Radio broadcast .. (1922-30)

PRODUCTION TESTING EQUIPMENT This device for measuring the resistance of wire-wound resistors within 1 per cent, is at- tached to each winding machine. By J. A. CALLANAN Stewart Warner Corporation In this, the third of a series of articles dealing with production testing of radio appurtenances, the discussion centers around the testing of a miscellany, which, while just as important as either capaci- tances or iron-cored items, does not come under either group. For example, the test- ing of r.f. coils is, in the writer's opinion, secondary in interest only to the testing of gang condensers. The acceptance limits must, of course, be held just as closely as those of gang condensers as they are used in conjunction with one another. Testing B. F. Coils In testing r.f. coils we have tried a num- ber of systems, including matching sets of coils, adjusting turns, etc., but have found that all of these resulted in manufacturing difficulties which made them impractical. Several years ago we tried the matched-set system in which every coil is graded ac- cording to the position of a condenser dial at which the tuned circuit resonates. The coil is then marked high, medium, or low and is used with a condenser conversely marked. Our manufacturing, repair, and service stations branches soon protested, and, while we operated with this method that year, the system was more costly than either the turn-adjusting system or our present one. The turn-adjusting method was very good, but it was not very fast and therefore required more labor. From these experiences our present system was evolved; it is neither costly nor slow and has been entirely satisfactory to all con- cerned. For those who may be interested in a similar procedure a detailed descrip- tion of our coil-winding and inspection system is given below. Upon receipt from the factory the coil forms are first dried to prevent a possible variation in size due to the absorption of moisture in shipping. They are next placed in a double split ring gauge which measures the circumference at either end of the coil form. A dial indicates the average circum- ference to the thousandth part of an inch. The gauge is necessarily constructed so that even if the shape of the coil form is distorted a true circumference reading is obtained. In our particular case we find that a difference in size of 0.003 inch makes a 5-turn difference in winding so we have made our factory acceptance limits plus and minus 0.003". This makes a difference of one full turn in apparent inductance, or, in other words, gives us three possible groups of coils, viz. minus \ turn of 150 turns (our standard), 150 turns, or 150 4- 5 turns, and each is interchangeable in the completed receiver. The tested coil forms are first punched and wound with the required number of turns as shown bv the inspection ticket, the coils being space wound to prevent error due to varying wire diameter. The coils are then doped and soldered after which endless belts or conveyors carry them along until they are placed in the receiver. Our coil test equipment consists of two types of machines, one for the "A" coils with relatively few primary turns and the other for "B," "C, and "D" coils which have sufficient primary turns to resonate below the broadcast band when connected in parallel with the tube capacity. The schematic diagram of the "A" coil ma- chine is given in Fig. 1, while the machine for testing B, C, and D coil is shown in Fig. 2 and the accompanying picture. These test fixtures consist of a crystal oscillator to which is coupled the coil under test, resonance being indicated by a V.T. voltmeter connected across the coil. 0-1 M.A DC .. B*90 C-t A- B- B+45 Fig. 1 — Circuit for checking lone-inductance r.f. transformers. A crystal oscillator supplies the power and the coil under test is connected in the grid cir- cuit of the V.T. voltmeter tube. The secondary is first checked alone and then primary and secondary are connected, series aiding, to test both windings and the mutual inductance between them. Fig. 2 — This circuit for checking high-inductance r.f. transformers operates in a similar fashion to Fig. 1 except that only the secondaries are tested, the limits being plus or minus one half turn. All primary coils are tested for resistance and only a small per- centage of the production receives an inductance test. The rejects run about 1 per cent. 274 • RADIO BROADCAST FOR MARCH