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Mac's Radio Service Shop: Dog Day Discussions
August 1952 Radio & Television News

August 1952 Radio & Television News
August 1952 Radio & Television News Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

The Dog Days of Summer run from July 3rd through August 11th. In fact, they begin tomorrow (as of when this is being written). John Frye usually wrote his monthly Mac's Radio Service Shop technodramas™ to correspond with the month - at least the season - in which it was published, so "Dog Day Discussions" is aptly placed in the August 1952 issue of Radio & Television News magazine. This episode dealt with how some TV manufacturers were dealing with the relatively new technology as the sets were being ravenously consumed by householders. Owners in fringe areas with weak signals or signals experiencing a lot of multipath interference were registering complaints about their very expensive (relative to today's cost) television delivering poor performance. One response was to issue retrofit kits to service shops for peaking reception sensitivity while filtering out nearby out-of-band noise. In those days you didn't simply throw out your old TV or radio and buy a new one. An interesting phrase used by Mac when Barney heaped praise on himself for having successfully installed a factory-supplied retrofit kit was, "Let the cobbler stick to his last." According to the World Histories website: "The proverb let the cobbler stick to his last means that one should do the work one is expert at, and not try to interfere in, or do, that of others. The noun last denotes a shoemaker's model for shaping or repairing a shoe or boot. The Old-English word was læste, from last, which denoted a mark or trace left on the ground by the foot."

Mac's Radio Service Shop: Dog Day Discussions

Mac's Radio Service Shop: Dog Day DiscussionsBy John T. Frye

The August afternoon heat was hot and oppressive, but both men in Mac's Radio Service Shop were too busy to notice it. Barney was fussing away at a little a.c.-d.c. chassis on the service bench, and behind him his boss was checking out a TV set that had just been returned to its cabinet. Mac kept switching the receiver from one weak signal to another and listening intently to the sound as the snow came and went on the screen.

"Now that," he finally said as he stepped away from the set, "is my idea of a really successful fringe-area conversion job."

"Let's not break our arm patting ourselves on the back, shall we?" Barney suggested sarcastically as he looked up from the receiver that was very obviously giving him a hard time. "What's so good about that particular conversion ?"

"The fact that it was really engineered in the laboratory of the manufacturer who built the set. This receiver was a good performer in the strong-signal area for which it was designed, but the intercarrier sound became very weak and buzzy when the set was used out here in this ultra-fringe location. When the manufacturer began getting complaints of noisy sound in fringe areas, he did not take the easy way out of explaining that the receiver was not intended for fringe-area use; instead he set his engineers to the task of seeing what could be done to help technicians clear up this condition in the field.

"The result is a kit that comes with all needed parts and is complete with detailed diagrams, drawings, and step-by-step instructions for performing some thirty operations on this particular chassis that will result in bringing up the sound without, in any way, impairing the picture."

"Thirty operations! Whew! That must take a lot of time and cost the customer a lot of dough."

"On the contrary, and that is why I like the conversion job. All of the time-consuming cutting-and-trying and de-bugging has been done by the engineers. The technician has only to follow the action-by-action instructions which are so clearly prepared that he can do this at a gallop. The complete rewiring job can be done in a couple of hours at the outside, and another half-hour is ample for the required realignment. The important thing, though, is how well it works. Notice that right now the picture is so snowy you can hardly make it out, yet the sound is clear and strong. Before, on a signal that weak, all you would be getting from the speaker would be a loud roar. When the customer gets this set back, he is going to have to quit calling it his 'Pantomime Special.'"

"What kind of changes did you make, anyway?"

"Put in two new traps; changed the loading resistors across the i.f. transformers; cut the first i.f. tube off the a.g.c. bus; put a new sound i.f. transformer in place of the former inductance-capacity coupling arrangement; and then realigned the video and sound i.f. channels and reset all traps."

"I gather you are not very enthusiastic about technicians working out their own methods of souping up sets for fringe-area reception." "If by 'souping up' you mean making unauthorized circuit changes or radical changes in alignment, you are right. It is not that I question the technicians' knowledge, for many of them really do have a sound grasp of both theory and practice; but practically none of them have the kind of equipment needed to do design engineering work. Modern service equipment is good, but it makes no pretense of equaling laboratory equipment; yet without such precision instruments it is impossible to measure the exact effects of a particular circuit change. When a technician is working without the original performance data and when he has only service instruments for doing his measuring, it is very, very easy for him to cure one particular fault but to produce two other faults in the process. You and I have seen some cases of that come into the shop.

"So-o-o," Mac went on, "I think the technician will do well to follow the advice of the old adage: 'Let the cobbler stick to his last.' I know it is a great temptation to him to display his knowledge and ability by completely reworking a television receiver, and by doing so he can often build up a cheap local reputation for himself as an electronic wizard who knows 'more about a TV set than the men who built it'; but no matter what the layman believes, there is often a great and important difference between changing a set and improving it, especially when accurate equipment for checking the complete results of any change is lacking.

"Personally, I am content to follow the manufacturer's recommendations with regard to changing the original design. If I spot what I believe is a weakness or shortcoming in that design, I do not hesitate to call it to the set-maker's attention and ask for advice in correcting it. Invariably I receive a courteous answer that usually includes specific instructions for straightening out the trouble. I have confidence in those suggestions because I know they have been worked out by men who make design their business, who know the receiver as they know the backs of their hands, and who have thousands of dollars worth of precision equipment for checking every move they make."

"Yeah, but aren't you afraid that people will say the guy who cooks up his own changes is a better electronics man than you are?"

