1A2 Key System - Components

Components

The earlier 1A2 technology consisted of Key Telephone Systems (Bell Systems Practice 518-215-100). Most Key Telephone Systems at that time (as late as 1980) consisted of key telephone units (KTU) which provided circuit features such as line interface, dial intercom, direct line circuits, etc. Also part of the system were the mounting facilities that hold the KTUs, and power supply. The common telephone sets at that time were the 565 (5-lines), 630 (17 lines), 631 (29-lines), 830 (9-lines), and 831 (19-lines). If the telephone set number started with a '2', such as '2565', then it was Touch-Tone.

The power supply provided 24VDC for relay operation, 24 VDC filtered for talk battery (intercom and direct-line services), 10VAC for lamps, 18 VAC for buzzers, and 90VAC at 20Hz for ringers (Bell System Practice 167-466-101). Lamp and signaling voltages were passed through an interrupter (KS-15900 L1) to create lamp flash (incoming line), lamp wink (hold), and interrupted buzzer and ringing. The mounting facilities varied according to the size and complexity of the Key Telephone System. Typical of early 1A2 systems was the extensive use of 583 and 584-panels to hold the KTUs. The 584C panel contained an interrupter and 13 KTUs. The 583 Panel did not have the interrupter and held 15 KTUs (Bell Systems Practice 518-215-410).

For smaller installations Key System Panels could be purchased that housed all components, including the power supply and connecting blocks. Typically these panels supported only six lines and a dial intercom (Bell System Practice 518-215-424).

Five pairs of wires were required for each line: The central office tip and ring, the station (telephone instrument) tip and ring, the A and A1 (control) leads, lamp and lamp ground, and the ring (signaling) pair. The cable to the telephone instrument required fewer pair: station tip and ring, A and A1 pair, and the lamp lead. The A1 ground was only used on the first button, it was not terminated afterward. The Yellow-Slate (grey) pair was usually used for the ringer (Bell System Practice 502-541-407).