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Three new circuits challenges await you from this "What's Your EQ?" feature in the September 1963 issue of Radio−Electronics magazine. "EQ," by the way, stands for Electronics Quotient (a la "IQ"). The first one is a relatively simple solving of two equations in two unknowns. Yes, I worked it out; in fact, my solution is via the "complete the square" method, whereas the author's solution plugs the coefficients in the quadratic equation. "Why No Voltage" is not too hard, and is easily understood based on the author's explanation. "Where's the TVI?" is more of a case study of an actual experience in locating the source of television interference. The culprit ends up being similar to the source of AM radio interference I got on my car radio in a certain area on my drive to work. It was worst on high humidity mornings. Turns out the problem was that same as the author's situation, only the damaged component in my case was a result of wear and tear - not sabotage.

Three puzzlers for the student, theoretician end practical man. Simple? Double-check your answers before you soy 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. (Solutions at bottom of page)

Write EQ Editor, Radio-Electronics, 154 West 14th Street, New York 11, N. Y. 10011.

Two Resistors

What is the value of R1 and R2? - Steve Stumph

Why No Voltage?

In checking a power supply as in the schematic, the dc output voltage was just about right, but there was no ac voltage from X to Y. The transformer was not overheating. With the power supply disconnected and the filter capacitors discharged, a resistance check was made from X to Y, proving that both halves of the secondary were intact. Why could no voltage be read between X and Y with the circuit operating? - Sid Elliot

Where's the TVI?

Very severe TVI: Heavy "bands of dots" or partial blackouts on TV screens all over town, whether on antennas or on the "cable", the community antenna system. (Their antennas were 3 miles from town and 1,000 feet higher.) Both channels affected were low-band stations, almost adjacent, to the east. Stations to the south were not affected.

Using simple methods, the source of the interference was located, reported and fixed. (Hint: This is a "fringe-area" town.) - Jack Darr

Quizzes from vintage electronics magazines such as Popular Electronics, Electronics-World, QST, 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.

Answers to Puzzles on p. 54

Two Resistors

The two resistors in series add up to 100 ohms: R1 + R2 = 100.

In parallel, they make 10 ohms:

R1R2/(R1 + R2) = 10.

Since we know that R1 + R2 = 100, we can substitute in the "parallel" equation and get R1R2/100 = 10, or R1R2 = 1,000. Now we have two equations in two variables, and we can solve them.

Expressing R2 in terms of R1 , we can write R2 = 100 - R1 (from our "series" equation above). This gives us R1 (100 - R1) = 1,000, or

100R1 - R12 = 1,000.

This is a simple quadratic equation. Let's rearrange it into standard form: R12 - 100R1 + 1,000 = 0. Now we can use the quadratic formula (see any algebra textbook).

If we substitute either R1 value into the "series" equation R1 + R2 = 100, or R2 = 100 - R1 (same thing), we'll get a value for R2, and the problem is solved. Notice how the two roots of the equation, 88.73 and 11.27, add up to 100. In other words, we can pick either value as R1, and the other is automatically R2. Try it.

Here is the Complete the Square Method (by Kirt Blattenberger):

Start with the author's easily obtained equation for R1 (could be R2 with same result) -

100R1 - R12 = 1000  =>  R12 - 100R1 + 1000 = 0

Rewrite as {(R1 - 50)2} +1000 = (-50)2

{R12 - 100R1 + 2500} + 1000 = 2500

(R1 - 50)2 = 2500 - 1000

(R1 - 50)2 = 1500

(R1 - 50) = ± √(1500)

R1 = ± √(1500) + 50

R1 = 50 ± √(1500) = 50 ± √(15 * 100) = 50 ± 10*√(15) = 50 * ±38.73

R1 = 11.27 Ω, 88.73 Ω

R2 = 100 - {11.27 Ω, 88.73} =  88.73 Ω, 11.27 Ω

Why No Voltage?

The power transformer had a "balanced" secondary - a rare unit in which the two secondary halves were wound side by side, so that each had equal resistance as well as equal inductance. But the leads on one of the halves had been reversed. Thus X and Y were effectively in parallel, and the circuit was working as a half-wave rectifier.

Where's the TVI?

Since the interference was being picked up on the cable antennas far above the town, it was obviously a very high-intensity source. Since it did not interfere with stations to the south, it had to be toward the east.

Going to three homes in town which had high-gain directional antennas and rotors, we shot bearings on the noise, turning the antennas for maxi-mum interference. These bearings were marked on a city map.

An aerial-navigation chart of the area showed a 67,000-volt power line running along the highway going north. Checked out with an auto radio, the cause of the interference was very apparent. Someone had shot an insulator, and the high voltage was arcing through the streak of lead. Because of the direction of the lines, as seen in the figure, they acted as a very efficient transmitting antenna to radiate the interference toward the town and community antenna.

Posted October 10, 2023