Mac's Radio Service Shop: Barney's Dog Show |
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Whether you are a DIY'er, tinkerer, hobbyist, experimenter, roustabout, odd-jobber, handyman, jack-of-all-trades, innovator, or any sort of electronics dilettante to any degree of adeptness from mere abecedarian to seasoned professional polymath, chances are you can learn something useful from just about all of John T. Frye's "Mac's Radio Service Shop" techno-drama stories. They appeared on a monthly basis in Radio & TV News magazine, and then continued when it became Electronics World in 1959. The varied collection of "dogs" that piled up on Barney's bench while Mac was on vacation provided a convenient excuse to touch on a wide variety of equipment types, from record players to radios, including sets that had fallen victim to "home-talentitis" at the hands of a not-so-capable owner. Mac's Radio Service Shop: Barney's Dog Show
For three whole days Barney had been operating Mac's Radio Service Shop all by himself while his boss took a brief vacation, and he was so swollen with self-importance that he could hardly stand himself. On this morning that Mac was due back, he had come to work a whole hour early to make sure that all tools and instruments were in their proper places, that the place was swept out and dusted, and, above all, that Mac would find it open and ready for business when he came down to open up. It was well that he did, for Mac himself showed up a half hour early. "Good morning, Flamingo Top," Mac saluted his helper. "I see we are still in business, anyway." "Natch!" Barney replied with a grin. "Your public never missed you. Have a good time?" "Swell, but I noticed a kind of funny. thing. There are times here when I feel I'd be blissfully happy if I never saw the innards of another radio or TV set; but I hadn't been away twenty-four hours until my hand began to sort of ache for the feel of a solder gun. The wife almost had to use her whip to keep me away from. a TV set with a bad case of vertical jitter in one of the motels where we' stayed. I'm sure I could have fixed it if she would have stopped mumbling, 'Remember, you're on vacation!' " "I was sure that would be the case," Barney said smoothly; "so that is why I saved this dog show for you." As he said this, he waved to a side bench on which several sets were piled; "You know you told me not to waste too much time on any 'dogs' that showed up; so any set that I couldn't make fly right within an hour, I put over there. Of course, I could have fixed them if I had taken the time, but I knew you were right about keeping up with the work." "Of course you could," Mac agreed without the sign of a grin. "Let's have at them." "Well first is this little 45 rpm record player with a funny case of hum. It's bad enough to be really annoying when a record is playing; yet it stops when the motor is switched off. My guess is that the a.c. from the motor field must be getting into. the amplifier some way. Listen to it." Mac cocked a critical ear for a minute and then remarked: "I don't hear the hum when the changer cycle is operating." "No; but I supposed that was because it might have a muting switch that shorted out the amplifier input while the records were being changed. Some of the changers have that arrangement, you know." "Not this one," Mac said as he picked up the compact little changer and started taking screws out of the Bakelite case so he could lift off the changer mechanism. "I've checked the filters, changed all tubes, looked to make sure the shielded lead to the cartridge is not shorting out or developing a poor grounding connection of the shield." Barney offered. By this time Mac had the changer mechanism off and was peering at the turntable drive mechanism. Suddenly he switched on the motor and carefully inserted a screwdriver bit against the revolving side of the motor shaft that drove a rubber-tired idler pulley. Then he replaced the changer on the case and set the pickup arm down on a record. There was not the least trace of the annoying hum. "The trouble was mechanical," Mac answered the questioning look in Barney's eyes. "There was a bit of hard dirt or gum on one side of the motor shaft, and every time this came around it gave the idler pulley a bump. The motor travels at a speed that does make the frequency of these bumps sound about like 60 cycle a.c. when the vibration is picked up by the needle resting on a record. When the change cycle picked the needle off the record, the vibration no longer reached the pickup and so the 'hum' stopped. I scraped off the gummy substance with the screwdriver bit held against the revolving shaft. Trot out your next dog." "Here's one that just won't align," Barney explained as he set a small a.c.-d.c. chassis on the bench. "Something is apparently wrong with the oscillator coil. Since it has 456 kc. i.f., I figure the oscillator ought to be setting on 2056 kilocycles when the set is tuned to 1600 kc.; but I've checked with the grid dip meter, and it only goes up to about 1800 kc. with the oscillator trimmer screwed clear out. By screwing this trimmer clear in, you can actually throw the oscillator over on the low side of the signal at the high end of the band; but then it won't track, of course." "Where does the oscillator sit at the low end of the band?" "Still a little low, but not nearly as bad as at the high end." "That sounds more like too high minimum capacity rather than inductance trouble," Mac suggested as he carefully examined the tuning condenser gang. Suddenly he picked up a small screwdriver and worked with it on the back side of the stator of the oscillator section, the opposite end from where the trimmer was mounted. "Now let's see if we can align it" he said as he switched on the signal generator. When the generator was set to 1600 kc., the oscillator was easily brought into line with its trimmer. The r.f. trimmer was adjusted at 1400 kc., and then the generator was moved to 600 kc. When the dial pointer was set at the 600 kilocycle mark, there was the generator signal. "What the heck did you do?" Barney demanded. "The oscillator section of the tuning condenser has two trimmers, one on each end of the stator," Mac explained. "You find that situation every now and then. The rear trimmer is set before the condenser is mounted and is not intended to need adjustment after that, for the front trimmer is supposed to take care of all ordinary conditions. That is why this back trimmer is practically inaccessible and easy to overlook. In this case, however, something made that back trimmer increase in capacity. Perhaps a trimmer plate was caught on a thread and suddenly slipped off; or maybe a little wax was between the plates and gradually softened and let the plates come closer together. Anyway, after this back trimmer increased in capacity, you could not get the minimum capacity low enough, even with the