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MECA Electronics Attenuators

Mac's Service Shop: Simple Things First
January 1960 Electronics World

January 1960 Electronics World

January 1960 Electronics World Cover - RF Cafe Table of Contents 

Wax nostalgic about and learn from the history of early electronics. See articles from Electronics World, published May 1959 - December 1971. All copyrights hereby acknowledged.

After many years of reading Mac's Service Shop sagas, a persistent theme seems to be Barney's refusing to refer to equipment schematics while troubleshooting, thereby often wasting valuable time. According to business owner and electronics sage Mac McGregor, assuming that what is typical for most sets will apply to all sets can and does create a fertile environment for frustration - and profit loss. Mac's advice to check "simple things first," has always been my troubleshooting philosophy - maybe because identifying the "hard things" has nearly been my undoing many times when the trouble is not simple. One of first things I do is check switches, connectors, and user-accessible potentiometers for proper operation (when potentially responsible for the problem, of course). I've written many times about how often a dirty connector is the culprit. Over time any non-hermetically-sealed connector contacts can develop an insulating layer that results in a high resistance contact or in some frustratingly difficult to diagnose instance intermittent contact. As an example, last fall when visiting our daughter in North Carolina she asked me to take a look at her few-year-old, 42" LCD television (out of warranty, of course) that had suddenly stopped working. She even recited to me my automatic response to any report of electrical failure: Turn off the power and exercise (unplug/plug) all available connectors a few times and then power it up. Not really expecting success, I laid it face down on the floor, removed the back cover, and worked all the many connectors on the huge PCB. While at it, I did a visual and smell test of the components, and checked the accessible power supply diodes with a multimeter. Lo and behold, we plugged it in and it fired right up, good as new. Yeah, luck played a big part there, but the 30 minutes invested paid off big time. I highly recommend the approach.

Mac's Service Shop: Simple Things First

Mac's Service Shop: Simple Things First, January 1960 Electronics World - RF CafeBy John T. Frye

Barney had been muttering to himself for the past quarter of an hour, and finally Mac, his employer, walked over to the boy's end of the bench to see what all the fuss was about.

"Oh how I hate intermittents!" Barney said fervently. "One time this little a.c.-d.c. receiver will play okay for hours on end; the next time it won't even start. Right now the only sound you can get out of it is a low hum. I've changed all the tubes one at a time; so I know tubes are not the trouble, but look at this reading on the oscillator grid of the 12BE6."

He touched the probe of the v.t.v.m. to the grid lug of the socket, and the meter indicated some forty positive volts.

"When the radio's playing, I get seven or eight negative volts here," the red-headed youth continued. "That's the normal grid-leak voltage developed by oscillator action across this 22,000-ohm resistor between grid and ground. But when the radio goes dead, this voltage suddenly switches from minus eight to plus forty volts. I was sure it was an intermittent internal short in the tube, but the same thing happens with a new tube. Now I've decided it has to be an intermittent breakdown in the socket insulation; so I guess I may as well start changing the socket - and that's a job I despise."

"Easy, boy, easy!" Mac said mildly. "Have you checked the diagram?"

"Oh come now, Mac," Barney said with a superior smile; "I'm a big boy now, remember, and this is just a little a.c.-d.c. receiver. Its simple diagram is tattooed on my brain. Think of how many hundreds of these things I've serviced."

"I'm thinking; and for everyone you've serviced, I've serviced ten; but I'm still going to look at the diagram," Mac said with a disarming grin.

He took the service sheet from the file and merely glanced at it before he slid it across to his assistant. As Barney stared down at the diagram, a brick-red flush crept up out of his collar and spread over his face, dimming his freckles to invisibility.

"Well what do you know!" he exclaimed. "That grid leak returns to the cathode instead of to ground, and the cathode goes to ground through the primary of a two-winding oscillator coil. I'll bet a nickel that primary is opening up leaving the cathode floating, and the high cathode voltage under these circumstances is conveyed through that 22,000-ohm grid leak to the oscillator grid."

A simple check with the ohmmeter confirmed this suspicion, and Barney was lucky enough to find the primary winding break right at the end where it tied to a terminal. A drop of solder solved the problem. As the crestfallen boy replaced the receiver in the cabinet, Mac observed:

"I don't know what I'm going to do with you if you don't learn to go to the service literature when the going gets tough. Here I've invested hundreds of dollars in that literature with the sole aim of helping us to do our job better and faster, and then you blithely ignore the whole thing and depend upon your alleged omniscient knowledge of how the circuit must be. Actually this business of returning the grid resistor to the cathode instead of directly to ground is very, very common; but your preconceived notion that it had to return to ground kept you from even thinking about that possibility. You were looking for a 'far-out' cause of the trouble, such as a defective tube socket. I'll bet you haven't found more than three tube sockets with broken-down insulation in all the time you've been working for me."

"I have in transmitters," Barney defended himself lamely.

"We're working on receivers, not transmitters," Mac relentlessly pointed out; "and I've told you over and over always to look for simple causes of trouble first. Save the odd-ball, freakish possibilities for investigation until after all the ordinary, likely things have been checked out. Hey! Wait a minute. What was that whistle?"

Barney had the set back in the cabinet and was tuning it back and forth across the band as a final check. As he crossed a station near the middle of the band a heterodyne was heard so loud that it blocked out reception of the station.

"That's just the second harmonic of the 455-kc. i.f. beating with the station on 910 kc.," Barney explained. "You can hear that little heterodyne on lots of sets. Probably this one doesn't have enough decoupling in the detector circuit and some of the i.f. is feeding back through the a.v.c. system to the receiver input."

