Radio Broadcast (May 1928-Apr 1929)

Solving an Employer s Problem AN EXAMINATION FOR RADIO SERVICEMEN By J. F. B. MEACHAM I ' ' INI RADIO dealers and others who regularly engage new men for their service staffs must cull out the good men from the poor and how to do it well is a problem that has not been generally solved. Many men charged with the task of hiring and firing have learned to their sorrow that not all who call themselves "radio experts" are really qualified. The first requisite for a good serviceman is that he have a good practical background of electrical and radio knowledge. One excellent way to find out if the applicant has this knowledge is to set an inclusive and formal examination which he must pass. The sample examination for servicemen which appears below is one which has been used with excellent results by the Q R V Radio Service, Inc., of New York. This examination attempts to determine the general knowledge of the applicant and does not go into all the problems which arise in the servicing of a.c. receivers. It has been found that men who can pass an examination of the scope of this one are usually quite capable of intelligently solving most service problems which they meet in the field. We present this sample examination with the suggestion that it may be helpful as a guide to other organizations who hire servicemen. We should like to hear from our readers who are using formal examinations of this type and would be glad to see copies of similar examinations which they are using. Section I — Fundamentals (ten credits) (Give the formula in each case and show your arithmetic.) 1. (a) If you desire to furnish a current of 3.5 amperes to a load through a resistance of 8 ohms, what must be the voltage drop across the resistance? (b) How much power is consumed in the resistance, in watts? 2. (a) If a voltage of 7 volts is impressed across a resistance of 125 ohms, what current will flow, in amperes? (b) In milliamperes? 3. (a) If a potential of 20 volts causes a current of 5 amperes to flow through a circuit, what is the resistance of the circuit, in ohms? (b) If a potential of 200 volts causes a current of 0.5 milliampere to flow through a grid-leak, what is the resistance of the leak, in megohms? 4. (a) What is the total resistance in ohms of three rheostats whose separate resistances are 3, 5, and 9 ohms, when connected in series? (b) When connected in parallel? 5. (a) What is the total capacity in microfarads of t hree condensers whose separate capacities are 0.5, 2 and 8 mfd. when connected in series? (b) When connected in parallel? Section III — Tubes (foub credits) 1. (a) What is the important difference between a 199-type tube and a 120-type tube? (b) Between a 201a and a 112? (c) Between a 112 and a 171? (d) Between a 171 and a 210? Section IV — Batteries (ten credits) 1. (a) Is it better to test the voltage of a dry cell under no load or under load? (b) Why? (c) What should be the no load voltage of a fresh dry cell? (d) Of a fresh 45-volt B battery? (e) How long may the average 45-volt bat Employers of radio servicemen have found that the most satisfactory means of selecting highly trained men is by submitting all applicants to a written examination tery be satisfactorily used? (f) Why can it not be satisfactorily used longer than that? 2. (Questions under 2 apply only to lead cells) (a) Is a voltage reading a sufficient indication of the condition of a storage battery? (b) Why? (c) Is a hydrometer reading sufficient? (d) Why? (e) If you know only the voltage and the ampere hour capacity of a storage battery, how would you compute the normal discharge and initial charging rate? (f) Of what is the electrolyte of a lead cell composed? (g) Why does the specific gravity of the electrolyte change during charge and discharge? (h) At what height should the surface of the electrolyte in a lead cell be maintained? (i) What trickle charge rate would be approximately correct for a set using five 201a tubes and one 112 tube which is operated for an average of four hours per day, the charger operating 20 hours? Section V — A.C. Power Units (ten credits) 1. (a) Can the voltage delivered to a set by a B-power unit be accurately measured by the ordinary batterytesting voltmeter? (b) Why? (c) Name an exception to your answer to (a), (d) What effect would be produced on the terminal voltage of the average B-power unit by the substitution of a 171-type tube for a 112-type tube in a set to which the power unit is connected? (e) Why? Section VI — Servicing (fifty credits) (In those of the following questions which refer to broadcast receivers, when no particular set is mentioned you may assume it to be an average five-tube factorybuilt set.) 1. In what order would you conduct a routine test of a broadcast receiver, assuming the entire pick-up system to be ok.? Answer briefly. Letter your answers as: (a), (b), etc. 2. If a set is noisy, and you do not recognize its source by the character of the sound, how can you determine readily whether it is in the set itself or is being picked up by the antenna-ground system? 3. (a) Which is most liable to open, the secondary winding or the primary winding of an audio-frequency transformer? (b) Why? 4. (a) What would be the effect on the output of an open in the grid circuit of the second r.f. tube in a broadcast receiver? (b) Of the first a.f.? 5. If one of the leads to a tuning condenser in a broadcast set is open, what is the effect? 6. (a) How would you determine the total plate current drain of a broadcast receiver? (b) Approximately what would the drain be of a set using four 201a's and one 112 tube, with the correct plate potentials and grid potentials applied? (c) If a test shows half the current you would expect under the conditions of (b) what is the most probable trouble, assuming the batteries ok.? 7. (a) When a low-pitched steady hum or howl is set up in a set, regardless of tuning condenser settings, what is the most probable trouble? (b) What is the cause of that trouble? (c) Name two remedies. 8. (a) If, in a broadcast receiver, one of the audio tubes appears to be burning with a brilliancy much greater than normal, what is the trouble? (b) Would the same condition produce the same brilliancy in an r.f. tube? (c) Why? 9. If the signal strength of a receiver was far below normal, but you could not find any trouble in the set itself, or the tubes or batteries, or loud speaker, what would be the sequence of your next tests, in detail? Let your answers be lettered as: (a), (b), etc. 10. If, with the tuned circuits of a set all out of resonance, an intermittent hissing and frying noise is audible in the loud speaker, what is the most probable trouble? 11. (a) If you find the plate potentials furnished to a set by a B-power unit to be very low, wi th the plate current to the set also very low, and you notice that the plate of the power-unit rectifier is running red hot, what it the most probable trouble? (b) If you found the plate voltages very low, but the plate current, measured in the minus B lead between the power unit and set terminals, abnormally high, what is the most probable trouble? 12. With the filaments of a set at normal brilliancy, approximately what potential should 90 volts of B battery produce at the plate of the first a.f. tube? 13. If a test shows normal potential from the B post of an audio transformer to filament, but none from the plate — of the tube whose output goes to that transformer— to filament, what is the trouble? Section VII — Diagrams (ten credits) 1. Draw a circuit diagram of a five-tube set, tuned and neutralized r.f., with detector regeneration, transformer-coupled audio, 171-type tube in last stage, with storage A, a. c. trickle charger, B-power unit, and automatic relay. Power-unit rectifier may be either thermionic or gaseous type. 2. Draw a circuit diagram of a superheterodyne having two stages of "intermediate" r.f., omitting the a.f. stages. Section II — Tubes (six credits) 1. Give data for the following tubes, for normal operation. Type 199 199 120 201a 201a 200\ 112 171 210 Use amplifier detector amplifier amplifier detector detector amplifier a.f ampl. a.f. ampl. Fil. V. Plate V. Grid V. Fil. Current Plate Current Plate Res. Ampl. Constant omit . omit omit @ april, 1929 . . . page 405 •