either sideband is selected. From this point on, you do not touch the b.f.o tuning. "Now you are ready to tune in an AM station the way it should be tuned. To do this, you simply turn on the b.f.o. and zero-beat the carrier of the phone signal. Then you turn the b.f.o. off. The station will be heard clearly and the full bandwidth can be utilized by the desired sideband to provide maximum high-frequency response. If you listen closely, it's easy to see that the 's's' can be heard much more easily in speech when the receiver is tuned in this fashion than is the case when it is tuned in the ordinary way. "What's more, you can flip from one sideband to the other without there being a bit of difference in reception, providing, of course, there's no QRM on one side or the other." "What if there is QRM?" "That's one of the beauties of setting up the receiver in this manner. If a station comes on near the one to which you are listening and puts a high-frequency heterodyne on the signal, you can usually lose the heterodyne entirely by simply switching to the other sideband. You don't have to touch the tuning when you do this. Neither do you have to retune when varying the selectivity of the receiver. In fact, you can give interference a real battle without ever touching the tuning dial. First you switch sidebands to try to get rid of the interference. In the event that stations are crowding in on both sides of the one you're trying to receive, you can narrow down the bandwidth of the receiver. If a single source of interference still persists, you can finally try to notch it out with the tunable T-notch filter." "Do you always have to use the b.f.o. to place the carrier correctly?" "You do to place it exactly, but after you get on to it you can come pretty close simply by tuning the receiver to the proper side of the signal being received. With my receiver, and a couple of others that are of the same general design, you tune to the high-frequency side of the signal when using the upper sideband position and to the low-frequency side in the lower sideband position. When I am just tuning around, I never bother to use the b.f.o. to set the carrier; but when I get into a QSO, I usually flip on the b.f.o. for a second or so and zero-beat exactly." "Is it always desirable to place the carrier down on the skirt of the passband?" "Practically always. I've found that when you're copying an extremely weak DX station you can sometimes pull it in just a little better by moving the carrier to the top of the passband. You know the same thing happens in ultra-fringe-area TV reception. Sometimes you have to sacrifice optimum normal-signal tuning - and even alignment-to get a picture at all; but these cases are exceptions to the rule." "I'll bet your knowledge of a TV set, which is actually very close to being a single-sideband receiver as far as the picture is concerned, helped you considerably in understanding the working of your new receiver." "You can say that again! In fact, I have a heck of a time trying to explain what is going on in our sideband receivers to another ham who hasn't swallowed at least a little TV theory." "That's fine," Mac said as he picked up his solder gun. "Now let's put some of your excellent grasp of TV theory to practical use. See what you can do with that portable set that has a nasty case of vertical jitter." "Do you always have to be so dog-gone practical?" Barney grumbled as he placed the set on the bench. "Well, Buster," drawled Mac, "I don't suppose I really have to be practical, but just as a matter of record it is your jam and cake I'm thinking about as well as my own bread and butter!"
Posted January 14, 2020 Mac's Radio Service Shop Episodes on RF Cafe This series of instructive technodrama™ stories was the brainchild of none other than John T. Frye, creator of the Carl and Jerry series that ran in Popular Electronics for many years. "Mac's Radio Service Shop" began life in April 1948 in Radio News magazine (which later became Radio & Television News, then Electronics World), and changed its name to simply "Mac's Service Shop" until the final episode was published in a 1977 Popular Electronics magazine. "Mac" is electronics repair shop owner Mac McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant. "Lessons" are taught in story format with dialogs between Mac and Barney.
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