"I wouldn't call that screech 'a little heterodyne,' " Mac observed acidly. "You'll never convince me a manufacturer would let a receiver go out in that condition. Take off the back."

When two screws were removed and the back, carrying the antenna, was pulled away from the cabinet, the heterodyne disappeared and the station could be heard clearly.

"Apparently the signal is being radiated directly into the loop," Mac said as his gentle fingers probed around in the receiver. Suddenly he held up the end of the broken little copper-ribbon ground lead of the 12AV6 tube shield and stared quizzically at the other technician.

"This just ain't my day," Barney sighed as he reached for the solder gun. "I must have broken that lead when I was changing tubes. It's easy to do. With the shield floating, the 455-kc. signal on the diode plates of the 12AV6 could radiate directly into the loop winding only an inch or so away. Once more I was looking for a more complicated cause of trouble than an ungrounded tube shield. Why don't you fire me?"

"Don't tempt me," Mac answered as he tried to scowl. "I probably would if it weren't for that Japanese radio sitting over there on my end of the bench."

"What's that got to do with it?"

"Well, a little old lady brought that in yesterday with the explanation that her son, quartered in Japan, had sent it home to her. It was a nice-looking little two-band, three-way portable. She had unpacked it and plugged it in, and it started to play right off, but after a few minutes it quit. She was sure some little thing had shaken loose in transit.

"Well, as we both know, almost every time we get tangled up with one of these foreign sets we lose money. Most of the time there is no adequate service literature on the set; the parts arrangement and circuitry are frequently different from that with which we are familiar; and we often run up against defective parts for which substitutes cannot be obtained. I explained this to the woman, but she said her son had written that one of the selling points of this receiver was that he had been told that all parts for it could be readily obtained in the United States. Against my better judgment, I took the set.

"When I plugged it in, there was only hum. I raised the line voltage a bit, and the set played. 'Aha, a bad selenium rectifier,' I told myself. After I finally found the rectifier tucked away on top of the tuning capacitor, I replaced it with a silicon unit and the proper additional surge resistance. Then I reduced the line voltage to 95 volts and plugged the set in. It played but, as I began increasing the line voltage, distortion started. At the same time, I noticed the filaments seemed to be burning brighter than they should. A check with the voltmeter revealed two volts across each one where there should have been 1.5 volts.

"At this point I happened to pick up the back of the set and glance at the very small printing on the label - no wonder most Japanese wear glasses! - and saw that the receiver was rated at 100 volts! I replaced the original selenium rectifier, set the line voltage at 100 volts, and checked the voltage across the filaments. It was a little below 1.5 volts, but the set would not play. I called some of the boys at the local radar station who were formerly in Japan, and they confirmed that 100 volts is a common nominal line voltage in Japan. In fact, less than 100 volts was usually present.

"What had happened was all too plain: when the lady plugged the set into her 120-volt line, it played for a short while; but the excess current soon paralyzed the tubes and prematurely aged the selenium rectifier, and the set quit. When I put in a new rectifier, this pushed the filament voltage still higher, and the set started to play; but it would not have done so for long because those poor filaments would have given up the ghost.

"When I tried to check the tubes, I found they were not listed for my tester. I called the local distributor about my problem, and not a one of the new tube checkers in the store listed the tubes. Neither could I find them on my tube list in the shop or in any of the brand-new electronic catalogues. The worst blow of all came when I could not find them in the cross-reference lists showing foreign tubes and American equivalents."

"What are you going to tell the little old lady?"

"All I can do is suggest she write to the manufacturer in Japan and ask him to list American tube equivalents for the tubes in the set. If he cannot do this, perhaps he can tell her where she can obtain the tubes in this country, or he can send them to her direct. With new tubes in the set, she can operate it on batteries without damage. If she wishes, we can get her a step-down transformer that will permit her to use it on a.c. I prefer doing this to inserting a heat-radiating, voltage-dropping resistor inside the set or trying to locate and install a suitable resistor-type line cord."

"I've got a question, Doctor," Barney announced with a shrewd look, "While ago you said the tubes were supposed to have 1.5 volts on the filaments. Since you had no diagram and were unfamiliar with the tubes, how did you know this?"

"I've been waiting for that," Mac said with a chuckle; "and I'd have been disappointed if you'd let it pass. Really it's quite simple: the set used a single flashlight cell for an A battery in portable operation. Okay?"

"Okay," Barney grunted.

"But the main point," Mac said seriously, "is that I was guilty of doing exactly what I've told you not to do:

I was ignoring the simple, logical possibility that a foreign set might be designed for a line voltage different from our standard 117 volts. If I had just glanced at that label right in the beginning, look at the time and trouble I could have saved myself. So-o-o-o, I can't very well fire you for being as dumb as I am, now can I?"

"No, I guess you can't," Barney said with a broad grin spreading across his pleasant, Irish face; "and you know something? I don't mind being chewed out nearly so much when the chewer-outer admits he makes mistakes himself!"

 

 

Posted May 22, 2019


Mac's Radio Service Shop Episodes on RF Cafe

This series of instructive 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 Radio & Television News magazine (which itself started as simply Radio News), and then changed its name to Mac's Service Shop after the magazine became Electronics World. 'Mac' is electronics repair shop owner Mac McGregor, and Barney is his eager, if not somewhat naive, technician assistant. 'Lessons' are taught in story format with dialogs between Mac and Barney.

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