Radio Broadcast (May 1929-Apr 1930)

z in the sockets when testing such single series circuits, and get at the contacts elsewhere. If the construction of the set is such that the socket contacts cannot be reached from above the panel when the tubes are in the sockets, time may be saved by testing the continuity of each tube filament separately with a C battery. If none of the tubes is open, then, to get at the socket contacts before removing the chassis from the cabinet, a piece of stiff wire bent into U shape may be substituted successively for each tube by inserting it across the filament prongs of each socket. After testing at one socket, the tube should be replaced in that socket, and the wire moved to the next socket to permit test there. Radiola 28 Circuit WHEN each tube in a series arrangement is paralleled with a resistance, such as in the Radiola 28 when operated from the 104 loud speaker, there are two series circuits which are parallel to each other throughout the length of the filament circuit, and are connected to each other between each tube. It should, therefore, be tested for continuity with the tubes out, but otherwise in the same order used when testing a single series circuit. As the continuity of the part of the whole circuit across which a tube filament is connected does not depend solely upon that tube, but depends also upon the parallel resistance, the opening of one filament is self evident because the others do not thereby go out. Similarly, if one section of the resistance opens, but the tube across it does not, the continuity of the whole circuit remains RADIO RROADCAST unbroken. In that particular case the tube filament will be carrying very nearly the current which was divided between it and the resistance section before the latter opened, so that it will be burning with abnormal brilliancy, a fact which is usually evident visually and hence removes the necessity for continuity testing. Special Cases IN SOME sets a series-parallel arrangement of tubes is employed. That is, in a set using six tubes, for example, there will be two series circuits containing three tubes in each, and the two circuits connected in parallel across the supply. Continuity testing of each of those circuits may then be done separately. That type of filament circuit is sometimes used in sets operated from a d.c. lighting supply. The serviceman should be very careful to refrain from changing tubes in that type with the power switch on, unless he is familiar enough with the circuit diagram of that particular model to know that it can be done without damaging other tubes. In some models there is but a single resistance used to drop the line voltage down to the voltage required for the two filament circuits. In such atrociously designed jobs, if one of the filament circuits is opened by removing a tube, or in any other way with the power on, the current drain is halved, the iR drop across the single resistance decreases, thus increasing the voltage across the remaining filament circuit. That increase of voltage in turn increases the current through the filaments,resulting in paralysis of all of them and often the burning out of one of them. X General Rules WHATEVER type of filament circuit is used, if proper voltage is not obtained at the set terminal strip, the next logical place to test is at the filament terminals on the power pack, regardless of what type that may be. If proper voltage is secured at that point, each leg of the supply from there to the set terminals should be analyzed separately as previously described in this article. If proper voltage is not obtained at the terminals of the power pack, then it becomes necessary to trace back through that supply unit. If the supply is d.c, one side of it goes back from the terminal strip of the pack directly to the line, while the other side goes back to the other side of the line through a fixed resistance, or a variable resistance, or both in series. One may, therefore, test for voltage from the terminal of the side which is connected directly to the line, to each end of the resistor or resistors in the other side and to any other joints which may exist. It is assumed in outlining these filament tests in receivers supplied from either a.c. or d.c. lines that the serviceman knows he has "juice" at the outlet used. When servicing any socket-powered receiver, if both filament and plate voltages are noticeable by their absence at the first socket from which a test is made, and there is no evidence of voltage at that outlet such as a lighted standing lamp supplied from it, then a voltage test should be made before resuming the regular routine. A common cause of "dead" d.c. -powered receivers is ignorance on the part of the housemaid, and often the customer, that when the little black plug which goes into the outlet is removed for any reason, it should be put back in exactly the same position. All servicemen who work in d.c. neighborhoods should carry small tags with them to be tied to the cord close to the plug, on which can be written: "Always replace plag with the notched side up." Then a small notch can be cut into the edge of the plug with a knife or file. The same thing applies to d.c. trickle chargers. If the polarity is reversed, the same thing will happen that happens when the polarity of the output of an a.c. trickle is reversed in its connection to the battery. The average customer is loath to pay a service charge to have a serviceman do nothing but reverse a plug in an outlet, especially if the call happens to follow closely one made to cure other trouble. No matter how carefully he may have been cautioned by previous servicemen about that particular thing, such is the nature of humans that in many cases the customer will not only commit perjury by definitely denying that he was so instructed, but he will also complain about the gross negligence of those preceding servicemen for their alleged failure to do so. As the d.c. supply for series filaments obtained from a.c. power packs is closely associated with the plate supply, testing of such supply between the power pack terminals and the line will not be considered in this article, but will be discussed in some detail in a future installment. The testing of parallel a.c. filament circuits differs from d.c. filament circuit testing only in that fewer stops are required. There are no rheostats to worry about, and in many sets the only point at which joints in the supply are not permanent soldered connections is the terminal strip to which the lugs on the cable from the power pack are screwed. When voltage tests made successively at the socket, set terminal strip, and transformer winding terminals to which the cable leads are soldered do not reveal voltage, then there can be only one trouble and one practical remedy (assuming, as before, that line voltage is getting to the primary of the transformer). The trouble is an open (or possibly shorted) transformer winding. The remedy is to replace the transformer. DEALER'S TEST ROUTINE CHART (FILAMENT CIRCUITS) Follow vertical arrows if proper voltages are not obtained across points written horizontally. If proper voltage is obtained at one of these points but not at the one described above it, then follow horizontal arrows Filament-parallel DC. TEST: Across Socket Prongs Filament-Series DC. TEST: Across Socket Prongs Filament-AC. TEST: Across Socket Prongs Across Strip Terminals £ t t I .. w I H + o e -a g v £ s Across Batter} Terminals Across Strip terminals in sets with parallel Resistors take Tubes out.In sets without such Resistors.test with Tubes in bo a 9 s El & + Across Strip terminals £ S 3, Eh bo Across Terminals of Transformer Filament Windings Across Power Pack Terminals E _ Elif £ 3 Eh "3 -II m ~ + & 1! a, c 2 + £ 2 S EH •£ £, a. v W Eh S g" c 6 « £ H E CD w E • muy, 1929 pugr 28 •