Questions and Answers

Radio Record, Volume III, Issue 17, 8 November 1929, Page 10

Questions and Answers

Receiver Neutralisation. W. N.M. (Westport) asks the following questions :- 1.'How can I neutralize my factorybuilt receiver? A.: If adjustments to the neutralising condensers do not affect a cure on the lower portion of the dial readings, try reducing the plate voltage on the radio frequency stages. Sometimes an additional 1 mfd. fixed condenser inserted across the B+ radio-frequency connection and the A- connection (at the receiver end of the battery cable) will stabilise a set sufficiently. For detailed method see "Listeners’ Guide." 2, What causes transformers to burn out? A.: A break in the primary winding of an audio transformer can be put down to various causes. For instance, excess plate current sometimes produced by using insufficient grid-bias may cause the trouble. A more common cause is the electrolytic, decomposition of the fine wire due to dampness and also eorrosion after the. use of unsuitable soldering flux. 3. Is it absolutely necessary to have 185 volts on the plate of a 112-power valve, and also is 44 volts grid bias sufficient? A.: No, it is not absolutely necessary, although you will not be able to obtain anything like the maximum undistorted volume when using less voltage. Also 43 volts grid bias is insufficient except when using low plate voltages. With 135 volts on the plate, a bias of -9 yolts on the grid would be required. 4, Can distortion he eliminated when fading occurs? A.: No. It is impossible to regulate either fading or distortion arising from this cause.

Battery Charging. T L. (Kaiwarra) has a_ battery * charger of the vibrating type, and asks if it is adaptable for use with the B. battery described previously in the "Radio Record" of "Megohm." A.: According to the slip enclosed, the charger will charge the five blocks connected in parallel at a slightly lower rate. This is not a great disadvantage. A 60-watt lamp will give anproximately a 100 m.a. charge, which would be quite suitable. There would be no advantage gained by cutting down the blocks below 24 volts each by a 60-watt lamp and follow instructions, Short-wave Adaption. I HAVE a four-valve receiver, using wet A and B batteries (writes W.D.A., Tarata), and I would Hike to ‘know whetrer an efficient short-wave set could be obtained without altering the wiring of the receiver in any way. A.: A short wave adapter would be quite suitable, and a description of how to build this unit can be found in the 1929 "Radio Listeners’ Guide," but. omit the bypass condenser between P of the valve and A+, » ee seecntenererenennnnennneney

Charging B Batteries. Jy I.C. (Fiji) sends a sketch of an * arrangement for charging B batteries ftom the 280 volt. D.C. supply, and wishes to know if it is quite suitable. A. Providing one or two details not mentioned in your letter are in order, the arrangement is-.quite suitable, Make sure that the positive of-the supply connects to’ the positive of the battery. Also it is presumed that the B batteries to be charged are accumulators, and not -the commercial dry cells. Battery Charging from a Windmill. HE charging of A batteries has always been a source of trouble to country listeners. "Is it possible," asks S.S. (Hawke's Bay), "to use 4 windmill for this purpose?" , A.: The idea is quite feasible provided that a suitable relay, which’ will regulate the charging rate, can be purchased or constructed. When the windmill comes to rest an automatic device will be necessary to break the circuit, otherwise the accumulator will discharge through the generator. The apparatus would have to be suitably geared to maintain the correct speed of the dynamo. This would necessitate more than an amateur mechanical or electri- cal knowledge. Moving Coil v. Linen Diaphragm, Vp HicH is the better for sensitivity.and the ability to handle volume, asks A.S. (Khandallah), the moving coil speaker described by "Pentode"’ or the linen diaphragm and its adaptations by "Mégohm?" TI am using a three-valve regenerative receiver. . -: A moving coil speaker has to be used with a powerful amplifier to redlise its full superiority, though such speakers have been used very successfully with smaller sets. Under these circumstances it would be better to utilize the linen or silk diaphragm speaker. . Where ean I obtain the casting for the M.C, speaker and what is the approxi-

mate cost? . A.: Precision Engineering Co. £3 10s. 3. What size baffle would be required to reproduce down to 30 cycles? : A.: At least two feet square. 4, Is a baffle required with the linen diaphragm?-No,