1A2 Model 2564 Phone Terminal Board

The part number for this board is labeled "636 A KEY", which can be seen between the Line 2 and 3 lamps.

Normally this board is partially obscured by wires.
The board pictured here is from an old parts phone I completely disassembled, with all the wires removed so you can clearly see all the terminal labels.

The lamps along the bottom are incandescent Sylvania 51A bulbs, which are available commercially, and tend to be pretty cheap for a box. But if a line bulb burns out, you can usually fix it in a pinch by replacing it with either the bulb from an unused line, or the one under the Hold button (which is often unused).

In the following, "(?)" means "unsure", as a lot of this stuff seems to be inferred, rather than explicitly stated somewhere.

Terminal Labels

• 1T/1R, 2T/2R, 3T/3R, 4T/4R, 5T/5R
  These terminal screws are for each line's tip/ring (T/R) signals; the numeric prefixes are the line numbers, the letters are for "Tip" and "Ring" (e.g. 1T/1R are Line #1's Tip/Ring pair)


• 1H, 2H, 3H, 4H, 5H
  These terminals are for each line's A lead signal; I imagine the "H" stands for "Hold" (?), since the A lead helps determine the Hold state, based on the fact the only other screw labelled with an "H" is the "LH" screw, which is definitely for the Hold lamp.


• "M", "X", and "N"
  These terminals are the common ground side of all the A lead switch connections, which all can be routed through the switchhook for various features. For instance, "N" is involved in the Hold circuit this way:

  
          ▒           Amphenol          ▒  Hold Key ▒  Terminal  ▒    Hook   ▒    Pickup   ▒
          ▒                             ▒           ▒   Board    ▒   Switch  ▒     Keys    ▒
          ▒                             ▒           ▒            ▒           ▒             ▒
          ▒                             ▒           ▒            ▒           ▒             ▒
          ▒       A Gnd (O-W) Pin 2 ─────────────────────(1B)──────────o e   ▒             ▒
          ▒                             ▒           ▒            ▒     │     ▒             ▒
          ▒                             ▒           ▒            ▒     ╳     ▒             ▒
          ▒                             ▒           ▒            ▒     │     ▒             ▒
          ▒                             ▒    ┌───────────(N)───────────o d   ▒             ▒
          ▒                             ▒    │      ▒            ▒           ▒             ▒
          ▒                             ▒   ━┿━     ▒     ┌─────────────────────────────┐  ▒
          ▒                             ▒    │      ▒     ├───────────────────────────┐ │  ▒
          ▒                             ▒    └───────────(M)────────────────────────┐ │ │  ▒ 
          ▒                             ▒           ▒     ├───────────────────────┐ │ │ │  ▒
          ▒                             ▒           ▒     └─────────────────────┐ │ │ │ │  ▒
          ▒                             ▒           ▒            ▒           ▒  │ │ │ │ │  ▒
          ▒                             ▒           ▒            ▒           ▒  ╳ ╳ ╳ ╳ ╳  ▒
          ▒                             ▒           ▒            ▒           ▒  │ │ │ │ │  ▒
          ▒  A1 Lead (W-O)   Pin 27  ────────────────────(1H)───────────────────┘ │ │ │ │  ▒
          ▒  A2 Lead (W-S)   Pin 30  ────────────────────(2H)─────────────────────┘ │ │ │  ▒
          ▒  A3 Lead (R-G)   Pin 33  ────────────────────(3H)───────────────────────┘ │ │  ▒
          ▒  A4 Lead (BK-BL) Pin 36  ────────────────────(4H)─────────────────────────┘ │  ▒
          ▒  A5 Lead (BK-BR) Pin 39  ────────────────────(5H)───────────────────────────┘  ▒
          ▒                             ▒           ▒            ▒           ▒             ▒
          ▒                             ▒           ▒            ▒           ▒             ▒
          



• L1/LG, L2/LG, L3/LG, L4/LG, L5/LG
  These are the lamp signals for each line. And the "LG" terminals the "Lamp Ground" side of each lamp. Usually these are all commoned together at the KSU for the 10VAC power supply.


• LH/LG
  These are the Hold button's lamp hot and lamp ground. The lamp under the Hold button is not normally used, but is available for special signaling purposes over spare 1a2 wire pairs.


• RR/RT
  Wired to the Y-S pair, these are almost always used for the bell/ringer. So the "R" prefix most likely means "Ring (bell) voltage".


• 1, 2, 3, 4, 5, 6, 7, 8, 9
  The numeric terminals seem to be used for spare (unused) wires on the mounting cord.
  The Y-G pair is usually wired to the 1+2 screws, and the Y-O pair is usually wired to the 3+4 screws, either pair often used for the buzzer, depending on the phone model.


• EB, EH, ET, ER, 1B
  These are apparently either "Extra" or "Exclusion circuit" terminals (?) which apparently are available for custom special purposes. Some phones can be configured with an "exclusion circuit".

  "1B" is often the A lead ground (O-W, pin 2), the common ground for all line's A lead signals. The "B" may refer to "Battery", i.e. just "B" would be Battery supply, and "1B" would be Battery ground. (?)



• SG
  SG is "Signal Ground". It is wired to BR-Y (pin 19) and is usually unused/spared off. Section 502-541-406 of the BSP says:
  "When keys are converted for signaling, the S lead of key involved provides the signal
  circuit and SG lead provides the common signal ground."

  Several other bell system documents refer to this as "signal ground":

  "Common signaling (Fig. 13) uses one converted line button to signal over several intercom
  circuits. The A1 or SG ground used to operate the signal circuits should be under control
  of the line switch to prevent false signaling if the customer is toying with the common
  signaling key even though on-hook."