Radio today (Jan-Dec 1938)

LOOK, MR. SERVICEMAN! • CHAPTERS in 40 CITIES • MEMBERS in EVERY STATE • and in ALL PARTS of the WORLD .... That's the Record of the RSA in only 6 Months! More and more, Servicemen have come to realize that it pays to belong — that the RSA is the one big constructive and progressive organization of Servicemen for Servicemen — that it is genuinely and actively helping every member. YOU GET ALL THIS (Material and Service that's Worth $75 and more!) YOU become eligible for participation in the Advanced Extension Course for Professional Servicemen — available to RSA Members only. ■£ YOU GET monthly Advertising and Business Promotion ideas to increase your business and make it more profitable. -^ YOU GET a subscription to "The Radio Serviceman"— the Association's monthly magazine. -^ YOU GET the benefits of being included as a "REGISTERED AND QUALIFIED SERVICEMAN" on record at National Headquarters for reference. -^ YOU GET a Certificate of Merit to hang in your shop. ■£ YOU GET a pocket card of credentials. -^ YOU GET the assistance of our big expert technical staff as a department in your service business. We will give you the right answer to your "impossible" service problems, -fr YOU GET (if you belong to a local serviceman's group) access to our National Speakers' Bureau — famous speakers for your meetings. ■£ YOU GET advance technical information on new circuits. It's as important to you as your job or your business! JOIN NOW and get in on all its benefits. RADIO SERVICEMEN OF AMERICA JOE MARTY, Jr., Executive Secretary, 304 S. Dearborn St., Chicago, III MAIL THIS COUPON! , RADIO SERVICEMEN OF AMERICA, INC. 304 South Dearborn St., Chicago, III. Gentlemen : I hereby make application for membership in the Radio Servicemen of America. Name Home Address City State Firm Name Address D I am enclosing $2.00 National Yearly Dues (Plus Nominal Local Chapter Dues) □ Bill me $2.00 National Yearly Dues. VIBRATOR TESTING the service profession. Many inquiries have been received for information, which would outline the proper method of using an oscillograph for testing a vibrator. The use of an oscillograph for testing vibrators is much less valuable than the dynamic characteristics of the mutual conductance method would be in testing tubes. The English reading emission tester has become by far the most popular method of testing tubes. There are a good many vibrator testers available but a simplified test would enable the service engineer to find out easily and rapidly the very things he needs to know about a vibrator. Earlier this article pointed out that vibrators should never need replacement until the contacts are worn to such an extent that the output of the power supply is unsteady or the vibrator fails to start at about 5% volts. The goodness of a vibrator may be tested by the value of the starting voltage the same as the goodness of a tube may be checked by the value of electronic emission from the cathode. Phrasing a vibrator test into English reading indications, vibrators which will start at 5.2 volts or less are "good" vibrators and will give many more hours of satisfactory service. Vibrators that start between 5.2 and 5.6 volts are "doubtful" vibrators and may be expected to fail in the near future. Vibrators that only start above 5.6 volts are "bad" vibrators and may be expected to give immediate trouble, usually when the car battery is low and not being charged by the generator. After the starting tests are made, the vibrator should be operated on 6 to 6% volts with a voltmeter connected in the output circuit. If the voltage fluctuates over a fairly wide range, the vibrator is definitely bad, but a fairly steady output voltage indicates a good vibrator. This test is equivalent to the "short's test" of tube testers. Vibrators which have been subjected to these two tests may be properly classified and the good ones used with confidence. Figure 13 shows a typical circuit of a tester which will provide the above tests. (See page 49.) The vibrator is first placed in the proper socket. The voltage is then adjusted by the potentiometer, with switch SI held in a closed position, to 5.2 volts. Switch S2 is then closed. If the vibrator starts, the starting voltage is 5.2 volts or less, indicating a good vibrator. If it fails to start, open switch 2 and readjust the potentiometer to 5.6 volts and again close switch 2. By adjusting the voltage to various values and opening and closing switch 2 the exact starting voltage of the vibrator and its corresponding condition may be obtained. 50 Radio Today