TESTING SETS

Hawke's Bay Tribune, Volume XIX, Issue 300, 4 December 1929, Page 10

TESTING SETS

If the average wireless set owner had a simple testing outfit he could perform many tests if his icceiver were not functioning properly. This would be of benefit, not only to himself, but to the radio dealer as well. Often some little thing is wrong that upsets reception completely, and there is a hurried call for the dealer’s assistant. “Max,” in the Christchurch “Press,” therefore gives the following hints, chiefly for battery models: — A simple tester consists of a 41-volt “C” battery, always incorporated in a set, and a pair of ’phones. Join one ’phone tip to one terminal of the battery and a foot or so of flexibii wire to the other end ol the “C” battery. The commonest failure is due to a faulty tube. Test these one at a time'by touching the free ’phone tip and the free end of the flex to the filament pins, after first removing the valve from its socket. The thick pin s are the filament ones and if the tube is correct, a good loud plink will be heard in the ’phones. Next try for a short between filament and nlate by touching one filament pin and the plate pin. and there should tie no sound. Do the same with the grid pm-.

With a. newly-built six-vavle set, “Max” once had smoke come from the variable plate resistance when it was nearly all out. The filament switch was immediately turned off and the tubes tested. The dealer, in his hurry, had omitted to test the valves himself and one proved io have a plate and grid touching, shorting the “B” battery. Therefore, test tt-.e plate and grid pins also, and there should again be silence. The home constructor will be inter ested to know that the screen grid

valve, as generally used for radio frequency amplification, is excellent in the first stage of audio proyided no high frequency magnification is used. Several stages' of radio might give too much volume for a screen grid valve to handle without distortion. If a straight detector and two of note magnification are employed, the conditions are good,' These valves have a very high amplification factor, but do not function well in the audio soc ket unless the same maker’s transformer follows immediately as the coupling to the second stage. A one megohm grid leak across the secondary terminals of the transformer is sometimes required if the set is designed for the short waves. This brings to mind another common sense tip, and that is to use valves and transformers of the same make or with the same types of impedances. The simplest way to illustrate this point is to mention that, as a general rule, British tubes and American transformers do not go well together, as the former are often of a much higher impedance. The converse also holds good, do not mix American valves and English-made transformeis. Some types match quite well, but if one is building a set, use the class of units for which it is designed.