Radio Broadcast (Nov 1926-Apr 1927)

APRIL, 1927 TROUBLESHOOTING IN A. C. AMPLIFIERS 583 CRACKLING NOISE IN LOUD SPEAKER ACRACKLING sound, much like static, frequently is heard when the buffer condensers (used with the Raytheon tube) are either omitted from the circuit or are defective (open-circuited). In either case the remedy is a pair of good buffer condensers. A defective Raytheon tube will also cause this same noise. ARCING BETWEEN HIGH-VOLTAGE PRONGS OF RECTIFIER SOCKET THE a.c. voltage between the two outside leads of the power transformer secondary is equal to twice the normal operating voltage of either half of the secondary which, in the case of the different transformers recommended by the writer in this series of articles, is between 500 and 600 volts. This high voltage is directly impressed across the two adjacent tube socket springs, and to prevent danger of a flash-over, followed by a steady arc, it is extremely important that nothing be done that will impair the quality of the insulation. For instance, just a little soldering flux or paste, between the filament springs of a rectifier tube socket — they are quite close together— is enough to start an arc, which, when once started, is capable of doing considerable damage. For this reason it is important to remove any traces of soldering flux around the terminals and contacts of the rectifier tube socket. Use a cloth dipped in alcohol for this procedure before connecting the complete unit to the electric light lines. ARCING IN STEM OF RAYTHEON TUBE IF A Raytheon tube flashes over and arcs between the two anode leads outside of the "cup," the useful life of the tube is over, as one of the lava insulating tubes has cracked. A new tube is the remedy. DETECTOR AND R. F. RESISTOR GET HOT AS EXPLAINED under another paragraph, care must be used when constructing an amplifierpower supply unit on a metal base to see that the metal does not at any point cut through the insulation on the wire and cause a short circuit. If the wiring is o.k., then look for a defective bypass condenser and replace it. FIXED RESISTOR ACROSS THE PLUS-B DETECTOR IF THE variable resistor controlling the detector plate voltage is turned to a very low value, or entirely short-circuited, the full amplifier B voltage is impressed across the 10,000-ohm resistor. An excessively high current will flow through the unit, and it will get hot. The variable resistor should never be so adjusted as to cause over-heating of the fixed resistor. POWER TUBE DOES NOT LIGHT WH EN the filament of the output power tube fails to light, and there is no doubt as to the tube being in good condition and making perfect contact with the socket prongs, make certain that the a.c. power is on before looking for further trouble. Possibly, the socket to which the amplifier-power unit is connected may be defective, or else the power may be temporarily off. If there is no doubt about the power and the condition of the tube, then check the wiring for an open or short-circuit. If the difficulty is still undiscovered, then temporarily disconnect one lead from each of the two buffer condensers. Should this latter step correct the trouble, then one or both of the condensers are defective and new ones should be secured. Should, however, the trouble still be present, the probability is that the power transformer is defective. MECHANICAL HUM OR VIBRATION A NOT altogether uncommon fault of some amplifier-power units, which are in other ways very excellent, is a mechanical vibration due to loose lamination in the core of the power transformer. If the transformer is assembled in some sort of frame, frequently all that is necessary to do is to tighten the frame bolts. In other cases, however, where the transformer is enclosed in a metal container sealed with wax, the trouble is more difficult to remedy. If the hum is very bad, then perhaps the best procedure is to return the transformer to the manufacturer. If it is only slight, but yet sufficiently annoying, then a simple remedy is to place the amplifier power unit in a closet, the cellar, or some other place sufficiently isolated to do away with the noise, and use cabled leads between it and the set. ELECTRICAL HUM IN THE LOUD SPEAKER AN ELECTRICAL hum in the loud speaker may be due to any one. of a dozen, or more, different causes. There is also considerable difference in the hum due to various causes, thus enabling the person experienced in the design and the construction of a.c. operated amplifiers to locate quickly the source or reason for hum. For instance, hum due to an unbalanced filament winding center tap is predominatingly a third harmonic of the fundamental power-line frequency, while that due to lack of electrostatic shielding in the transformer, a ninth harmonic, and that due to incomplete filtering of the B supply, a second harmonic. The novice, however, will probably have to go right through the list of possible causes and remedies until the difficulty is located and corrected. First, and most important, the negative A terminal, either on the amplifier, set, or storage battery, must be grounded. If there are any other ground connections to the receiving set proper, remove them, or, in the case of sets with which the removal of the regular ground connection will interfere with the proper operation of the set, insert a i-mfd. bypass condenser in series with the ground lead. Secondly, remove any lamps or other electrical appliances from the vicinity of the set or amplifier. Merely turning off the light, in the case of a lamp, will not do. Either remove the lamp as a whole or else remove the lamp cord plug from the base outlet or socket. Thirdly, see that the grid-bias voltage control, if variable, is not set at too low a value of resistance. Adjust for best quality, most stable operation, and minimum hum. Fourth, do not run the a.c. cord from the amplifier near the antenna lead, the set proper, or the input lead from the set to the amplifier. Make the line cord as short and direct as possible without coming close to the set. Fifth, ground the transformer, choke coils, and condenser cases, or frames. Sixth, place the amplifier-power unit several feet from the set unless either the set or the power unit, or both, are completely shielded. Seventh, so place the filter chokes with respect to the power transformer as to prevent any undesirable magnetic coupling. Eighth, interstage coupling between either the radio or the audio stages, especially if the detector tube is involved, may result in a disagreeable hum. All of the amplifiers illustrated in connection with these articles have been so constructed as to minimize any possibility of such trouble. If a hum still exists after all other attempts to stop it have failed, then, with the amplifier and the set in operation, pull out the first audio tube. If the hum then stops, it is probably being caused by interstage coupling, due to poor layout of parts, failure to cable all direct-current leads, failure to use bypass condensers and resistors, as shown in the various circuit diagrams, long grid and plate leads, or magnetic coupling between audio transformers. Regeneration due to a regenerative detector circuit is confined to one stage and does not produce a hum. Interstage regeneration, due to any of the above causes, does produce a hum. Follow the different layouts described and trouble from this source will not be encountered. " MOTOR-BOATING " THE reason for and the methods of curing "motor-boating" were completely covered by the writer in the article in power amplifiers in the February Radio Broadcast. To recapitulate briefly, the two most important points to be watched are the proper adjustment of the grid bias control, and avoiding the use of excessive detector or r.f. voltages. Radio Broadcast Photograph COMBINING COMMERCIAL UNITS This illustration shows how a standard B supply unit, such as the Mayolian may be used with the audio amplifier designed by the author, to obtain a combination amplifier-power unit. The amplifier consists of) an impedance-coupled stage and two stages of resistance-coupled amplification and includes an r.f. choke, a grid choke in the power stage to prevent motor-boating, and a tone-filter. Where the power supply is other than 60 cycles a.c, it is often difficult to obtain suitable transformers and chokes for the home construction of a power-supply unit. B supply units for 25 and 40-cycle a.c. are available, however, and when used as here shown, give the equivalent of a combination power amplifier and B supply