D4 Digital Channel Bank Family: The maintenance bank

The D4 Maintenance Bank (D4MB) is a centralized facility for testing D4 channel units as well as a place for the hot monitored spare of all common equipment used in the D4 channel bank. At least one D4MB equipped with D4 channel units is recommended for each office. The D4MB contains a test set and two digroups of common equipment that simulate the D4 digital system. When a channel unit needs to be tested, it is inserted in the test position of the D4MB and the craftsperson tests it by following simple instructions listed on 3×5 plastic cards. These tests are performed on any given code of channel unit, usually without changing any options in the channel unit under test. Voice-frequency (VF) and signaling tests are performed on the channel unit under test. These tests assure the craftsperson that the unit under test will not affect or impair the transmission of other channel units already installed in working D4 channel banks. The D4MB is an inexpensive and simple D4 plug-in unit test vehicle for craft: use. It has proven extremely reliable in determining whether a unit is good or defective. Testing with the D4MB has greatly increased the office personnel's confidence in both channel units and common units that are to be used in a working D4 channel bank. As a result, significant cost savings have been realized using the D4MB by virtually eliminating unnecessary shipping and retesting of good units that might have been previously returned to the factory for repair.