Mac's Service Shop: Always Something New |
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After discussing the virtue of not letting someone else's opinion on the likely cause of a problem direct your own actions when troubleshooting, service shop proprietor Mac McGregor asks Barney about issues he has run into related to the area's having recently had television channel 13 broadcasting added to the area. When he mentions the trouble cause by homeowners leaving excess lengths of twin-lead 300 Ω lead−in wire coiled up behind the TV set, it brought to mind my own mentioning a couple days ago that very scenario in comments made on the "A Two-Band Piece of Wire" article. You and I would be tempted to criticize people for making such an "obvious" mistake, but most people then - and now - have no knowledge of the particulars of routing such cable. A switchover to coaxial cable with the advent of cable TV removed most of the need for being concerned over installation, other than assuring tight connects, proper terminations, and keeping cable length as short as possible to minimize signal loss. Other than that, properly shielded coax cable can be run just about anywhere. Mac's Service Shop: Always Something New
I saw either a dirty robin or a rusty blackbird on my way to work this morning," Barney announced to Mac, his employer, as the two of them worked side by side at the service bench. "You're always seeing robins about a month before anyone else does," Mac scoffed. "The poet had you in mind when he wrote, 'The wish is father to the thought.' However it is hard to be sure about color at times - which reminds me that something finally happened to my color set." "Nothing trivial, I presume." "Nothing really serious, but it was rather interesting. All at once during a color show the picture went out of focus. As you know from working with me, one thing I can't stand is a picture out of focus. It grates on my nerves the way a crooked picture on the wall annoys a housewife or a missing motor upsets an auto mechanic. And let me tell you right here and now that an out-of-focus picture looks lots worse in color than it does in black-and-white. Not only do outlines become blurred, but the colors themselves seem to suffer contamination. "I quickly found I could bring the picture back into focus with the focus control, but it would not stay that way. Every few minutes I'd have to reset the control to restore sharpness to the picture and the control was extremely critical in operation. The color set, of course, uses a separate focus control rectifier to produce the required high voltage, around 4000 volts, needed on the focus anode; so I trotted out the high-voltage probe of the v.t.v.m. and checked both the high voltage and the focus voltage. The high voltage was just a trifle low and the focus voltage was erratic. With a technician's perennial hope of solving things easily, I changed the focus rectifier, high-voltage rectifier, damper, horizontal oscillator, and horizontal output tubes. None of these new tubes had any effect, but changing the high-voltage shunt regulator tube did bring up the anode voltage. However, the focus was still erratic; so I was convinced the trouble was what it looked like in the first place: a bad focus control." "From force of habit I checked my service literature on the set to see if possibly there were any suggested changes in the focus circuit. Sure enough there were. A whole new arrangement had been worked out that was called the 'Focus Control Protection Circuit.' Obviously my set was not the only one that had lost a focus control. The replacement control was the same as the old one, but one resistor was cut out of the circuit, two new ones were added, and the high voltage for the focus rectifier was taken off a different transformer terminal. Later I had a talk with a service manager of the manufacturer and he told me that he thought the rather frequent break-down of the focus control in the original circuit was due to momentary flashovers in the kinescope or other components, and the new circuit was designed to shunt some of the instantaneous heavy current produced by these flashovers around the control." "I suppose you had to pull the chassis." "Nope. The control is inside the high-voltage cage at the rear of the chassis, but when four self-tapping screws are removed and the high-voltage and yoke leads are unplugged, the whole high-voltage cage lifts right off, allowing easy access to the points you need to reach. The whole job took only a few minutes, and now the set is easily brought into focus and stays that way." "It's a good thing you thought to check on any circuit changes." "It's always a good idea to do that, especially when a part goes out for no apparent reason or when the same part fails repeatedly. You can save yourself a lot of callbacks and other grief by doing so." "Hey, what were you doing here in the shop late last night? When Margie and I were going home from the show I saw a light on back here." "You might say I was proving how stupid I was," Mac said with a sigh. "There's a guy over at the relay factory who does part-time servicing. He took on a set a couple of weeks back that had him stumped; so he wished it off on my friend, Bob Sprain, who works in the lab there. Bob quit the service game when he went into engineering a couple of years back, but he still has his equipment; and this other fellow had done Bob some favors so that he kind of had him across a barrel. "Well, after Bob fooled around with the thing a week he decided he could use a little help, too; so last night he gave me a call and explained the deal. Bob has given me a hand time and again; so I was glad of a chance to pay him back a bit. I told him to bring the set down here and we'd see what we could find out. It was one of those situations all technicians get into now and then. You can