Radio broadcast .. (1922-30)

iriiiiiHIIIlllllrtmili Construction and Operating Data A RADIO DEALER'S TUBE-TESTER By JAMES W. BLACKWOOD AN ESSENTIAL PART of the dealer's and serviceman's equipment has always been some simple tube-test- ing device. Such an instrument must be inexpensive, portable, and yet accurate enough to judge the quality of a tube. On the other hand, to test all types of tubes for all conditions which might affect their performance requires expensive and complicated equipment which is beyond the needs of the dealer, since he is prima- rily interested in whether the tube will perform normally in his customer's receiver. What the dealer or serviceman needs is not some exhaustive test on tubes but a simple test to de- termine whether the tube will oper- ate at all, and if so how well. He needs to know only three things: (1) Is the filament intact? (2) Are any of the elements shorted? (3) Is the tube satisfactory in other respects? Either (1) or (2) above cause non- operation; (3) may result in poor operation. A testing arrangement which the dealer or serviceman can use to classify tubes on the basis of these three general divisions is described in this article. The apparatus can be divided into three circuits: the first, for testing short circuits and open filaments or heaters on all tube types; the second, for testing the operation of three- and four-element receiving tubes; and the third, for testing the operation of rectifier tubes. For convenience, each of these has been arranged to work directly from the 110-volt 60-cycle supply, available to dealers and servicemen everywhere. Short and Filament Tester The short-circuit test-set (Fig. 2) con- •ists of two sockets to accommodate four- and five-prong tubes and a system of lamps to indicate when a short circuit occurs. The 110-volt a.c. supply, which is connected across a resistance-type voltage- dividing arrangement, pro- vides the energy necessary to light the lamps when a short circuit occurs. The key, Kj, in the circuit is for closing the filament or heater circuit through two 10-watt 110-volt lamps in series and gives an indication of open filament or heater circuits. When key, KI, is pressed lamps D and E should both light with equal brilliance, proving the filament or heater circuit is complete. If they do not light, or if only one lights, or if one lights brighter than the other, a short or an open filament is indicated. It is suggested that the owner of the tester have available a convenient method of check- ing this circuit periodically. This can easily be done by providing old bases with short circuited grid-plate, grid- filament, plate-filament, etc., terminals and placing them consecutively in the socket to check the lighting of the proper lamp. The list of equipment in Table I is offered as a suggestion for those who may wish to build the tester. Any equivalent apparatus, however, will give satisfactory results. The tube is inserted in the proper socket and the clip attached to the external terminal, in the case of screen-grid tubes. For a normal tube no lamps should light 0-200MA-. SOCKET This ilnin in ^ shows the layout of parts on the panel of the rectifier tube tester. Table I Ri, R2—Two Carter resistors, type H. 20-ohm Hj—One 80-ohm resistor (Two Carter type H were connected in series) R4—One 300-ohm Electrad resistor, type B A, B, C—Three dial illuminating lumps and sockets, 6-volt, 0.1-ampere D, E—Two lamps and sockets, 10-watt, 110-volt Ki—One key or switch, S.P.S.T. (Push button w'M do) Kt—One General Electric toggle switch, single- pole (Catalogue No. 269943) Si—One tube socket, X type St—One tube socket, five-prong (Y type) One adaptor, c-199 (ux-199) tube to x-type socket One adapator, oil (wn-11) tube to x-type socket Two clips for connecting to control grid Two fuses, five-ampere Box, panel or circuit board. / / CX-12 CX-326 CX-345 C-324.. C-327; CX-299 CX-322 CX-300A CX-310 KEY OR PUSH BUTTON "• NOTE:- / CX-220 ( PRIMARY RESISTANCE X = CONTROL GRID CLIP AND LEAD The arrangement of tube sockets, meters, voltage control, etc., of the tube-tester is indicated in this panel layout. until the key is pressed after which the two 10-watt lamps D and E should light up with equal brilliance, their circuit being completed through the filament or heater circuit of the tube. If any one or more of the lamps light before the key is pressed, a short circuit is indicated. If, upon pressing the key the two 10-watt lamps are not of equal brilliance, a short circuit is also indicated. The 10-watt lamps will usually burn very dimly when the key is pressed since they are in series across the 110-volt circuit and have, therefore, only 55 volts each. If, before either key is pressed, any lamp or lamps light the trouble may be found from the following table: l^amps lit Type of short A only Grid-cathode (control grid) B ' " (control or screen grid) C ' Plate-cathode A and B Screen grid—control grid A and C Control grid—plate B and C Grid-plate (control or screen grid) E only Heater—cathode A and E Heater or filament—control grid B and E Heater or filament—grid (con- trol or screen grid) C and E Heater or filament—plate To test the actual operation of a tube, after it has been deter- mined that there are no shorted elements or an open filament or heater circuit, the circuit shown in Fig. 1 is used. It is a modification of that published in January, 1929, RADIO BROADCAST. The filament (or heater) is provided with raw a.c. from a transformer. At the same time a.c. voltage is put on the plate. On the positive half cycles the tube takes current which flows through resistors (R.2 and R 3 ) which puts a negative bias on the grid of the tube. Thus a certain plate current flows. When a key (Ki) is pressed part of this resistance (R 2 ) is shorted out of the circuit, thereby changing bias and changing plate current. This change in plate current may be taken as a measure of the value of the tube. With a 15-milliampere full scale meter. MI, in the plate circuit, us- ing bias resistors of approxi- mately the values given, all types of tubes can be tested with satisfactory results. A 10-watt, 110-volt lamp, LI, used as a protective resistor is included in the circuit to protect the meter in case a shorted tube is accidentally inserted in the socket. A plate- filament or grid-plate short in a tube inserted in this tester will cause the 10-watt lamp, LI, to light and the meter MI to vibrate slightly about the zero adjustment, the needle following the 60-cycle current passing through the meter. However, it is recommended that in every case tubes be tested for short circuits before inserting in this tester. The circuit requires a special transformer having a 100-volt primary and a secon- dary tapped at 1.1, 1.5, 2.5, CX-112A CX-350 CX-171A CX-301A CX-340 144 • JANUARY 1930