Tips and Jottings for the Constructor

Radio Record, Volume V, Issue 27, 15 January 1932, Unnumbered Page

Tips and Jottings for the Constructor

__. Loudspeaker Buzzing. N not a few cases peculiar and very unpleasant "zizzes" in loudspeakers. of the. balanced armature type may be traced-to actual vibration of the baffle board. Some of these instruments produce in themselves a great deal of vibration, and these vibrations are transmitted to the board., The. result is that. whenever of the natural frequency oL the board occurs, resonant effects are observed. One way of overcoming this difficulty is to make a stand for the unit and its cone and to place it behind the baffle so that the front edge of the chassis just clears it. Protecting Ebonite. F an ebonite panel is clamped in a vice wifhout any protection its surface will almost.certainly be damaged. To prevent this obtain a lump of cobbler’s wax and ‘cut two pieces of thick card to’ fit. the’ jaws. Rub one side of each card with the wax so that its surface is quite sticky. Put both cards in the vice withthe waxed sile toward the jaws and clamp up tightly. When the vice is opened the cards will then adhere to the jaws, They may be taken off with little trouble, and they will form a perfect cushion for any work which is soft enough to take an impression from the bare jaws. Cutting Ebonite. HEN an ebonite panel is cut, it will be found that the cut edge is very rough in appearance in comparison with the finish of the original edge. To obtain a similar finish for the new edge, the panel must first of all be clamped in a vice and the marks from the saw removed with a medium flat file. Work the file diagonally across the edge, taking care not to cut too deeply at any one point. Next lay the file flat along the edge and push it straight along with firm strokes, keeping the whole length of it pressed down on the work. Now wrap a strip of emery cloth round the file and work with this in straight lines backward and forward until all the file marks have disappeared. Repeat with the finest emery paper. Finally moisten a piece of rag with turpentine, rub this into the edge, and polish off with a clean, soft rag. . A Good R.F. Choke. A RADIO-FREQUENCY choke for shortwave -work may be quite simply made in the following way. On an ebonite coil former 14 inches in diameter and about 2 inches long wind about 70 turns of number 34 d.s.c., securing the ends to terminals mounted in suitable positions at the end of the former. The whole is then mounted on a baseboard by means of a strip of ebonite which is passed through the former and screwed down at each end. ee

Curing Microphonic Howl. HE simplest and best ‘cure for microphonic: howl is to apply . "damping" to the glass bulb of the detector valve. This.may be accomplished either by covering the bulb completely with sticking plaster, or with a soft lead cap. A half-inch nut stuck to the. bulb with plasticene or chewing gum is also quite effective. "Starting" Small Nuts. ’ SIMPLE way of "starting" small nuts which are situated in inaccessible positions is to break the point of a lead pencil and then inserting the blunted point into the nut. A little manipulation will suffice to "start? the nut on its thread, after which the pencil may gently be removed and the nut screwed home tightly with the fingers or a small pair of pliers. Drilling Ebonite. . FPONITE should always. be drilled slowly and with not too much pressure on the drill. If the drill is rotated too rapidly the ebonite will chip instead of leaving a clean-cut hole. When the drilling is finished, give the panel a rub over with methylated spirits. This will remove all finger marks and restore the panel to its original condition. A Useful Adjunct. AN old shaving brush with the pristles trimmed down until they are about an inch in length is an excellent adjunct to the radio worker’s outfit. It can be used for removing traces of flux from a wired up panel, for removing brass steel filings from inaccessible positions, for cleaning soldering irons and for a number of other thnigs. Slow Motion Dials. [HERE are several points to note when buying slow motion tuning dials. he first is that an end stop is provided after each half revolution. This ensures that the dial will not slip on the condenser central and upset the reading. A second important point is to see that there is no backlash, while finally make sure that the method of fixing--to the condenser spindle (and panel if necessary) will result in a. firm and secure grip. Sensitivity Test. AN excellent test for sensitivity for headphones is to put them on in the ordinary way, holding one of the tags between the. lips. Now rub the other tag with a key, file, or other piece of metal. If a rubbing noise corresponding with the movement of the key is heard, you can be sure that the phones are sensitive. es te:

Making Soldered Joints. THE ability to solder -joints efficiently is undoubtedly half the battle in set construction, Certainly it is possible to obtain .satisfactory results from the set with no soldered joints, but the chanees- of possible failure or of undesirable noises occurring in the. finished set are greatly. diminished where soldering has been employed .. throughout. After -all .there is nothing very difficult about making a soldered connection if the following points are borne.-in. mind. Never attempt. to work with the iron. too hot or too cold. ‘The. best indication of the.correct. temperature is to hold.the bit against a piece of paper. It should be sufficiently. hot just to scorch it, but. not to burn it, _. With the iron correctly heated, file the end until it is quite eleand dip it int a tin lid on which has been app’. @ a small amount of fluxite, and, g apply a strip of solder to the nished portion of the bit. ‘The solde should run quite easily and should leave the bit end brightly nickeled in appearance. Next clean the tw9 parts which it is desired to solder, apply a small amount .of fluxite, and then, by holding the bit on the prepared parts, the two should weld together quite easily. Bushings for Metal Panels. " OW that metal panels are becoming more popular the subject of terminal bushings is of some importance. Ebonite terminal bushings, can, of course, be obtained from most dealers at reasonable prices: Nevertheless, this article can be made very cheaply indeed from a piece of ebonite tubing whose internal diameter is sufficient to take the shaft of the terminal. Tn order to convert such a piece of ebonite tubing into efficient bushes with metal or wood panels, cut sections from the tubing, each section being long enough to enable approximately 1-8in. of it to protrude on either side of the panel. For use these sections should be soaked in hot water, for 2 few minutes in order to soften them and they should then be inserted quickly in position and the terminal nuts screwed home as rapidly as possible. In this way the tightening pressure of the nuts will compress the 1-8in. of the bush which protrudes on each side of the panel, thereby creating a flange which will effectively insulate the terminal. Testing Condensers. HEN a large fixed condenser (i%1 or 2mfds. is connected . atross "B_-" and a "B+" tapping it is possible to remove the wander-plug from; the latter tappings without the set im~ mediately ceasing to operate. The smaller the current being taken the longer will the set go on working, and also the larger the condenser the greater will be the period before signals will fade out. This phenomenon is, of course, due to the reservoir effect, or storing capabilities of the condenser. It forms a convenient method of testing large fixed condensers of 5 mfd. and upward. All that has to he done is to connect the condenser to he tested across a "B-++-" and "B-." switeh on the set, and then remove the wander-plug supplying the particular — tap and note if reception continues for a while. =)