Useful Tips and Jottings

Radio Record, Volume III, Issue 17, 8 November 1929, Page 29

Useful Tips and Jottings

A Terminal Substitute. GMALL springs, made from phosphor bronze wire, from handy connectors for the constructor when making ‘temporary connections or when carrying out experimental "hook-ups." These are wound in a’ solenodial shape on a cylindrical former about din. in diameter. A good electrical contact ‘may be made by pressing the ends of the wires to be joined between the turns of the spring. This method of making connectiotis is also applicable when it is necessary to join two or more pairs of head-phones in seriés or parallel. we. A "Permanent" Crystal Detector. WHo2RE a crystal used in a set is of the type, which requires a very light pressure of the whisker on its surface, it is very annoying to find that , the slightest vibration, such as that * eaused by a person walking across the room, upsets the adjustment. This trouble wan -be easily avoided by stretching a little muslin cap across the gurface of the crystal. The whisker is then moved into contact with the crystal as usual, and the fine mesh of the muslin holds it permanently in position. Incidentally, the surface of the crystal is kept free from dust, which is injurious to its sensitivity. A Non-Corrosive Soldering Flux. SPIRITS of salts (ie., hydrochloric 5 acid) should never be used as a flux in radio constructional work, because of its corrosive effect on copper. The set would probably function quite " efficiently for a time, but it is quite likely that after a period the wiring. would be completely eaten through A good plan is to use "hard" solder in eonjunction with a paste flux made by dissolving resin in methylated spirits. ' Repairing Ebonite Panels. OrpD ebonite panels which have unhwanted holes drilled in them can be quite effectively repaired by the use of threaded ebonite rod. To stop a 8-8in. hole, for example, a piece of 7-16in. ebonite. rod may be used. The hole is tapped 7-16in., and a_ short length of rod previously smeared with Chatterton compound, is screwed in. The rod is cut off flush, and after the panel has been well rubbed down with knife powder and oil, the repair is alymost indistinguishable. A Precaution in Screen Construction. WHaENn designing a completely screened receiver, it is a good plan to make up full-sized cardboard patterns of the parts that will afterwards be made from sheet metal. The exact dimensions of the portions to be cut away, and the positions of all holes required, can thus be determined, without any possibility. of error. If firstclass results are desired, drilling should always be completed before shaping is commenced, The cardboard patterns enable this to-be done efficiently. To Remove Surplus Flux. \ FREQUENTLY, after wiring and sol- . dering a set, it is found that thin layers of flux remain spread over the adjacent surfaces. The panel must be cleaned if no lenkage is to result. A little benzine or methylated spirits applied with a small paint-brush will en-

able even inaccesible positions (such as those which exist between valve pins or coil sockets) to be thoroughly clean‘'B Battery Efficiency. TRIOTLY speaking, when the valves of a receiving set are not incandescent, no current should flow from the B battery. When the set is not in use however, it ‘will always be found advisable to disconnect either ‘one or both of the B ‘battery leads, This eliminates the possibility of panel leakage, which, though, perhaps, very slight, yet in time would seriously impair the usefulness of the B battery. ’ Mounting Panel Meters. IN a flush-fitting meter is to be mounted on a panel by means of three bolts passing through a flange round the meter, it is as well not to drill every hole before mounting. One hole should first be drilled, and the meter bolted into the desired position. The next two holes can now be drilled, and the mounting completed. If all the holes are drilled first, the holes in the meter and in the panel probably will not coincide, unless very ac--eurate measurements are made.

Instrument Protection. THE efficiency of sensitive measuring instruments may be easily impairby placing them. too near powerful motors, dynamos, loudspeakers, or mains units. Powerful permanent magnets, such as those used for movingcoil loudspeakers, ,create strong magnetic fields around them. For this reason, close’ proximity to any of the above instruments is to be avoided. A delicate watch, for example, may quite easily be. magnetised, and thus ruined. Drilling Large Holes. [THE process of drilling holes of large diameter in ebonite or wood is not an easy one for those who do not possess a good fret-saw. Good results may be obtained, however, by utilising -the following method. With a pair of sharp-pointed dividers, scribe a circle of slightly less diameter than that. required. ‘This is to allow for drilling and finishiig. Next, make a succession of fine drill holes around the circumference thus marked. When the centre has finally been drilled out, the indentations should be trimmed down with a sharp knife. A smooth finish is finally obtained by running first a fine file and then glass-paper round the edges of the hole. For Transformer Protection. H® burning out of a low-frequency transformer can often be attributed to a too sudden application of current, and, in order to avoid this happening, the filament current should be turned off via the filament rheostats, so that this becomes a gradual change, Alterations in high-tension voltage can then be made as desired, after which the valve filament may once more be heated in.a gradual manner. If this expedient is adopted, sudden rushes of current in the anode circuit will be prevented, and risks of transformer damage are thus considerably reduced.