 The custom terminals had been designed for JOSS as the engineers felt that the Model 33's "telecommunications functions are a mystery to the untrained person" and would be too confusing for the occasional users to bother learning.In practice, the Selectric-based mechanisms proved too fragile for the constant use they saw and were frequently being repaired in the field by IBM servicemen.In 1970, a full third of the terminals were completely rebuilt, and the output from the computer slowed to improve reliability.