Radio Broadcast (May 1929-Apr 1930)

RADIO BROADCAST. radio-frequency stage, or a broken lead to coil or tube. To test for an open circuit in any transformer coil, place the voltmeter, in series with a 22|-volt battery, across the section to be tested. No voltage reading indicates an open. If a reading is obtained, this shows that connections or coils are not open. The test tells nothing about shorts. Locating short-circuit: Any coil or piece of electrical apparatus has some resistance and, therefore, should show a voltage drop. A lower voltage reading will be obtained when testing through a coil than v/hen the ends of the test clips are touched together. When the leads are touched together, the full voltage of the battery is read, but when the leads are placed across a piece of apparatus the battery voltage, less the volt drop across the coil, is read. If no drop, not even a small one is shown, a short is indicated. A fairly low resistance voltmeter should be used for this test so that enough current will flow to give a readable drop across apparatus under test. Testing condensers: In testing condensers for shorts or breakdown, the same test for open circuits will apply. In this case, however, a reading will indicate that the condenser is shorted. A condenser in good con n"BU' Total Fil.Emission Fig. 6 — D. C. tube-tester circuit. dition is an absolute block to direct current. Therefore, if a reading is obtained, the condenser is bad. This test is made best with a high potential, say about 135 volts, as occasionally a condenser will test o.k. on low voltage but will break down when the set-operating potential is applied to it. Bad by-pass condensers will create short-circuits in grid and plate supplies and, therefore, will show up by a zero plate or grid voltage reading. With a double-range portable voltmeter, and the shop work bench wired up as shown in Fig. 1, the voltmeter may be connected to the permanent battery clips for series readings. Or, if the bench is busy, a separate battery and the portable meter may be used at another location. Tube and Set-Tester Several good tube and set-testers of standard types are on the market. It is generally safest to purchase such apparatus from one of the well-established concerns. Many testing sets, some standard and others more or less novel, may be made profitably in one's own shop or laboratory. A combination voltmeter and oscillator, as shown in Fig. 3, is a convenient piece of equipment for use in a service shop. Its uses are to test voltages and to provide signals during hours when there is no broadcasting. Only the best materials and meters should be used, and it should be made as small and light as possible. The coil, for the condenser values shown, may consist of No. 30 d.c.c. wire tightly wound on a form 1.75 inches in diameter, to 250 turns and tapped at the center. After being wound, the coil should be coated with collodion to hold the wire in place. Meters and other parts should be arranged to make the leads as short as Fig. 5 — View of the test bench, oscillator, and tube-tester described in this article. possible. If a 199 tube is used, the batteries may be placed inside the box. A 4|-volt C battery will suffice for the filament, and a small-sized B battery may be used for the plate. These will last a long time. Note that the B battery functions both as plate power for the oscillator and for operating the voltmeter in making continuity tests. The oscillator must be calibrated from a suitable standard. The following paragraphs describe some tests that can be made with this test-set. To use the apparatus as a straight highfrequency oscillator, insert tube, throw switch Si to "on" position, and couple test lead to a receiver. Coupling may be increased or decreased by changing the amount of wire twisted together in the lead. At resonance between tester and receiver the greatest " dip " in the grid milliammeter will be noted, but coupling should be loosened enough to prevent the dip from pulling the oscillator off the frequency to which it is set. The oscillator Fig. 7 — Typical B and C supplyunit of an a.c. receiver. may be modulated for audibility by closing Ss and causing the buzzer to vibrate. Continuity and voltage-drop tests may be performed with the oscillator shown in Fig. 3 by connecting the circuit and resistance coil, or other piece of apparatus being tested, to posts 2 and 3. This places the B battery and the high range of the voltmeter scale in series with the circuit or part to be tested. A practical serviceman would gradually discover a number of other ways to employ this set advantageously. A slightly different oscillating set for testing purposes is seen at the left in Fig. 5. A diagram of this is given in Fig. 4. It is self-explanatory. The milliammeter, in the plate circuit in this set, gives an indication of resonance by dipping. With this apparatus calibrated for standard frequencies, it may be used with a single coil of wire having no center tap for in dicating resonance as a pick-up wavefrequency meter. W ith the center-tapped Hartley coil it is useful as a "driver." For instance, in checking on the coils and condensers of the different stages of a neutrodyne receiver, if the milliammeter dips the same amount for each stage, when tuned to resonance with each tuned circuit, this is an indication of continuity through each coil, the dip being caused by absorption into the coil circuit. If each tuned circuit tests the same, this also shows that they are properly designed to tune to the same frequency. The batteries are mounted inside the oscillator cabinet. At the right of the oscillator, in Fig. 5, is a tube-tester, made and used by students of the Loomis Radio College. This is a standard arrangement of plate voltmeter, plate milliammeter, and filament voltmeter. Something of the kind is indispensable in every radio service shop. Weston meters, type 301, flush, were used. The resistor is made to be used with the plate voltmeter and is supplied with it by the manufacturer. Some difficulty was encountered in locating suitable jack switches, or push-button jacks, for this set. These were finally devised from parts of telephone jacks, the "push-buttons" consisting of the knobs and brass shafts of Eby binding posts. With the filament rheostat adjusted to show a voltmeter reading of the correct filament voltage, the milliammeter reading of plate current, for various plate voltages, indicates the condition of the tube. If desired, a meter of the same make, showing filament current could be included in such a tester. The tests described apply to all types of receivers, but, due to the differences in circuit design, as indicated in Figs. 7 and 8, a.c operated receivers also require additional tests. However, these are beyond the scope of the present article. jg5V.for UX171-A > 8 „ g 1.5 V. for UX226A.F. [g 2 5 V. for UX227 , 8 S1.5V. for UX226R.F. (A) (B) Fig. 8 — (A) Method of obtaining negative C bias; (B) usual system of securing filament voltages in an a.coperated receiver. • JUNE -19 2 9 • • 89