MAKE YOUR OWN FIXED CONDENSERS

Radio Record, Volume I, Issue 21, 9 December 1927, Page 12

MAKE YOUR OWN FIXED CONDENSERS

> -A COMPLETE SCHEME FOR CONSTRUCTORS

With a set of fixed condensers made an the plan set out below, it is pos-. sible with a certain amount of trial, to’ get the best possible tone from the audio side of a receiving set, and those who have constructed the Browning Drake four-valver are advised to give this idea a full trial, when previous results will in very many cases be improved upon. The owner of a set of condeusers need not be greatly concerned as to the exact value of any of them, so long as he has tried several, and found a particular one to give the best results. ‘Megohm" commenced making fixed condensers on this plan five years ago, and has used them with puccess ever since. For experimental work they must be both durable and capable of being quickly changed. ‘Mica is usually rather scarce in New Zealand, and most listeners will be obliged to take what they can get, and quite likely many will have to split up a thick piece. This is easily done by inserting a sharp knife-blade at one corner, and running it round until a layer splits off. These layers should not be made too thin and "papery,’’ but should have the stiffness of, say, a visiting card, with rather less thickness. All should: be made as nearly the same thickness as possible, but double thickness is advisable for outside plates. The standard size of mica adopted for the plates is ljin. by lin., and the tinfoil to be used is to be cut into slips 08in. wide and L.7in. long. These are shown exact size in the diagram. piece of tinfoil is attached to each thin piece of mica in the position shown, projecting at one end, shellac cissolved in methylated spirits being used to stick the tinfoil to the mica. Any number of plates can now be put together, each one stuck to the next with shellac, and the projecting tinfoil placed at each end alternately. A piece of the thicker mica is placed on each side of the assembled plates, and the whole put under a weight of two or three nounds to set. Iind clips are made from yverv thin copper sheet cut 0.4in. by 0.9in., as shown, and folded in two over an edge of suitable thickness. The projecting tinfoils are now turned over on one side at each end to lay on the outside plate Then a small quantity of -fish glue or seccotine is put on each end of the other outside plate and a copper clip pushed carefully on each end and nipped close with pliers. In order to keep the condenser as solid as possible, smaller clips can be put on the sides in the same way. One of the outside plates should "be num-

bered in reverse with white or other paint, showing the number of tinfoil plates. This number is then placed inwards, and will show through the mica, and never rub off. There is such variation in the thickness of mica whether bought ready or split at home that it is impossible to give ,

capacities with amy accuracy, but, roughly, an ayerage two plates may be about .00025 mfd. Experimenters will require a few capacities smaller than two plates. Two tinfoils 0.6in. wide will give a 14 plate; two 0.5 wide, al plate; two 0.2 wide, a half plate, and so on. It should be noted that good, transparent mica with a ruby tinge should be procured if possible, as quality is (Continued in Next Colum.)