Circuit Quiz
June 1966 Radio-Electronics

June 1966 Radio-Electronics

June 1966 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.

Being that this Circuit Quiz appeared in a 1966 issue of Radio-Electronics magazine, the amplifier components shown are transistors, rather than vacuum tubes. I have to admit to not doing very well on it. One of the challenges is first determining what the intended function of the circuit is supposed to be, then you figure out what is wrong with it. Spoiler alert: I'm going to use circuit A as an example. It is declared to be a voltage regulator circuit, and the deficiency is the lack of a stable voltage reference. The architecture is typical of a voltage regulator with the common base setup used to increase the current supply. However, there is no reason to necessarily assume the DC IN is not itself already regulated, and the function if merely to increase the current supply capacity. If that is the case, then the circuit seems sufficient as shown. Maybe the fact that there is no problem otherwise should tell you make an assumption about the designer's intention and look for something that would be suspect under that condition. Anyway, that's my excuse and I'm sticking with it ;-)

Circuit Quiz

Circuit Quiz, June 1966 Radio-Electronics - RF CafeBy Marvin Jack Moss

Each of these circuits has one error that will keep it from working as it should - or from working at all. Can you name each circuit and find the trouble? Assume component values to be consistent with proper design of the circuit.

 

Answers below.

 

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.

RF Cafe Quizzes

Vintage Electronics Magazine Quizzes

Vintage Electronics Magazine Quizzes

 

These Are the Answers.

A. A voltage regulator such as this would not regulate very well without some sort of reference voltage. In this case, a logical choice for a reference would be a Zener diode placed between the emitter of Q2 and the positive line return. Resistor R3 would no longer be needed.

B. No current can flow through the splitload phase inverter if there is no ground return for the base and emitter resistors, R2 and R4.

C. This transformer-coupled audio amplifier will operate in class B unless capacitor C1 is removed and connected in parallel with R2. R1 would then be connected to the junction of R2, C1 and the input transformer.

D. One little item that some of us fail to notice occasionally is the polarity of the supply we connect to our transistor circuits. The RC-coupled amplifier here uses a p-n-p transistor, so the collector should be negative.

E. If you used this circuit as a balanced-bridge meter amplifier, you'd have some difficulty in setting the meter to zero with no DC input. By moving the emitter of Q2 to the right end of R9 and connecting R8 to the wiper of R9, you can balance the circuit.

F. This could be a conventional push-pull audio amplifier, except that Q2 is in the circuit upside down. Its collector should go to the output transformer and the emitter should connect to the emitter of Q1.

G. Here is a two-stage RC coupled amplifier. Depending upon circuit values, voltages at the terminals of Q1 and Q2 could very widely. However, if we assume that the two stages are biased identically and that the potential between collector and emitter is several volts, then the collector of Q1 will be several volts more negative than the base of Q2. Thus, capacitor C3 is in backwards, which can make it pass high leakage currents. This will change DC bias levels, causing highly distorted output or damage to Q2.

H. This typical i.f. amplifier stage for a radio will overload easily, since with an increase in the signal fed to detector diode D, the avc voltage would be negative. This would tend to forward-bias the stage and increase its gain. Oscillation might result. The way to check this would be to reverse the diode, putting a positive (reverse-biasing) avc voltage on the transistor.

I. This is the familiar dc-to-dc converter used so often today. This particular unit would hardly be acceptable, though for there will be no output. Diodes D1 and D3 short the transformer on one half cycle and D2 and D4 short it on the other. To obtain de of the polarity indicated, you must reverse diodes D1 and D2. As it is now, Q1 and Q2 probably would not even oscillate, because the shorted output loads the circuit very heavily.

 

 

Posted April 7, 2023