Radio Broadcast (May 1929-Apr 1930)

RADIO BROADCAST Fada 14b dynamic chassis. We had one set on the floor which was normal in every respect but terribly noisy, so bad, in fact, that some locals were blanketed. This is a common occurrence in our location, but in this case I ' smelled a rat. ' Acting on a hunch gained through experience with Elkon trickle chargers, I disconnected the voice coil leads and plugged in a magnetic loud speaker, leaving the a.c. loud speaker leads in the circuit. The noise persisted but, on pulling out the a.c. plug to the loud speaker, it disappeared. With the dynamic chassis out of the cabinet and field coil energized, if I touched the chassis the interference was increased to a tremendous degree, making any reception impossible. This led me to believe the rectifier was producing some sort of r.f. and the metal chassis was acting as a radiator, because when I touched it, the radiating surface was increased. The remedy was evident: ground the loud speaker case. This may be accomplished with a one or two-microfarad condenser between the case and one side of the a.c. line." Elgin D. Mobell, Service Radio Laboratory, East Orange, New Jersey. Hum from dynamic loud speaker: "After hooking up a dynamic loud speaker with a 100-volt d.c. field to a high-grade amplifier supplying the current to this field, I was greeted with a loud deep-pitched hum. This hum seemed to be entirely in the field winding. "After making several unsuccessful attempts to remove this hum, I hit upon a simple remedy that entirely eliminated it. This may prove of interest to those who are struggling with this condition, as I understand it as a common fault. "Simply connect a 200volt fixed condenser of from one to six microfrads across the loud speaker field. Theoretically the larger the condenser the better results. The writer used a four-microfarad unit which removed the hum to a point where it was inaudible a foot from the loud speaker, whereas without the condenser it could be heard all over the house." E. R. Hatheway, Worchester, Mass. Mr. Hatheway's experience is along the arguments adduced in the Aerovox Research Worker, March, 1929, which can be secured free of charge by writing to the Aerovox Wireless Corporation, 70 Washington Street, Brooklyn, N. Y. An article in this publication shows, by means of interesting curves, how it is possible to eliminate such hum almost entirely by shunting an Aerovox 1500 or 2000-mfd. "A" condenser across the field winding. By use of high-capacity condensers it is possible to reduce the ripple level considerably below the minimum secured by hum coils. Reducing hum by tube shielding: "A considerable reduction in hum can often be effected by shielding one or more of the tubes in a receiver — generally including the detector tube. "A good many manufacturers are turning out a.c. sets in which the tubes are Original and effective! Black type on a yellow card makes this reminder easily located among the contents of the average desk drawer. placed adjacent to one another in a straight line. If the rectifying tube is close to any other tube, especially the detector tube, it introduces hum in the set. A very simple remedy for this trouble is to take an ordinary cylindrical bakingpowder can and solder a lead to the bottom. If the can is then slipped over the rectifying tube and the lead connected to the ground post, the reduction in the hum will be gratifying." Geobge Eabl Speibs, Revere, Mass. Ballast cap reduces hum: "In many of the modern electric sets using a heatertype detector tube (227) a bad hum will sometimes be noticed in the loud speaker. If an ordinary ballast tube cap of the type used for microphonic tubes is placed on the detector tube, and a wire from the ground attached to the cap, the hum will be found to diminish or disappear entirely." W. Gobden Genneb, Jb., Great Neck, N. Y. 36$ Radio Service' Frank J. Shannon 5315 HAVERfORO AV£ PHIUDrLPHI RADIOLA SPICIAUST AND AIL MAKES WORK GUARANTEE!* ep This Card For Future Reference Test and service laboratory of E. F. McLaughlin, of Rockland, Mass., dealer in Atwater-Kent, Radiola, Kolster, and Majestic receivers. Prominent portions of this equipment include a modulated oscillator and a tube reactivator. Home-made lube ballast: H. J. Goddakd of Ellendale, N. D., does the same thing a little more economically : "A bad case of hum in an a.c. set that refused to respond to the usual treatment, was quieted by wrapping the 227-type detector tube with tinfoil and grounding this to the chassis." Miscellaneous cases of hum: The "April installment of the 'Serviceman's Corner' discussed an interesting case of hum which recalled to my mind a similar trouble which I ran into some months ago. I was called upon to service a newly built power pack. It hummed badly, although it worked well otherwise. Apparently the circuit was connected properly as the plate, grid, and filament voltages checked correctly. "After increasing the capacity of the detector by-pass condenser, the hum reduced materially, however, not sufficiently. It was apparent at once that something was wrong in the condenser bank. Upon careful inspection I found that the condenser bank consisted of two groups of condensers. The lead from the rectifier tube ran directly to the group of condensers which contained the detector by-pass condenser instead of first going through the choke. Thus, the hum was induced capacitatively into the detector circuit. By simply re-arranging the leads from the choke to the condenser groups the hum was eliminated." Fbed F. Oeblein, Philadelphia, Pa. Poor arrangement of parts causes hum: Chables H. Jenkins, Jb., Radio Service, of Audubon, N. J., is both lyric and effective : "'Hum'! How often have I heard it and how elusive it is sometimes. "In power amplifiers where the audio system and the power-supply apparatus are on the one board or very close together you will often have a hum if the audio system is built of good transformers and the loud speaker a good dynamic cone. If the audio transformers are too near the chokes or transformer in the power supply, you will get a hum regardless of the fact that the transformers may be shielded and grounded. The greater the amplification the more likely you will hear the hum. Changing the relative positions does not help a great deal. There is only one sure answer, i.e., allow plenty of room between the power supply and the audio system. "Another frequent cause of hum from good amplifiers that I have constructed, is the value of the grid leak. Generally speaking, a high-resistance leak in the detector circuit will permit more hum to come through than [a low-value unit. Of course, the lower the resistance of the leak the less sensitive is the detector, but with sets having two or more stages of radio-frequency amplification a 1-meg. leak usually holds down the hum and does not impair the operation of the set. " Another cause of hum is the voltagedivider system. Too low a resistance shunted across the high-voltage output places a load on the rectifying tube and causes hum." Hum in A-K Sets: "The most common hum trouble that I have experienced in Atwater-Kent electric sets has been traced to a loose nut that makes connection with the common negative return to the transformer and the ground connection. "This is one of the two screws that hold the bakelite terminal plates down to the ground connection in the transformer, choke, and condenser box, and the trouble seems to be caused by the warmth • JULY • 1 929 • • 175