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1a1 1a2 schematics needed

Started by RB, June 19, 2019, 12:39:40 PM

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Can someone post a schematic for a 1a1/2 key system?
Can't for the life of me find one ???


which one do you need? the TCI Library has them, but you need to know which model you need the information on....



you need to go to the TCI Library and download a copy of the KSSM. its the Key System Service Manual. it will give you pretty much everything you need. also there is a copy of the 1955 1A1 manual in the library, that will help too!


What KSU do you have? Because that also helps to know.


1A1 & 1A2 is a pretty broad description. KSU's, power supplies, telephones etc. I think you need to be a little more specific on exactly what you are looking for. Whatever it is, it will be out there somewhere.
Harry Smith
ATCA 4434

"There is no try,
there is only
do or do not"


OK, did not realize I was asking such a broad question.
specifically, I am interested in the area of the lite and buzzer circuits.
I wish to see how they are powered, and how they are routed to each phone.


I'm a little late to answer this, but just joined today, and thought: well it's never too late, lol..

First, there are some good textual descriptions of 1A2 on the web (warning: one I wrote, lol):

When I first started learning about 1A2 phones and how they interact with the KSU (control unit), I started with this "condensed functional schematic" from a Bell System Practices document (518-215-125) on the 400D line cards:

It might be hard to read at first, as it uses a lot of Bell System symbols that take some time to get used to. But it shows a simplified view of how a single line is managed: the phone line comes in as a tip/ring (T and R) pair of wires (blue), and connects to the 1A2 phone (orange) through the KSU (just about everything else in that diagram), showing the 400D line card (purple) and the line card's connector (green).

For the phone (orange) it shows the important signals:

  • Voice path is the tip/ring pair ("T (STA)" and "R (STA)") wired through the phone's "pickup key" switches (buttons on the phone that select the different lines) and "switch-hook" (the phone handset's cradle switch that closes when the receiver is lifted) and to the phone's "network" (abbreviated "NET.") which splits the tip/ring pair into 2 wires for the mic and two wires for the earpiece.
  • A lead: this is a signal that is grounded when a line is selected, shown inside the phone as the single wire "A1"
  • Lamp: the incandescent lamp circuit, show as the lamp next to its ground symbol, "LG". There's a lamp under each line button that lights up when the line is in use, flashes when ringing, and "winks" when a line is on Hold. This lamp is powered by the KSU's 10VAC power supply ("LAMP PWR SUP") and is controlled by the 400D line card, which switches between the various flashing signals provided by the KSU's "INTERRUPTER"; an electro-mechanical device that can be thought of as a rotary switch attached to a motor
  • Ringer or Bell: labeled "AUD SIG" for audio signaling, in 1A2 the bell is a completely separate circuit, which rings the bell at 105VAC / 30HZ, powered by the KSU's audio signal power supply, and the ringing controlled by the INTERRUPTER's "RING SEQUENCE" to generate the one second ring followed by 3 secs of silence.. and this signal is in turn switched on or off by the 400D line card's "C" and "B" relay contacts

Of interest in that diagram are some symbols that may seem strange even to those familiar with electronic schematics: "X" and "|" symbols represent "normally open" and "normally closed" switch contacts, and when combined with thin vertical rectangles that represent a relay coil, can define various components of a single relay:

As an example, in the 400D KTU (purple), you will see a "B" thin rectangle which is the coil for the "normally open" contact for all the "X" symbols with a "B" next to it.

If all that's a bit much, some time ago I made this diagram of the phone and KSU with even more reduction in detail that is perhaps a bit clearer using more common schematic symbology for switches:

Here's a wiring diagram for a typical 1A2 phone's amphenol connector (the 50 pin connector you see at the end of all 1A2 phone's mounting cords):
..a diagram that's based on various Bell System documents, such as:

Anyway, as others have mentioned, there's a lot of documentation on all this in the form of "Bell System Practices" documents, which all have 9 digit numbers (###-###-###) and are now organized pretty well by various folks on the net into PDFs so that they can easily be found by a google search for the number, or going on those websites directly.

Some websites I have bookmarked that have Bell System Practices:


These documents are mostly intended for phone installers who are either installing or troubleshooting phone systems. One has to really immerse oneself in it to put it all together.


Great post welcome to the forum erco!