READING CIRCUIT DIAGRAMS

Sun (Auckland), Volume 1, Issue 7, 30 March 1927, Page 7

READING CIRCUIT DIAGRAMS

(By "Raytheon.”) Tn roanv enthusiasts and especially *hnse who have just recently joined the increasing body of listeners-in. a radio SSoit diagram appears more or less f,V.. a "jigsaw puzzle. For the help d assistance of such a table of mbois used for radio components in ’'“usual radio ele< tricul circuit diagram, more “theoretical” diagram, is

here presented. As from time to time such circuits may be reproduced in these columns, it is recommended that this copy be retained for future reference. It will be a surprise to some how quickly the essential portions of a diagram can be read by using this,

on h e whiof y ° U Be ® a circuit even the one which appears most intricate. time these notes appear, therf. will remain one more day only before the old years' radio license lapses. It is understood that last week the "rush" for renewals had commenced in earnest, and. it is pretty safe to say that the particular counter at the post office will for a week or two show every indication of brisk business. Frayed cord tips on flexible tinsel cords may be repaired by winding the tinsel ends with fine copper Wire, and

then flowing - solder over them with a rather cool soldering iron. TRANSFORMER BURN-OUTS This is one of those little troubles that occur with the constructor or owner just at a critical time. You : may not be able to replace the trans- ) former for a few days, but in the i meantime would certainly like moderI ate reception. This small hint may do. In the majority of cases it is the primary of the transformer that goes. The plate lead from the valve is removed and a small clip inserted in the circuit containing a resistor of 100,000 ohms. The other end of the resistor is connected to the “H” battery. A fixed condenser of any value from .0025 mfd to about .5 mfd will do in emergency, and is connected from the plate of the valve to the grid terminal on the transformer. This really turns the stage into resistance coupled, the secondary of the transformer acting as the grid leak. Although the volume may not be quite so strong, it will be found satisfactory in the meantime to work the loudspeaker. SWINGING AND FADING OF SIGNALS Most of us have experienced the phenomena of swinging or fading of signals. At first both effects seem the same, but when they are analysed it will be found that they are quite different. Fading is beyond the control of both the transmitting and receiving station. On the other hand "swinging” can very often be rectified at the receiver. When a signal fades the intensity of the energy received varies in accordance with the degree of fading. On the other hand, when a signal swings the amount of energy received will remain practically constant, but it is the wavelength that changes. This may be due to the aerial swinging in the wind, not only at the receiving end, but also at the transmitter. Especially may it be the case if the latter operates on low-power. If such is the case, the wavelength change can be followed up or down on the dials. If it is caused through a swinging aerial at the receiver the moral is, tighten up. RESISTANCE-COUPLED AMPLIFIERS To those who may contemplate building, such an amplifier thes. few words may be a guide, and they may also help those who might perchance have experienced some difficulty in the working of the same. The determining factor of quality in this amplifier is the combination of the coupling condenser and grid leak. If the resistance of the latter is small, the coupling condenser is made large while with a small coupling condenser the grid leak resistance may be increased by a proportional amount. If a 100,000-ohm resistance is used in the plate circuit, the following combinations of grid leaks and coupling condensers may be used to obtain a practically flat characteristic curve down to 50 cycles:

One of the greatest troubles with resistance amplifiers is due to the choking effect which somethnes takes place in the valves. This is most likely to occur when one of the valves is being overloaded. With this class of amplifier it is essential to see that the correct "C” battery bias is used on the grids of the various valves. It the circuit characteristics and the amplification of the valves are known it will be found about right. I. sing a. ewough to handle a grid swing of 40 volts peak value.

Grid Leak. Coupling Condenser. 0.2 megohm 0.06 m.f.d. 0.5 0.025 1.0 0.012 2.0 0.006