Sometimes a few turns in the secondary winding of a transformer experience a partial short, which in turn causes a voltage drop across the secondary. The usual symptom of this condition is transformer overheating caused by large circulating currents flowing in the shorted windings.
The easiest way to check this condition is with a voltmeter. Take a reading on the line or primary side of the transformer first to make certain normal voltage is present. Then take a reading on the secondary side. If the transformer has a partial short or ground fault, the secondary voltage reading will be lower than normal.
Replace the faulty transformer with a new one and again take a reading on the secondary. If the voltage reading is now normal and the circuit operates satisfactorily, leave the replacement transformer in the circuit, and either discard or repair the original transformer.
