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What's Your EQ?
February 1964 Radio-Electronics

February 1964 Radio-Electronics

February 1964 Radio-Electronics Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Put on your thinking cap again and take a shot at these trio of new circuit analysis problems that appeared in the "What's Your EQ?" feature in the February 1964 issue of Radio-Electronics magazine. These days, I'm guessing not too many people are familiar with the characteristics of neon bulbs, and even way fewer with vacuum tube circuits. Neon bulbs were one of the earlier forms of voltage references since once ignited, the voltage drop across them is fairly constant, sort of like a gaseous Zener diode - except there was zero current flow prior to ignition. Although I didn't know for sure, I figured that even with its high input impedance, the VTVM would have an effect on the circuit, or Mr. Collins wouldn't have bothered to include it. Read on to discover why. "The Innocent Black Box" ended up having a different solution than my assumption. Oh well. "A No-Signal Stinker" has a low-tech solution, which I also didn't think of. My suggestion was that some joker turned the 2.5 MΩ bias potentiometer down.

What's Your EQ?

What's Your EQ?, February 1964 Radio-Electronics - RF CafeThree puzzlers for the student, theoretician and practical man. Simple? Double-check your answers before you say you've solved them. If you have an interesting or unusual puzzle (with an answer) send it to us. We will pay $10 for each one accepted. We're especially interested in service stinkers or engineering stumpers on actual electronic equipment. We get so many letters we can't answer individual ones, but we'll print the more interesting solutions - ones the original authors never thought of.

Write EQ Editor, Radio-Electronics, 154 West 14th Street, New York, N. Y. - 10011.

Answers to this month's puzzles are on page 62.

Neon-Bulb Circuit - RF CafeNeon-Bulb Circuit

In the circuit shown, one neon lamp requires a minimum of 74 volts for ionization and four in series require a minimum of 296. After ionization, the drop across each lamp is 59 volts and the total current 0.3 ma.

Assuming that the lamps are non-conducting when switch S is closed (because 266 volts is not enough to start conduction), what change will occur when a vtvm with an input resistance of 10 megohms is connected across terminals A and B? Also, what reading will the voltmeter have and what will happen when the voltmeter is disconnected from the circuit?

- Kendall Collins

 

The Innocent Black Box

A black box has only two terminals. When I use an ohmmeter to check the resistance in the R x 1 range, the meter reads 25 ohms. When I change the range to R x 10, the meter reads only 10 ohms. The meter has been recently checked for calibration and is good. The black box does not contain any voltage source. What does it contain?

- H. D. Varadarajan

The Innocent Black Box - RF Cafe

 

A No-Signal Stinker

In this old Stromberg-Carlson radio (1210), there is no signal through the 6SC7 stage. The tube is new and good. B-plus voltages are OK, all resistors and capacitors are good, and measure close to rated values. What's wrong?

- Jack Darr

Quizzes from vintage electronics magazines such as Popular Electronics, Electronics-World, QST, Radio-Electronics, and Radio News were published over the years - some really simple and others not so simple. Robert P. Balin created most of the quizzes for Popular Electronics. This is a listing of all I have posted thus far.

RF Cafe Quizzes

Vintage Electronics Magazine Quizzes

Vintage Electronics Magazine Quizzes

Vintage Electronics Magazine Quizzes

Answers to What's Your Eq?

This month's puzzles are on page 57

Neon-bulb circuit

The purpose of the voltmeter is to start the four neon lamps on a voltage lower than the combined firing voltage of four lamps. For several microseconds after the vtvm is connected across terminals A and B, the IR drop across both the ballast resistor and the voltmeter is low, and as a result, most of the supply voltage is applied to three lamps. This voltage level is above the combined firing voltage of three lamps and the lamps fire.

When the three unbridged lamps are lit, the rise in current produces an IR drop that exceeds 74 volts across the 10- megohm voltmeter resistance. This causes the bridged lamp to fire. After four lamps have fired, the maintaining voltage across each lamp is 59, and the IR drop across the ballast resistor is 30 volts. For practical purposes, the voltmeter reading is 59 volts. When the voltmeter is disconnected, the lamps will continue to glow and the total current will decrease by approximately 6 micro-amperes.

 

The Innocent Black Box

The black box contains a flashlight bulb. An ohmmeter allows a high circuit current of 50 to 80 ma (normally) for reading low ohms in the R x 1 range. This heats the lamp filament and the meter reads hot resistance. But in the R x 10 range, current is very low and meter reads almost cold resistance.

 

No-signal stinker

Only one possibility here. (Note that there is no voltage drop across those big plate resistors!) Grid voltage is zero, cathode voltage zero, so the tube isn't blocked. The tube heater is not burning! Since the tube is good, this is probably a socket trouble. This is a metal tube, so you wouldn't see it at first; you'd have to wait for it to get warm. Check: measure heater voltage on the ends of the base pins, not on the socket terminals.

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Copyright: 1996 - 2025
Webmaster: Kirt Blattenberger,
   BSEE - KB3UON - AMA92498

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a lady's voice announced "You've Got Mail" when a new message arrived...

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