pdf GTE Section 57 Part 505 - 555 PBX Switchboard Circuit Popular
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GTE_Sec57_Part_505_Sep59_555_PBX_SwitchboardCircuitDescription_Schematics.pdf
Sept 59. Describes the operation of various circuits of the 555 PBX switchboards. These include the cord, operator's telephone, dial, CO trunk and station line circuits. See Part 506 for maintenance info.
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