Mac's Radio Service Shop: Mac's "System" |
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Reading an episode of "Mac's Radio Service Shop" always has me wishing I had been born a couple decades earlier and had taken the path of running an electronics repair shop. For sure it was no picnic either from the standpoint of needing to keep abreast of constantly changing product designs and finicky customers, but the thrill of the hunt (for the cause of "trouble") and the satisfaction in knocking them out (the "shooting") is something people like us (you, too, I assume, since you're reading this) understand. Back in my USAF days as an air traffic control radar repairman, day-to-day routine system alignments and preventative maintenance could be pretty dull, and most problems were fairly easily resolved in an hour or two. However, every once in a while a real doozy of a case would crop up that would have a full shift or two of fellow technicians agonizing over it until the cause was finally discovered. Often, as with some of the scenarios Mac describes, an intermittent component or connection ended up as the culprit. One time I remember having a particularly bad time with the signal path from the transmitter to the rotating antenna on the roof of the equipment trailer (AN/MPN−14 mobile ASR/PAR radar). We had deployed the system from Robins AFB, Georgia, to Loring AFB, Maine, while the base had its fixed system upgraded. We lowly sub-E-5 non-coms could not solve the issue, so finally a posse of E-8+s were sent to show us how it's done. They did - by changing out the entire waveguide path from the transmitter to the antenna. Problem solved... never did know why that solved it, but it did and that's all we cared about. I'd love to be able to go back with the knowledge I have now and have a chance to apply the accumulated school of hard knocks lessons learned since then. Wouldn't we all? Mac's Radio Service Shop: Mac's "System"
It was really too nice an afternoon to work. The whole early May day had been a sneak preview of late June. Outside the open door of Mac's Radio Service Shop the warm sun beat down on the concrete sidewalk that had been washed clean by a shower the night before and sent shimmering heat waves rising through the quiet air. Only the ever-busy, worm-stretching robins seemed to feel industrious. Inside the service shop that noon hour there certainly was no out-of-tune display of ambition. Mac himself occupied the high stool that was precariously balanced on two legs so that he could lean back against the wall of the service department. Across from him Barney, the apprentice, sat on the service bench with his long legs doubled up and his arms clasped about them so as to form a rest for his chin. "Hey, Mac," the boy finally drawled. "Uh huh," Mac acknowledged. "A guy down at the radio club last night asked me what kind of a system of troubleshooting you use. He looked at me kind of funny when I said I didn't know. What system do you use?" The stool came down on four legs with a thud as Mac straightened up. "That darned question always makes me mad!" he exploded. "A lot of people can't rest until they get any form of activity boiled down to a neatly-labeled little 'system.' The next time he asks, just tell him we use the looking-and-thinking system." "Don't you believe in using a regular method in running down trouble?" "Depends on what you mean by 'regular method.' If you mean working logically from observed symptoms to likely causes of trouble, I certainly do; but if you mean a religious and exclusive following of the voltage-checking system, the resistance-measurement system, the signal injection system, the signal-tracing system, or the touching-the-grids system - which I now see labeled with the hoity-toity name of 'circuit disturbance method' - the answer is a flat and emphatic No." "You don't use any of these?" "I use all of 'em - and a lot of other 'systems' besides. As far as I am concerned, I need everything I can lay my hands and brains on in running down trouble in a receiver. When an intermittent and I go to the mat, it is a fight to the finish with no holds barred. Those devils employ every trick they can hatch up to keep you from finding what ails them, and if you fight fair by following some sort of electronic Marquis of Queensberry rules, they'll tie you up in knots. "Just look at how dirty they play: a mica condenser, that ought to be like Caesar's wife, above suspicion, will become leaky just often enough to escape an investigation until you have exhausted every other possibility. And then there are those sets that behave like angels on the bench but start cutting out just as soon as you have them all bolted down in the cabinet. When you are dealing with fiendish cases like those, it is no time to be hobbled with a single system. You need every method of diagnosis you can conjure up and a strong dash of inspiration besides." Barney drew a careful bead on the doorknob with the solder gun as he asked, "You say you need to use everything?" "That's right. Take the eyes: when I turn on a set, the first thing I do is to look at the rectifier to see if it glows that gorgeous shade that indicates gas or if the plates turn a cherry red because of a shorted filter condenser. "The nose comes in mighty handy, too. One whiff of the inside of a cabinet in which a power transformer has been too hot is all a service technician needs to tell him that he had better do some checking before he turns that set on." "And I suppose you use the master's touch to fix them," Barney suggested. "Modesty forbids my saying 'Yes,'" Mac replied with a grin; "but the sense of touch is really a very useful service tool. It will locate a stone-cold metal tube with an open filament or spot a leaky filter condenser that is running a temperature and should be replaced; moreover, a finger probed lightly around the rim of a speaker with a rubbing voice coil will quickly tell you where it is rubbing and which way it should be moved to free it. "I don't think I need to go into any illustrations of the hundred and one things your ears