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Mac's Service Shop: New Breed of Test Equipment
September 1972 Popular Electronics

September 1972 Popular Electronics

September 1972 Popular Electronics Cover - RF CafeTable of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

When semiconductor junction devices - diodes and transistors - first came on the electronics scene, many people were mystified by their operation and had no idea how to work with them in terms of designing, troubleshooting, or understanding circuit function. These Mac's [Radio] Service Shop articles provided a great service to readers by always addressing contemporary issues whether they be customer relations, new types of radios and televisions, troubleshooting new kinds of circuits, shop procedures, documentation interpretation, circuit operation, or as in this installment, test equipment operation. As with so many things that appear in these vintage electronics magazines, what was a big deal decades ago is commonplace now. The Kurz-Kasch Junction Verifier (JV-1505/6) described here is a cruder version of a function built in to most DMMs today - even the el-cheapo ones sold at Harbor Freight. The lowest cost model of the JV cost about $31 in 1973, which is the equivalent of about $190 in 2019 money.

Mac's Service Shop: New Breed of Test Equipment

Mac's Service Shop: New Breed of Test Equipment, September 1972 Popular Electronics - RF CafeBy John T. Frye, W9EGV, KHD4167

"Man!" Barney exclaimed, stopping short in the doorway of the service department, "I knew we had lots of business, but I didn't think we'd have to install a traffic light on the service bench."

Mac, the owner of the shop, looked around with a grin and then turned back to the little (4-3/4" x 4-1/4" x 2-3/4") blue-cased instrument resting on a tilt bail on the bench so its brushed aluminum panel looked up at him. That panel contained only an on-off switch on the left, two binding posts at the bottom center, a red one on the left and a green one on the right, and directly above these two large cut-outs in the panel in the form of opposite-directed diode symbols. Both were flashing on and off at a rate of three or four times a second. The bottom one with the arrow pointing to the right was flashing a bright green; the top one with the arrow pointing to the left was flashing a bright red, giving rise to Barney's comment.

"'Junction Verifier, Electronics Division, Kurz-Kasch, Inc., Dayton, Ohio.?'" Barney read from the little panel over Mac's shoulder. "What the heck is a 'junction verifier'?" (Kurz-Kasch, Inc., Electronics Div., 1421 S. Broadway, Dayton, OH 45401)

"Just what it says: a device to verify the operating condition of any junction in most solid-state, semiconducting junction-type electronic devices in use today, including silicon, germanium, or selenium diodes, zeners, bridge rectifier assemblies, npn and pnp transistors, LED's, FETs, and tunnel diodes. With this defective diode you see I have connected to the binding post terminals, both displays are flashing to indicate no rectification is taking place across the shorted junction and current is flowing in both directions. You get the same display by simply putting a piece of wire between the binding posts. When I remove the shorted diode and replace it with an open-circuited diode, like so, neither display flashes because current cannot pass in either direction across the open junction."

"Does that thing only give you the bad news? How about the good news?"

'Junction Verifier, Electronics Division, Kurz-Kasch - RF Cafe"I'm coming to that. Let's connect this good diode with the marked cathode to the green binding post and the anode to the red post. That is the normal way to connect a junction to the instrument: anode to the red post, cathode to the green post. Notice now only the bottom green display is flashing, verifying that the junction is good and that the diode is properly marked resulting in 'conventional' current flow from left to right. But now let's reverse the diode so the cathode is connected to the red post and the anode to the green post. See, now only the red display is flashing. This indicates the junction is still good, but the current is flowing from right to left. That is what would happen if we tried to test a diode with the cathode improperly marked."

"I get it," Barney said. "The verifier tells you four things about a diode: (1) if the diode is shorted, (2) if the diode is open, (3) if the diode is rectifying, and (4) which way current is flowing through the diode. Right?"

Checking Transistors. "Right as far as you go. The verifier actually tells you these four things about almost any semiconductor junction, no matter if that junction constitutes a simple diode rectifier or is one of several junctions contained in a single solid-state device. Let me show you what I mean," he said, attaching matching test leads to the red and green five-way binding posts of the verifier and removing the back of an inexpensive little transistorized radio lying on the bench.

"Suppose we want to check the transistors in this Japanese receiver for which we have no service data. The transistors, which we do not recognize, are soldered in. I touch the green lead to the collector and the red lead to the base of a transistor. The green display starts to flash, and I immediately know two things: the collector-base junction of the transistor is good, and the transistor is a pnp type in which the collector voltage is supposed to be negative with respect to the base.

"Next, I shift the green lead to the base and the red one to the emitter. Again the flashing of the green arrow tells me the junction I am testing, the emitter-base junction, is good and that the base is properly the cathode of this junction. If, in our first test, the red arrow had started flashing instead of the green one, I should have known we were dealing with an npn type transistor that requires a positive voltage on the collector. If both displays had flashed simultaneously, I would know the junction was shorted. Had neither arrow flashed, I could be sure the junction was open. In either case, the transistor would be bad. So you see the verifier serves as a transistor tester and a transistor-type identifier."

