AN UNFAMILIAR EXPLANATION

Evening Post, Volume CXIII, Issue 127, 2 June 1927, Page 14

AN UNFAMILIAR EXPLANATION

Many owners of receivers" have experienced the breaking down of transformers on their amplifiers. The symptom is either a sudden failure of the set to work at all, or terribly noisy operation for a while and then total failure. Some makes of transformer seem rather more liable, than others to the trouble, but it is reported in respect to a great many makes. Many explanations of the. trouble have been given. Usually it is called "burning out," implying that the plate-current in the primary (which is invariably the coil affected). has fused the wire. Sometimes it is attributed to corrosion by soldering flux because there are usually signs of corrosion about the break; but no decent manufacturer would dream of using a corrosive flux in such work. That the breakdown of: transformer primaries is a general puzzle is shown by a recent long-drawn correspondence in an. English wireless jourmil, in which a large variety of .views waß expressed. I am indebted to Mr. K. H. Thow, of Standard Telephones and Cables, for an explanation which is based on the investigations of his firm and which is on the face of it far more reasonable than any I have seen published. It is that the breakdown is, in most cases, flue tot to "burning out" or to fluxcorrosion, but to the corrosive effect of electrolytic action. .An examination of s low-frequency amplifying cir- j euit will show that, as the primary of the transformer is connected to the positive pole of the high-tension battery and the secondary to the negative, '..the full potential of the battery is continually applied between the primary and the secondary, and if the insulation is not. perfect a small current will pass through the separating .medium. If there is any moisture here,.electrolysis will take place, and the primary coil will be surrounded by an active-(oxygenated) compound of a-cor-. erosive nature. Similar action may also occur between the: primary coil and the .core of the transformer, if this is t arthed or not well insulated. Mr. Thow states that careful examination of a large number-of disabled transformers showej the general symptom that the breaks; occurred in the turns of wire immediately against the *'cheeks" of. the bobbin on which the primary Is. wound, and were usually accompanied by visible evidence of eorzosion. / Such accidents^ can be prevented in two ways—by special precautions in manufacture, and by protecting the transformer' against the' constant strain imposed by the battery. The former remedy lies "with the maker, and is a. matter of careful insulation and impregnation of - the windings. The secondary coil, it will be noted, does not suffer, although the potential strain upon its insulation is the same as that on the primary, because the electrolytic process brings oxygon to the primary and hydrogen to the secondary.' The owner of a set can protect his transformer against electrolytic breakdown by disconnecting the plate battery as well as the filament battery when the set is not in use. The potential strain will exist during the few hours the set is in use, but will be off during the twenty hours or more each day when it is "closed down." it is well, however, to take proper precautions when disconnecting the plate battery, to ensure that accidental short circuits and wrong connections are not madel by the loose wires;.otherwise a precaution against a mishap which may never occur may easily;cost several valves or a ruined battery;- 1 The beat thing to do is to install a special switch.