can tell you about a sick set. If you just know how to listen, it will babble all its symptoms to you. I must admit, though," he said musingly, "that I have never been able to figure out a way to use the sense of taste to help fix radios. I wish I could." "If you forget how the polarity of a bias cell goes, you might try touching a couple of leads from it to your tongue," was Barney's bright suggestion. "I think the positive lead - or maybe it's the negative one - is supposed to taste salty." "I'll remember that valuable suggestion," Mac promised dryly. "Seriously, though, all of the well-known methods of running down trouble have good qualities, and I want you to be perfectly at home with any of them. Quite likely anyone of them, if followed carefully, would eventually spot a trouble; but the service technician can make much better time if he has all of the methods at his finger tips and can change freely from one to the other as he tracks down a receiver fault. Each of the systems should be looked on as a service tool. Just as you pick up first one tool and then another when you are working on a mechanical job, so you should be able to switch from one system to another when you are cornering an electronic trouble." "Let's have a 'ferinstance,' " Barney suggested. "Well, for instance, suppose you turn on a set and start checking the voltage. You find voltage at the rectifier cathode and the plate of the output tube, but there is none on the i.f. tube plate. It is a good idea to turn the set off at once because of the possibility that there may be a short somewhere that will burn up a resistor - perhaps inside the i.f. - or do some other damage. You simply switch over to the ohmmeter and use the resistance-checking method to find out if your trouble is an open i.f. transformer primary, an open plate-isolating resistor, a shorted-bypass condenser, or maybe a shorted tube." "That makes sense," Barney observed judiciously. "Another fault with being married to one system," Mac' went on, "is that it tends to kill a guy's originality. A good service technician ought to be constantly trying to find better ways of doing his work. Just last week I had a convincing demonstration of that. For twenty years I've been trying to cook up a really good way to service those sets that are plagued with a rustling sound. You know what I mean: the kind of sound you get when an i.f. winding is opening up or an output primary is going out or a grid cap solder connection is poor or you have a noisy plate resistor. "Those rustling sounds are produced by small, instantaneous current fluctuations. They are too quick to show on a meter, and you can't go probing around with an ohmmeter without running the risk of temporarily 'healing' the trouble you are trying to spot by running the ohmmeter current through the coils or resistors. Even a signal tracer cannot be used to advantage unless it is of extremely high gain and very, very quiet. "The other day, when I had one of these sets, I suddenly got the bright idea of putting the vertical input of the scope across the units I suspected while the sweep was working on a low frequency and drawing out about a three-inch horizontal line. With the input across the output transformer, every little rustle would shoot up a little hair-like filament from the line. These little offshoots kept right on showing up as I moved back through the receiver until I reached the grid of the audio stage. On the plate I saw them; on the grid I didn't. When the tube was yanked, they were still there for a few seconds until the rectifier cathode started to cool - it was an a.c.-d.c. set - so the tube was not at fault. When the mica r.f. bypass was cut loose from the plate, no little spikes were present across the resistor. That condenser was leaky only when a certain critical voltage was across it. It checked OK with the battery voltages ordinarily used with an ohmmeter. "From now on I intend to use the scope to track down all of these 'rustlers.' If my mind hadn't been burdened with the idea that a scope was only good for aligning, waveform analysis, and distortion-spotting, I might have discovered this much sooner. The less hidebound your service technique is, the more likely you are to discover new and better methods of doing your work. "Really it all goes beck to how you feel about radio servicing. Personally, the thing I like about servicing is that it is a thinking business and cannot be easily systemized. The problems you encounter are far too complex and varied and changing to allow you to settle into a dull routine; and, by the same token, they are not to be solved with the greatest efficiency and dispatch by the application of any neat little universal formula or procedure. I am confident that the next set I put on the bench - and incidentally it is high time we started putting some sets on the bench - will be a little different in some detail or other from any I have worked on before - say," he broke off, "what the heck are you trying to do with that solder gun?" "Just following your advice and working out a new and better method of servicing," Barney explained blandly. "It always gripes me when I want to see if a voice coil is open and I have to unsolder the output transformer secondary from across the coil before I can test it. About half the time I lose the flexible voice coil lead out of the terminal and have a tough time working it back. Now I've got a better way. When you pull the trigger on this gun anyways near the output transformer, you get a strong hum from the speaker if the transformer secondary and the voice coil are both OK. It must be a.c. induction from the gun transformer." "My boy, I am proud of you!" Mac beamed. "You are really using that old flame-colored noggin. That makes me feel good." "That's not what makes you feel good," Barney observed. "What does, then?" "Getting the 'systems' out of your system," the boy said with a knowing grin as he slid from the bench.
Posted July 20, 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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By John T. Frye