A Diagnosis of Radio

Radio Record, Volume IV, Issue 52, 10 July 1931, Page 16

A Diagnosis of Radio

The Technical ; Editor. (Continued from last week.) ’

Variable Condensers : VARIABLE condenser is one the capacity of which can be varied easily. There are several different types such as tuning, midget, differential and compression. Let us look for a2 moment at the tuning condenser. This is usually a fairly large piece’ of apparatus with one set of metal plates mounted with another set of

plates, affixed to a spindle so that they can be meshed to any degree with the first plates. Of these there are three distinct types, the straightline wavelength, the straight-line frequeney and the straight-line capacity. in the straight-line capacity the dial readings are in direct proportion to the capacity. Thus, if the dial is graduated from 0 to 100 and it is reading at 25, the capacity of the condenser is a quarter of the maximum; fn other words, it is a .0005 condenser. The amount in the circuit is .000125. The straight-line wavelength type has plates which are cut off and curved upon the entering side. At first the capacity increases slowly, but as the plates go further into the mesh, the capacity increases more rapidly. it is arranged so that the dial graduation is in proportion to the wavelength. The.capacity varies as the square of the dial reading. The plates of the straight-line frequency type condenser are still more sharply tapered. Increase in capacity is even slower than the straight-line wavelength type, but the increase becomes greater and greater as the dial travels toward 100. The condenser is arranged so that its capacity is increased with the dial readings at such a rate that stations separated according to kilocycles are evenly spaced from each other all the way across the dial, that is, the same number of broadcast frequences as between zero

and 10 on the dial and as between 90 and 100. This even separation of the stations is undoubtedly an advantage. The same can be said of the straight-line wavelength line, only in this case it would be the- number of wavelengths. Nowadays condensers are made in ah different shapes to suit the needs of individual manufacturers. © The condenser generally used by constructors is straight-line frequency, ard is us illustrated in our sketch. In the compression type condenser ithe plates are pressed together by means of a screw. They are sepirated by mica or some similar dielectric. They ‘are very much smnalier than the average variable condenser and ‘can’ be used for a variety of purposes. They are specially recommended for wave-traps. The differential condenser has two fixed or stator plates; and one set: of rotary plates. It is.a. straight-line capacity type, that is to say, the capacity is directly proportional. to: the dial reading. Whereas air is used to separate the plates of the tuning condenser alnost invariably the differential condenser employs thin ‘sheets of mica. The mica must be of a high quality to withstand the constant friclion with the moving plates. Generally speaking, the two sets of fixed plates

are connected to the reaction coil and the moving plates to earth, although there are many other ways they . ean be connected, Midget Condensers: These are similar to the ordinary variable condenser,

but are much smaller. They are: put ‘to‘a variety of uses in the modern set. When condensers are connected in series it is necessary to add together the reciprocals of the separate capacities, which gives the reciprocal of the CONDENSERS IN SERIES

total capacity. The reciprocal of a number is one divided by the number. When condensers: are connected in parallel, the total capacity is equal to the sum of the separate capacities. ~The Crystal "CRYSTAL" in. radio parlance is a general name given to. certain kinds of minerals, and is not to be confused with the product of the glass manufacturers. The types of crystals that are most widely used for radio purposes are galena, zincite, and silicon. These crystals have the property of allowing current to pass in one way only. The current which is sent out by a broadcasting station flows in more directions than.one. Its voltage rises to a maximum positive peak and then falls to zero, then reverses to a maximum negative peak. In this stage it cannot be heard by the ear. It must be "rectified" or made to flow in ‘one direction only, and this is the function , of the erystal-it allows the positive halves of the wayes to go through, but prevents the negatives. To make a contact with the crystal, if it is of the galena type, 4 fine wire, which will not easily corrode, is employed, and this is known as the "cat whisker.’ . With zincite, silicon, car-

borundum: and other minerals, a different means of contact has to be found. , For carborundum a steel point is used, but it is fixed in 4 certain posi-tion-the most sensitive one-for few erystals are of even sengitiveness and we must find the best point. Zinecite and silicon are’ usually employed together; that is, one is used as a cat whisker, and thesé are combined, usually in the semi-permanent type of erystal. A galena and cat whiskér is the most sensitive combination, but the most sensitive point cannot be held for long. A loud signal may often throy it out of adjustment. For this reash the semi-permanent -type, with borundum or permanent .crystal, has: become very ‘popular. In’ the carborundum type a battery and a potentiometer can be employed, and this will. improve the results, Here are a few hints concerning erystals :- 1. Never handle the surfaces with the hands. A film of grease covers them. 2. If it is desired to clean crystals, dip them in methylated spirits. 8: In the semi-permanent type always pull back the plunger before exploring for a more sensitive point. 4, Carborundums can be knocked, very hard in order to find a better point. They can even be thrown 01 to a concrete floor and not be damaged. 5. A erystal set can be tuned with a very sensitive voltmeter. If a meter of the 1000 or 250 volts per ohm type is available, put it on to the lowest , veading (no greater than 0-7). and con: | nect either side to the phone terminals, and take out the phones. You will °

‘that the needle will show maximum reading when the set is tuned to the optimum point. If a very sensitive milliameter is available, it can be vsed in the same way as a voltmeter. . ‘You will get a lot of fun out of trying the different points on the coil and different combinations of the whisker and erystal in this way.