Testing Audio Transformers

Radio Record, Volume II, Issue 6, 24 August 1928, Page 27

Testing Audio Transformers

Maxx queries that are sent in dealing with sets.that have gone ‘‘dead," or nearly so, point to breakdown in audio transformer windings as being the cause of the trouble. It is quite a simple matter to ascertain whether a transformer is in good order or not, though many listeners do not appear to know how it is done. Every owner of a receiver should either make a special testing piece or be able to put it together for use at any time. The only requirements are a pair of headphones, one dry-cell of 14 yrolts, which ean be a "dud" that has done service on filaments or grid-bias, so long as it shows not much less than one volt. A coupl: of yards of flexible wire. [* an old pair of ’phones can be set aside for the purpose, a small flashlight cell can be attached on top of the headband and put in series with the ’phones, ready for use at any time. Otherwise the cell can be kept ready with connecting wires and clips or terminals, to which a pair of ’phones ean be quickly attached. This tester is useful not only for transformers, but for testing any other part of the circuit where lack of continuity is suspected. .. In whatever form the tester is put together, it is simply the cell connected in’ series with the headphones with two loose, flexible leads that can be applied to any parts to be tested.

Testing 2 Transformer. (THE breakdown of a transformer ‘is sometimes a puzzling . occurrence, for although:'a break in the primary will probably cause a complete stoppage of: signals, a break in the secondary may.only cause weak signals or distortion. With a break in the secondary, no grid bias is applied to the grid of the valve, so that distortion and blocking are likely to occur. In the case of the first audio transformer, if the input to the grid of the yalye is small, this distortion might not be noticed for some time. It is quite possible to receive weak signals from the local station with a break in either a transformer or resistance amplifier, and this should be a hint to apply the test to either. OMETIMES the break in a transformer is only partial. and in such a case reception will be varying in.strength or intermittent. and noisy. Crackles like violent static will probably be heard. At the same time it must be remembered that such crackling can be caused by defective joints in wirin;. or hy a dry B battery that has become qry." ; nnecting wires should be taken off transformers for the purposes of t Sting. First apply the two free ends of the tester (shown as arrowheads in the diagrams) to the two primary terminals of the transformer. A click should be heard in the ‘phones that is vot quite as loud as that heard wheu the two free ends are touched together. If this is so, the winding. is

intact. If the winding is broken, a very minute click will probably be heard, which is the broken winding receiving a charge from the battery just in the same way as a condenser can be charged. The secondary winding is tested in the same way. ‘The click that indicates continuity may not be quite as loud as that from the primary, but it should be distinct. If extremely faint, the same indications apply as above. Some listeners may be puzzled _ fo know which are the primary and secondary connections of a transformer. The two primaries may be marked 1P,

OP; P. and B positive, or P and HT. The two secondary terminals may be marked IS, OS; G, GB, or G and F. NOTHER test to show that’ the transformer is functioning correctly is to connect the ’phones alone across the secondary. place them on the head. then connect the dry cell across the primary, making and breaking contact. Good loud indications heard in the ’phones indicate that all is in order. Suspected joints or connections in any part of the circuit are tested by applying the tester to each side of the joint-loud clicks indicate ~ood coutact. No clicks indicate that resoldering is required. Fixed condensers are tested for breakdown in the same way, if of small capacity.