Radio Broadcast (May 1928-Apr 1929)

THE SERVICEMAN'S CORNER SERVICEMEN have not been slow to write us of their unqualified approval of "The Serviceman's Corner." And what is more, they have sent us a great number of excellent contributions, many of which appear in the paragraphs below. We welcome contributions, all of which, if accepted, will be paid for at space rates. Melvin L. Shook, of Shook & Jones, Akron, Ohio, writes that a large proportion of their service calls are on old sets. "So far, our experience on the a.c. sets has largely been the replacement of tubes. One of our greatest difficulties is in securing circuit diagrams of standard receivers which we are called upon to service It eliminates a lot of work when you have the diagram with the constants to go by. As practising servicemen we are constantly called upon to service all types of sets. Consequently, we greatly appreciate your 'Service Data Sheets on Manufactured Receivers.' " It is doubtless true that a large amount of service work is not done by the dealer from whom the set was purchased. Service organizations and individuals doing service work will be wise to collect systematically all the data on all types of sets which turns up from any reliable source. How to Tell Failing Rectifier Tubes: Failure of the rectifier tube is best indicated by a decrease in the plate voltage supplied to all tubes in the receiver. It may be distinguished from failure in the last audio tube by comparing the voltage readings obtained on the remaining tubes with the plate voltage reading on the power tube. When the rectifier tube is at fault, all readings will be low; when the power tube has failed, all readings will be high. E. T. Cunningham Inc., inform us that the average life of the cx-381 is 1000 hours or more when the transformer voltage does not exceed 700 volts and the plate current is limited to 85 mA. maximum. Tubes of this type operated from transformers not made recently may be operating out of 750 volts, with a resulting lessening of tube life. Another important precaution is operating the filament at its rated voltage. The tolerance here is not greater than plus or minus 5 per cent, (see Radio Broadcast for January, 1929, page 181.) Replacing a Fixed Condenser in A. K. Sets: "I have had considerable difficulty in procuring a condenser of proper physical dimension to replace the fixed by-pass condenser in Atwater Kent sets," writes G. A. Thurling of Springfield, Massachusetts. "This condenser breaks down, shorting the B-power circuit and making reception very weak, or altogether impossible. "The fixed condenser used in a Ford ignition spark coil makes an excellent substitute. Discarded Ford coils may be purchased for These pages mark the third appearance of our special section for the practising serviceman. To say that this department has been enthusiastically received would be putting it mildly. Scarcely any innovation in Radio Broadcast in the last five years has attracted so much favorable comment. We are welcoming carefully considered contributions which, if accepted, will be paid for at our regular rates. A number of contributions describing set testers have been received and we are not at present in terested in others unless they are designs of great originality and especial merit. Contributions are especially desired which describe the solution of especially baffling problems encountered which service manuals do not cover and similar items calculated to be of the widest interest to others working in the field. Typewrite your contributions and address them to the Editor, Serviceman's Corner, Radio Broadcast, Garden City, New York. — The Editor. as little as twenty-five cents at most Ford service stations. This condenser fits snugly in place of the usual condenser in Atwater Kent models." Testing a.c. plate voltage: Servicemen testing the plate voltage on tubes operated from a.c. often make the mistake of placing their voltmeters across the B supply just as they would in a d.c.-operated amplifier, for example. This does not give the true plate voltage, but the plate plus the grid voltage. In order to read the true plate voltage of an a.c. -operated tube, especially where the tube gets its C bias by the plate-current drop through a resistor, the meter must be con i Voltmeter across'here j reads' Ep+ Ec >. (220 Volts for A 171) Fig. 1 — These diagrams show hoiv to measure accurately the plate and grid voltages in a.c. -operated receivers • february, 1929 . . . page 255 • nected between the plate terminal of the tube's socket and the filament terminal of this tube. It does not matter which of the two filament terminals is used. See Fig. 1. Increasing Response on the Longer Wavelengths: My contribution is on the subject of boosting volume on the long-wave end of the dial on sets employing a resistor across the grid and ground as an untuned coupler for preventing the length of the antenna used from affecting the gang control of the radiofrequency stages following. Such sets as the Aero Seven, Graybar 310, R. C. A. Models 16, 17, 18, Knight 6-7, Monroe 8-9, and many others can be improved with this simple kink. Take any solenoid coil such as is used to cover the broadcast band in conjunction with a 0.00035-mfd. condenser and connect one end of the coil to the antenna post of the set and the other end to the ground post. Coils having a diameter greater than two inches seemed to work best. Some old Lorentz wound coils out of an ancient Freshman Masterpiece set worked excellently. The primary coil was ignored, but care was taken that the ends did not short. The results were not so good on two Atwater-Kent sets the coils were tried on. probably because A.K. uses a choke instead of a resistor across the antenna and ground. The most effect of the coil in all cases was on the long waves, just where most one-dial sets need a little more energy. — J. P. Kennedy, South Bend, Indiana. Interference Elimination: In one locality a great deal of motor noise was picked up by a Radiola 18 and it was found that by removing the ground the trouble was reduced to such an extent that it was not objectionable, while in another case with the same type of receiver the trouble seemed to be due to a faulty ground. When a wire was shunted across the water meter the interference practically vanished and the volume picked up at least 50 per cent. — K. R. Tantlinger, Cumberland, Md. Trouble in a Radiola 17: Operation would be normal for about twenty minutes then a tremendous noise would drown out the signal. This noise would continue and then normal operation would obtain. Tubes were tested and proved in good condition, but as an additional precaution were tested in another set. Routine tests for continuity were made, showing proper results. The trouble was finally remedied by replacing the grid condenser. The condenser after removal was tested for a short circuit with 250 volts, but it showed no current, passage. As I was unable to disassemble it without injury, I do not know what the trouble was. [The test made would indicate a short circuit, but not an "open." It is just possible that the mica grid condenser in question had an open cir