International projectionist (Jan-Dec 1936)

INTERNATIONAL PROJECTIONIST December 1936 FIGURE 1 1. Horn receiver attaching piece. 2. Spacer ring with empire cloth above and below. 3. Diaphragm retaining member with spider-sup ported cone. 4. Diaphragm composition insulating and retaining rings. 5. Duraluminum diaphragm. 6. Speech coil. 7. Case cover plate constituting electromagnetic pole, Ps. 8. Main case, the centre of which constitutes electro-magnetic pole, Pl, 9. Wool, not to be disturbed. 10. Field winding. continued use the diaphragm, instead of vibrating firmly, is so weakened that although it appears normal it crinkles like a piece of cellophane paper, and at the instant part of the diaphragm is traveling in one direction, another part is actually traveling in the opposite direction. A diaphragm found in this condition, checked by listening, is worn out for normal use and should be replaced. Sometimes if a gun is fired in front of a horn the diaphragms will be split, the horn acting as a sound-vibration collector much the same as an ear trumpet. Even on football fields many feet from gun fire diaphragms have been damaged so that reproduction therefrom was seriously impaired. Diaphragms can be burst wide open by a single blow with a hammer on the horn that the units are working into. Care should be taken not to damage the receiver by sound vibrations of enormous magnitudes. Receiver diaphragms so damaged should be replaced. A heavy rust will sometimes form on the pole pieces of receivers used in airdromes or receivers left in unheated closed houses. This rust is often so thick [over .003"], that the movement of the speech coil is not free. Sometimes these receivers can be cleaned, but usually the speech coil is damaged so that diaphragm and coil should be replaced. A single iron filing in the receiver will always work its way into the highly magnetized gap between the pole pieces and the speech coil, thus damaging the coil. If the iron filing rubs the speech coil while receivers are being used, the coil and diaphragm usually need replacing, as part of the turns are frequently shorted. For this reason a receiver should only be dismantled on a clean table or bench. If a receiver is removed from service and the mouth cap cannot be found, do not put a cork in the mouth. The upper compartment of the speaker is supposed to be airtight ; if so the air pressure from the cork will spring the diaphragm so that it will thump back and forth in service like an oilcan bottom. The reproduction from a receiver so damaged sounds similar to the reproduction from a normal receiver driven by a Class "B" amplifier. A cloth over the mouth secured with a rubber band is sufficient to protect the receiver against dirt. Simple Checking Aids Eighteen inches of rubber tubing, four inch of copper tubing, some rubber tape or a single-hole rubber cork, and a lighted cigarette are the only materials necessary for a simple yet quite satisfactory method of checking the diaphragm and the upper chamber of the receiver. Remove the entire upper part of the receiver by removing the screws holding the case cover plate, 7 (Fig. 1) to the main casting, 8. [Do not remove screws holding parts 3 and 7 together.] After these screws are removed the plate will probably still be held in position by dowel pins. The plate should be pried off evenly with screw-drivers, care being taken not to burr the surfaces or let loose iron burrs fall into the speech coil. After the top part is removed the rubber tube is corked into the hole in the receiver attaching piece, 1. The other end of the rubber tube should be operated with the mouth only, glass blower fashion, and not with a pump. A pump is noisy and too powerful. With a puff of smoke the receiver upper chamber can be tested for air leaks. This is important. For example, if a hole 6 inches in diameter is cut in the very end of a 14-foot air column horn, no effect will be noticed. If a 2-inch hole is cut in this horn two feet from the receiver, the effect is quite noticeable, as is a hole y% in diameter in the horn at a point close to the receiver. An air leak in the upper chamber of the receiver should be sealed if it can be done without obstructing the operation of the diaphragm. Now, by blowing and sucking on the rubber tube alternately in rapid succession, observe the clearance between the speech coil and the pole piece — which should be over .003" preferably .005" all the way around. Use a piece of thick bond paper as a gage, as anything stiffer may damage the delicate diaphragm or speech coil. A diaphragm in a perfect receiver will be almost absolutely quiet in its operation. If it makes considerable noise when operated back and forth by successive air pressures and rarifications, the diaphragm has either been damaged or used longer than its normal life. An excessively noisy diaphragm should always be replaced. Always remember that a receiver is a precision piece of apparatus which has been properly adjusted and damped before it leaves the factory. The damping wool in the pole piece cavity, if the receiver is of that type, should never be disturbed. Replacement of Diaphragms Diaphragms are on sale and installed by radio shops, but the writer recommends that receivers used in theatres or other public places be shipped to receiver manufacturers for replacements of diaphragms. These manufacturers do precision work and have elaborate test equipment. Many diaphragms installed in the field appear to be in good condition but fail to meet test requirements placed on receivers for strenuous theatre use. Sound service men often have instruments to test the characteristics of the entire sound equipment up to the receivers, but very few have the instruments and equipment to thoroughly test receivers. At this writing probably more receivers will fail to meet reasonable test toleration limits than all other units or apparatus put together. As a rule, receivers that have had strenuous use for over three years should be removed from service and sent to a reliable concern for replacement of diaphragms. The time required to test such units is likely to be wasted, as such a small percentage are still good, and those that are barely acceptable have very little life left. When putting new receivers into service, loose nails, bolts, nuts and supporting chains should be eliminated. Horns should be supported with steel cable instead of chains. Loose parts around the horns will vibrate to produce the same sound as defective receivers. Care should be taken also to properly pole receivers. Polarity, Poling and Phasing Wherever there are two .coils in parallel so connected to a current sup 2_OOhms %% ecch L FIGURE 2 Diaphragm travel check: if the direction of windings is the same as shown by the figure, the currents are traveling in parallel and the diaphragm will be drawn back.