Strong started recording the audio from the series when he was just 14, using a domestic reel to reel, quarter-inch, tape recorder, the only way of preserving audio recordings at the time. The first recordings were made via a basic crystal microphone, hanging over the television speaker with a plant pot placed on the top of the T.V. to keep the microphone in place.
Following The Daleks' Master Plan, episode 7, Strong, a keen electronics student, managed to wire the audio input into the Tape recorder, directly to the audio output of the Television set. A highly dangerous procedure that breaks every rule of electrical safety but one that resulted in recordings that were crystal clear.
In fact, the surviving recordings are so clear that they often exceed the quality available on the surviving film prints of the episodes, and as a result, a number of DVD's of early episodes contain audio taken from Strong's recordings rather than the film print.
Strong was one of a small number of early fans who recorded audio from the now missing stories. However, he is believed to be the only one to record directly from the Television, resulting in the superior quality of his recordings.
With Thanks to Stephen Cranford