Radio broadcast .. (1922-30)

PART III— Apparatus Designed and Used by the Stewart- Warner Corporation for Testing Radio-Frequency Coils and Chokes, Loud Speakers, and Wire- Wound and Fixed Re- sistors. The calibrated condenser is of the trans- mitter type and is equipped with special bearingstoresistwear.The "A" coil machine (Fig. 1) is made so that the secondary is first tested alone and then by a throw-over switch the primary and secondary are connected in a series-aiding circuit. In our particular case where a variable an- tenna compensating condenser is connected across the "A" coil secondary, the induct- ance limits are rather broad, plus and minus one turn for the secondary winding and two turns for the primary and secondary in a series-aiding circuit. The "B", "C", and "D" coil machine dif- fers only in that the secondaries alone are tested for inductance with limits of plus and minus J turn, the primaries being tested for d.c. resistance with a simple ohmmeter. We have found this d.c. reading to be just as satisfactory an indicator of the primary as an inductance test, though, as in other branches, an inductance test is run on a percentage of the primary windings. Our production department re- ports that rejections run approximately 1 per cent, with this system. The 0-15-mA. meter indicates the condition of the crystal oscillator while the 370-mmfd. variable condenser in the tank circuit controls the power output. The condensers in the V.T. voltmeter are to adjust the circuit to R- Fig. 3 — In testing r.f. chokes with this apparatus the reso- nant frequency is checked by adjusting a 750-mfd. condenser. All chokes resonating in or near the broadcast band are rejected. High-Inductance r.f. transformers are tested with this device. It contains a crystal oscillator coupled to the coil under test and a V.T. voltmeter to indicate resonance. The circuit is given in Fig. 2. REGULATING RESISTORS B+90V. B- Fig. 5 — Carbon resistors are tested by passing through them a current approximating normal and measuring this current with a milliammeter and the voltage across the resistor with a voltmeter. resonance when a standard coil is con- nected, while the arrangement for connect- ing the coils in the circuit is shown in the picture. These test sets use one 6-volt A battery and two 45-volt B batteries. Testing R.F. Chokes The testing of r.f. chokes is very similar to the testing of r.f. coils except that, as the limits are broader, we do not find it necessary to use a crystal oscillator. As shown in Fig. 3 the coil under test is coup- led to an oscillator whose frequency is made variable by means of a 750-mmfd. tandem variable condenser. Resonance is indicated by a simple V.T. voltmeter and limits are painted on the dial. The operator rejects chokes which resonate near or in the broadcast band while those resonating between 300 and 500 kc. are accepted. This test set uses one A battery and one 45-volt B battery. Loud speaker coil testing is accom- plished by first testing for shorted turns with the test fixture described in last month's article and then measuring the resistance with an ohmmeter. Very little trouble is experienced here because auto- matic counters are used in winding. The testing of wire-wound resistors on a production basis is not difficult as far as testing is concerned but is often an ex- pensive item because of the quantity used in the average receiver. A good bridge or ohmmeter costs from $50.00 to $100.00 and can only be used by one operator, so where there are 25 to 50 girls winding re- sistors, if each were to have a tester this would become quite an item. We have de- signed a simple bridge circuit costing about $10, one of which is attached to each wind- ing machine. As this enables the operator to wind exactly the right amount of resistance wire our resistor rejections are less than one per cent, and this is due to breakage and not to incorrect resistance. As may be seen by referring to Fig. 4 and the picture, the test fixture consists of a bridge with two standard legs and a high- and low-limit arm, either of which may be inserted by a throw-over switch. This switch also serves to break the battery circuit. The binding posts in the front of the fixture are so that it may be connected to the winding machine while the contact points are used in testing completed re- sistors directly. The arrangement is sensi- tive to differences in resistance of 1 per cent, and is not affected by battery poten- (Conlinued on page 301) JEWEL PTRN. N?54 —50-0-50 GALVANOMETER '''A STANDARD"R'/^C HIGH LIMIT'R" O 0 + SUFFICIENT VOLTAGE TO GIVE 10-0-10 READING " N<? 1425 FEDERAL ANTI-CAPACITY SWITCH SWITCH POINTS C&D SHOULD MAKE CONTACT BEFORE A&B Fig. 4 — Wire-wound resistors are checked within 1 per cent, by means of this bridge system which is attached to the wind- ing machine. Changes in bat- tery potential do not affect its accuracy. RADIO BROADCAST FOR MARCH • 275