preach 'unbusiness-like practices' and 'not helping competition' until you're black in the face; but sometimes you get into a spot where you have to be a real stinker to say, 'No.' "In the beginning the complaint was simply no raster. The original guy soon discovered there was no high voltage. Somehow, in fooling around, he found that when the horizontal yoke was disconnected the high voltage came right up; but when the yoke was connected, the high voltage and the boost voltage both disappeared. "Well, this fellow is apparently the sort who jumps at conclusions; so he ups and installs a new yoke. This made no difference at all. Then he concluded the horizontal output transformer must be at fault; so he put in a new one of those. That didn't help either and that is when he gave it to Bob. "As Bob said, replacing the yoke and transformer automatically cut the ground from a lot of beautiful theories about what might be wrong. On the other hand, I had to check and make sure no mistakes had been made in installing the new parts. Experience has made me suspicious about such matters. Worse yet, what Bob had told me conditioned my mind into thinking that connecting the yoke somehow imposed an unusually heavy load on the transformer that pulled down all the output voltages, including the rectifier filament voltage, so that nothing was left. "Finally, though, after I had checked and found all connections on the new transformer and yoke correct, I called a halt to my aimless fumbling around and forced myself to think of the set as just another one that came in with the original symptoms. I started checking it as I would any such set and found that everything worked normally right up to the grid of the horizontal output tube, no matter if the yoke was connected or not. But then I discovered a very substantial amount of horizontal pulse appeared between 'B-plus' and ground when the yoke was connected. Most of this disappeared when the yoke was removed. That, of course, was the tip-off on what was wrong." "Open output filter capacitor," Barney guessed. "Right. With the current-storing or bypassing action of this capacitor removed - whichever way you like to think of it - the plate circuit of the output tube could not get the strong pulses of current it demanded to supply power to a load. The whole thing acted like a transformer whose primary was being fed from a voltage source with poor regulation, When the secondary demand load increased, the voltage dropped. "When this capacitor was replaced, everything worked fine; but I've been brooding ever since about how easy it is for a technician to forget what he knows especially when something or somebody gives his mind a push in the wrong direction." "You know something?" Barney asked with a quizzical smile. "It makes me feel good all over when you make a mistake. What with you showing me up all day long here at the shop and then going home and watching those 'brains' on TV quiz programs coming up with answers that no one has any business knowing, I feel pretty stupid most of the time. It helps a lot when you pull a boo-boo." "OK; so then I'll be big-hearted and pull some more," Mac promised with an understanding grin, "But I've been meaning to ask you if you are running into any unusual problems since channel 13 came on the air with that fine signal they're putting out?" "Nothing too unusual. I'm pretty well convinced that everyone in this area should be getting 13 better than 8. On our own field-strength tests, 13 is putting in more than twice as much signal; so-o-o-o, when I hear someone say they are not getting it well, I work on the theory something is wrong in their set or installation, and there usually is." "What's the most common difficulty?" "Simply that the oscillator is not adjusted properly on channel 13. In many cases this channel was never set up in the beginning, since channel 13 was blank in this area for years; and in other cases the oscillator has just drifted off. At any rate, a simple twist with an alignment tool takes care of it." "There must be other cases not so easily disposed of. At least you're away from the shop long enough." "Oh there are!" Barney said hastily. "For instance, I've found that those people who insist on leaving a coil of surplus lead-in piled up on the floor behind the set may have been getting away with it after a fashion on channels 6 and 8, but channel 13 is another story. I chop out the extra lead-in, and if they insist on having it available so they can move the set around, I fix them up with a lead-in extension with connectors all around so that it can be spliced into the line when needed or left out when not. "Another thing I run across fairly often is one of those line-tuning gimmicks made of a piece of tinfoil wrapped around the line. This was adjusted so that 6 and 8 came in OK, but very often it cuts channel 13 way down. Usually I just take it off and throw it away. If the customer is the bullheaded type, I show him that it will have to be shifted up or down the line to get good reception on 13." "Have you had any trouble from too much signal?" "I was just coming to that. In quite a few instances the customer complains of picture pulling on channel 13 only. In most cases this is simply the result of overloading because the a.g.c. control is not properly set. Most of the sets in this ultra-fringe area are running nearly wide open most of the time. Re-adjusting the a.g.c. control normally sets things straight unless, of course, there's something wrong with the a.g.c. circuit itself." "Well, you can say this for radio and TV servicing," Mac observed philosophically: "it is often a headachy, frustrating, maddening business, but it's never dull or boring. There's always something new coming up!"
Posted February 27, 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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