Mac's eyes twinkled in a grin as he answered, "That won't seem so important when you get a little older. I don't want to be known as a good 'electronics man.' I just want to be a first-class technician whose primary job it is to see that radio and television sets keep playing as well as they did the day they left the factory. Maybe the frustrated-engineer type of technician feels apologetic about being 'just a technician,' but I don't. I think the service problems I solve require just as much intelligence and knowledge of practical theory as do the design problems worked on by the engineer, but the schooling, technique, and equipment that each of us uses is different. It is like the difference between the obstetrician and the general practitioner: one brings the babies into the world, and the other keeps them in it; and who dares say which has the more important job?

"You know it's a kinda funny thing, but almost every technician has a favorite story about a case in which a design engineer tried his hand at servicing and was a miserable flop; yet nine out of ten of us secretly fancy that we could give the engineering boys some pretty sharp suggestions about their job if we were just a mind to."

"Well, you're not talking to me," Barney said as he turned back to the little set on the bench. "I'm about ready to admit that I'm a complete idiot who knows nothing about either design or service. I've spent a half a day on this stinker and have not helped it a bit."

"What's the beef?"

"It picks up ignition noise. Noise from a car as much as a block away will ride right in over a broadcast station as though you were listening on ten meters. In every other way the set is normal. Sensitivity and selectivity are good. All voltages check right on the nose. I've put in a whole new set of tubes."

"Did you use the signal tracer to see where the noise seems to begin?"

"Yes, I can first hear it on the grid of the i.f. tube. That makes me think it starts in the 12SA7 mixer. Of course the oscillator rush drowns out everything else on the plate of that tube."

"Got any theories?"

"None that seem to lead anywhere. I keep wondering how the set could be receiving a broadcast station on around a thousand kilocycles and at the same time be sensitive to ignition noise that likely peaks up around fifty megacycles."

"Try putting a resistor of about 1000 ohms in the lead from the oscillator grid to the oscillator tank circuit," Mac suggested.

Barney's diagonal cutters were at work. almost as soon as Mac finished speaking, and the resistor was clipped in place. A pleased grin spread over Barney's freckled face as he tuned across the band without hearing any sound from the cars that were passing up and down in front of the shop.

"Give!" he commanded his boss.

"Well I'll confess that I did not just whip up the answer to that one right now," Mac said. "I sweated over just such a problem for a whole day before getting the answer. The trouble is a parasitic oscillation in the oscillator grid circuit that peaks up somewhere around thirty megacycles. The ignition noise mixes with this high frequency oscillation and goes right on through the i.f. system along with the signal produced by the beat between the regular oscillator and the broadcast station. That resistor simply serves as a parasitic suppressor."

"Well I'm certainly glad that is out of the way," Barney commented. "Now I can get started on putting a new power transformer in that console."

"Are you sure it needs a new transformer?"

"Can't you tell by the smell?"

"I can tell the transformer has been overheated, but that does not always call for a new transformer. Those jobs will often take a lot of punishment. Pull the rectifier and check the voltages of the two filament windings. If both are normal, let the set run for a few minutes with the rectifier out of the socket. Listen carefully for any frying sounds coming out of the transformer and notice if it heats up. If either of these symptoms is present, you are safe in telling the customer he will have to have a new transformer; but if it runs cool and makes no noise, find out why it got hot before.

"Usually you will find a short in the "B-plus" circuit that is causing a heavy overload on the transformer when the rectifier is in its socket. Cathode type rectifiers often develop cathode-to-plate shorts that cause transformers to overheat quickly. Filament leads sometimes become shorted, especially around dial lamp sockets or where a 'hot' filament lead passes around a sharp-edged chassis projection or right where these leads issue from the hole in the transformer shell. That is why I wanted you to check for low filament voltages, indicating a possible short, before letting the set run.

"Once the cause of the overloading has been located and removed, put the rectifier back in the socket and let the set run for at least a couple of hours, noting carefully if the transformer overheats. Remember that most transformers eventually reach a temperature where the hand can barely stand the heat. If everything seems normal, do not replace the transformer. It may not hold up, but the odds are that it will."

"How come so many transformers go bad in the summer time?"

"The hot, humid weather is an important factor. When the surrounding air is around ninety degrees, the transformer has more difficulty in getting rid of the heat generated inside itself. Then, too, lightning often comes in over the power line, even jumping an open switch, and breaks down the insulation between adjoining turns. Once a transformer has shorted turns, it will heat up rapidly with or without external load and must be replaced.

"Whether you put in a new transformer or not, be sure and clean off any of the sticky mess that runs out of a hot transformer onto the chassis and cabinet. If you don't, a strong, pungent odor will be given off by the set as soon as it gets warm, and this may last for weeks.

"If you do have to replace the transformer, do your durndest to get one with the exact voltage windings as the original and also with the identical current ratings or just a little higher. Do not go all out on the 'safety factor' and try to use a transformer with much heavier current ratings than will be needed, for this will mean that voltages in the set and cost to the customer will both be higher than necessary. Always fuse the primary of a new transformer. A two-ampere fuse is about right for consoles, and a three-amp fuse will carry most TV sets. Putting in this fuse is just good insurance on your service job. A new transformer is a pretty costly affair to the customer, and he is going to be very unhappy if he has to repeat the operation as long as he has the set. I often drill a nest of half-inch holes in the cabinet shelf directly beneath the transformer to give it added ventilation -"

"Can't we talk about a cooler subject," Barney broke in plaintively, as he wiped the sweat from his forehead with his handkerchief.



Posted July 2, 2021

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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