In-Circuit Tests, Too. "What impresses me," Barney observed, "is that you made these tests with the transistor wired into the circuit. Can you always do that?"

"Almost always. A junction may be tested in-circuit as long as the parallel resistance is 1500 ohms or more or the parallel circuit capacitance is 10 microfarads or less, which is practically almost always the case. Lower parallel resistance or greater parallel capacitance may cause the verifier to indicate erroneously a shorted junction."

"A couple of times you said the instrument would test 'almost' any junction. What are the exceptions?"

"It will not test junctions having more than 2 volts forward voltage drop. About the only place you find these is in high-voltage rectifier stacks. Neither will it reliably test junctions having reverse voltage drops less than 4 volts. That means it will not test low-voltage Zeners, such as the 3-volt type. Finally, some voltage reference diodes, such as the 1N821 and the 1N937, are designed to operate in a forward conducting mode, in contrast to zener and avalanche diodes. To insure their correct circuit application, these devices are banded on their anode ends, as opposed to cathode banding on true Zener and conventional forward-conducting diodes. Such reference diodes rated in excess of 2 volts will appear as open elements on the junction verifier because tests with an ohmmeter will show normal forward resistance of about 10,000 ohms and infinite reverse resistance."

"Do you know how the junction verifier works?"

"Only in general terms. It is said to use a unique digital technique that sends pulses of current, not exceeding 5 mA, in both directions through the junction, samples both forward and reverse currents, compares the two, and displays the result on the diode symbol screens of the front panel. I may as well admit I peeked inside and saw two IC's and a host of discrete components mounted on a glass epoxy board. Since the circuit is a new one for which a patent has been applied, it is naturally not published."

"What does the thing cost?"

"This ac model, the JV-1505, costs $44.95. A battery model, using four 'C' batteries, the JV-1506, is priced at $30.95. But actually the interesting thing to me about the junction verifier is that it typifies a new line of service instruments coming on the market - instruments designed to meet the problems of the modern technician and to impress favorably his customer."

"Why do you accent the word 'modern'?"

New Types of Testers. "In the old days the work was done almost entirely in the shop. It mattered little how bulky, heavy, and hard to understand the instruments were. They were only seen by the technician, and he knew what they said. But today much of the work is done in the home. Those instruments have to go out into society, and they should have what the auto industry calls 'pizzazz,' or flair. They should please the eye of the customer and impress him with their performance.

"Note, for example, the compactness and light weight of this little junction verifier. Every ounce and every cubic inch of space counts when you have to take the instruments to the receiver. Also look at the attractive, textured, chip-resistant blue case and the brushed aluminum panel. Above all, though, notice how movement and color are employed in the readout. Those flashing colored arrows catch the eye of the customer just as they caught your eye when you came through the door. That is showmanship.

"But the showmanship is backed up with solid utility for the technician. The instrument has versatility that insures it against obsolescence. Just as a crescent wrench is always useful, no matter how many box sockets and end wrenches you have on the bench, so will an instrument that speedily checks a single junction always be in demand. And it is speedy. You can check junctions as fast as you can apply the test probes.

"You might say the same thing about the new digital voltmeters," Barney observed. "They are rugged, easy to read, and are bound to impress the customer with the computer-fast speed with which their display changes."

"Right you are, and I'm glad you used that adjective 'computer-fast.' That is the key to what I am talking about. The important thing is the association that takes place in the mind of the customer when he sees some of the modern service instruments working. That DVM readout reminds him of the displays he has seen during rocket launches, and he knows that the space program incorporates the best there is in electronics.

Other Examples. "The new. compact, solid-state scopes are another example of the same thing. The customer is always intrigued by the moving patterns seen on a scope screen. After all, he has seen these instruments used in science laboratories and in the operating room, and he equates their use with precision and technical excellence. It is most reassuring to him to see his TV set adjusted with the use of a scope. But, again, a modern dc scope is the most versatile instrument a technician can take into the home. It can serve as an ac or dc voltmeter, a signal tracer, a distortion analyzer, a curve tracer, a phase-shift detector, a frequency meter, or what have you. A modern scope gives real meaning to that common phrase: 'the usefulness is limited only by the ingenuity of the owner.' "

"Why are we getting these new instruments now?"

"I think the major debt is to the space program and the computer industry. For example, I am sure this junction verifier came about through the research Kurz-Kasch did in developing their digital logic testing probes which they claim will replace wide-band scopes and DVM's now used for this purpose. The readout of these low-cost probes is in the form of three colored lights. And we know that IC applications in space age instruments and computers have lead to the DVM's and the solid-state scopes."

"Well," Barney said as he got into his shop coat, "I just hope the space and computer boys continue throwing an occasional glance toward our needs. Service instrumentation can stand some 'pizzazz' as you call it."



Posted January 3, 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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