front trimmer screwed clear out. I managed to back the adjusting screw of the rear trimmer off a couple of turns, and then everything was all right. What are you looking for on that set?" he broke off as Barney scrutinized another little set he picked off the bench. "I'm just making sure this one has no trick trimmers," Barney muttered. "I can't make it track, either. In this case, though, the oscillator tracks with the dial all right, but the r.f. section will not track with the oscillator. If I peak up the r.f. at 1400 kc., it is way off at 600 kc., and I have to back the trimmer clear out there to get any sensitivity at all. The i.f.'s are right on the nose, and I can't see a thing wrong with the loop antenna. What do you make of that?" "Hm-m-m-m, it sounds as though you have too much capacity on the low end of the band, doesn't it?" Mac reflected. "Apparently when you tune from the high end to the low, the capacity of the loop-tuning portion of the condenser increases faster than it should. I see you've bent the outside rotor plates out as far as you could." As he talked, Mac was carefully looking at the tuning condenser. Once more he picked up a screwdriver and carefully pried sideways on the end of the stator section of the r.f. tuning condenser. The set instantly came to life on the low end of the band. The bent-out rotor plates were returned to their normal position and the r.f. trimmer was adjusted at 1400 kc. Then the set was tuned to 600 kc. Moving the r.f. trimmer screw in either direction now made the output fall off, indicating that the r.f. circuit was tracking perfectly with the oscillator. "The stator section of the r.f. tuning condenser had shifted," Mac explained, "so that the rotor plates no longer were centered between the stator plates. In fact, when I looked at the closed condenser, it looked as though the plates were nearly rubbing. This reduced spacing will produce a whopping increase in the condenser's maximum capacity. I simply pried the stator plate mountings over until the rotor plates looked evenly spaced between the stator plates, as they should be. Incidentally, I might mention this happens pretty often. In fact, whenever the r.f. section refuses to track with the oscillator, the first thing to do, even before you start bending plates is to see if something like this has not happened." "Well, so far you've straightened out everyone of my puzzlers with nothing more than your little old screwdriver," Barney answered; "but here's one I'll bet will make you use something else. When this set came in, it had a very bad case of home-talentitis. A screen bypass had gone out, and then the owner proceeded to turn every trimmer he could find before giving up and bringing it in. There are a lot of trimmers, too, for the set is an AM-FM combination. I quickly located the bypass and replaced it, and then I realigned the set. Had a heck of a time doing it, too, for the trimmers were so far out that the generator would not shove a signal through until I fussed around with the adjustments quite a bit. Finally I got it aligned, and it seemed to be working all right until I tuned in a strong station. Then it distorted pretty badly. It does the same thing on every strong station, although it sounds OK on all FM stations and on weak AM stations." "What checks have you made?" "Naturally I decided something was overloading. Since the setting of the volume control had no effect on the distortion, I reasoned the overloading must be taking place in an r.f. or i.f. stage. That sounded like something was wrong with the a.v.c. circuit; so I checked the a.v.c. bus, and there are no leaky condensers anywhere along it. Thinking one of the tubes might be gassy, I tried all new ones without this making any difference. Next I got out the signal tracer and went through the set. The distortion definitely begins in the last i.f. stage. This stage, since it doubles in brass as a limiter on FM, does not have any a.v.c. applied to its grid; so it would be easy to overload with a strong signal. Listen to how it acts." Barney ran the signal generator into the antenna connections of the receiver. As he gradually increased the output of the generator, the 400 cycle tone coming from the speaker rapidly rose in volume to a certain level; then it actually decreased as the generator output kept on going up. "What does the a.v.c. voltage measure?" Mac wanted to know. "It comes up to about three or four volts on a strong station, which does not seem like enough to me," Barney replied. "Still, there's nothing wrong with any part of the a.v.c. circuit that I can find. Here's one peculiar thing that might be a clue: when I take the r.f. probe of the v.t.v.m. and go down through the set, the r.f. voltage keeps increasing steadily until I come to the secondary of the last AM i.f. transformer, and there it takes a big dip. I'd expect some step-down, but when I have fifteen volts across the primary of this transformer, there are only about two or three volts across the secondary. Do you suppose something's wrong with the transformer?" Without answering, Mac picked up an alignment screwdriver. "Oh no, not that again!" Barney groaned. Mac hooked the signal generator across the input of the i.f. amplifier and set it to the 456 kc. frequency specified in the service data pasted on the chassis. Then he began to turn an i.f. alignment slug with his screwdriver. At first the signal heard in the speaker took a slight dip and then began to rise steadily. He reduced the generator output, peaked the trimmer, and disconnected the generator cable. Now when the set was tuned to a strong local station, not a bit of distortion was present. "That was a peculiar one," Mac conceded, "and it all came from the set's being so far out of alignment in the beginning, As you were trying to force a signal through it, you probably happened to align from the front end back - almost always a bad practice. By the time you reached the last adjustment, the final i.f. stage was badly overloaded and operating in a non-linear fashion. Because of changes produced in the badly distorted modulated envelope delivered by this stage to the detector and a.v.c. rectifier, tuning the final slug in the proper direction produced more a.v.c. voltage and so decreased the signal fed to the overloaded stage, but it also actually caused the audio signal delivered by the detector to decrease. This led you to set the tuning slug far out of alignment, and so the set could not produce sufficient a.v.c. voltage to keep the final i.f. amplifier from overloading. Don't take it too much to heart, though; that could have happened to anyone." "I don't mind that so much," Barney muttered darkly; "but I'm just wondering whether or not I like the idea of working for a screwdriver mechanic!"
Posted May, 22 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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