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1A, 1A1 & 1A2 Key System Differences

Started by HowardPgh, January 10, 2013, 04:31:00 PM

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What is the difference between these types of PBX?
What is required to operate them?


I am in no way the expert here.  I have two 1A2's and none of the earlier systems.  Here is what I understand, and there are several others here that know a heck of a lot more about them than I do.  So if I am putting out anything wrong, I welcome any correction.

1A and 1A1 and 1A2's are all key systems, rather than PBX's  In fact, it is not uncommon for key systems to be running behind a PBX.

1A key systems are the very early version, and is all relay with no active electronic components, and has a different method of placing calls on hold and releasing the hold than the later versions.  A phone has to have a key mechanism that is just for 1A to operate, and I don't believe 1A key sets (phones) will work on 1A1 or 1A2.

1A1 and 1A2 have the newer "A" lead control for holding/releasing.  1A1, I believe is still all relay controlled with no active electronics.  1A1 and 1A2 keysets I believe are compatible with each other.  1A2 uses transistors and more sensitive components.
-Bill G


It's true that the early 44x and 46x phones work only on 1A. Later model 44xs and 46xs (such as 464GG) as well as 564s can be used with 1A, 1A1 or 1A2.

1A2 uses plug-in line cards, one for each line.
"C'est pas une restauration, c'est une rénovation."--François Martin.


Here's an example of 1A Key showing two 1A Key Telephone Units=a hold relay and line relay for each line.
"C'est pas une restauration, c'est une rénovation."--François Martin.


Here is one of the latest versions of 1A2: a Western Electric 620A panel for 8 lines. It currently  has 4 line cards.
"C'est pas une restauration, c'est une rénovation."--François Martin.


The hold circuit on 1A is line powered I beleive .The relays are as shown in the 105? app box. 1A1 and 1a2 the hold circuit is isolated with a power supply.You can change a 1A set to work with 1a1 or 1a2 but will need to  have another switch pile-up.Note it has been years since I messed with this stuff


I agree with everything written here.

1A is Tip Ring Balance & Hold with no lamps I believe. All relay and many small KTU's put together to get any number of lines. Presumably no lamps at all.

1A1 and 1A2 are Tip, Ring, A, A1, Lamp Ground, Lamp. 1A1 is all relay and it was an additional relay to get "wink hold" where as 1A2 is electronic cards with some relays on them still and "wink hold" is built in.

Who has a picture of 1A1 handy to add here?



Here is a quick scan from the 1969 Bell System Key Systems handbook of two 1A1 KSUs.

Fig. 9 includes the rotary dial intercom 207C KTU below the line pickup/hold package above.  Under the cover the big square box looks similar to the unit shown in Fig. 10.
Adam Forrest
Los Angeles Telephone - A proud part of the global C*Net System
C*Net 1-383-4820


While the relays are different the AE 10A1 was a very similar layout. There was a group of relays like this that did 3 lines and then there were single line plates as well. We used 19" racks then and a 3 line plus 2 single line plates and a wink hold plate nicely fit into the 19" racks. So each "shelf" or section would do 5 lines. I think I have 2 "shelves" in my garage so I am good for 10 lines of 10A1.

I also have some "6 packs" of AE 10A2 mounted in a similar apparatus cabinet to the one that Adam just posted which does 6 lines and has an Intercom and Power Supply inside as well. We also used "13 Line Panels" (also 10A2) which as the name suggests, would do 13 lines and had an Interupter on the shelf and fit into a 23" rack. I have some of that hanging around as well. I hope to get a small set-up of each of these going soon in my new Phone workshop.



love this subject... I have a 1A1 system and use 400, 500, and 2500 series phones on it. I have a WE 462 that can't be converted to 1A1 service and is just for the 1A system which I have in storage. like its stated, the difference is in the phones and how they were wired... Why W.E. didn't make EVERYTHING convertible is beyond me... just like everthing else, its the technology....always improving it and adding features... I have a 200 series KTS that has a 202, 207, 210, and a 212 KSU inside it and it runs great... had to tweek the tension on a wink relay to get that going, works now and sometimes I wonder if it will ever burn out..... but it works.  the main thing that all the systems have in common is all the wiring...whew! just trying to remember how you did it make me tired... I had the Hold lamp wired to be a power indicator, and disconnected it when fixing something else....set it aside....and now I can't remember which wire goes where... oh fooey!   so that's a future project...  I could go on for hours about this subject, but will stop here...   John


This is so interesting reading about 1A and 1A1 systems. I've worked on 1A2 and have had a 1A2 system in my home for many years. To accommodate standard non 1A2 phones I've made A lead control circuits, but would sometimes go on hold when ringing in, even with an integrated delay circuit to keep A active while the 400E line relay is idle. I recently went back to basics - made a system I used for hold back in the 70's. It's quite simple, basically using CO line current to hold a latching relay. When I read how the 1A systems operate it seemed similar to what I'm using now.

I modified the 1A2 keysets that I still have to put a momentary + on the A lead when line 5 is pressed. A resistor provides enough current to hold the relay until going off hook again. When on hold, the lamp will dim slightly to indicate hold is active. One keyset with a standard 500 type network doesn't release it for some reason, so I converted another key to short T & R. All other phones (1A2, and modern standard phones) work fine.

I don't need a flashing ring or hold indication, so the interrupter and 400E line card are no longer used. It seemed strange actually pulling the 400E out. If anyone is interested I've attached a basic schematic of the hold/line status circuit. My old version of Visio with electronics templates doesn't work on Windows 8 (like other things) so I used Powerpoint and exported it. It would be good to have an electronic component template for Powerpoint, for now I drew my own - crude as they are.



Anyone remember the "Com-Key" system?
This was an attempt to do what a 1a2 system does, same cables, but there was no KSU, the subscriber sets ARE the KSU.
Later sets only use 4 or 6 wires, so the 25 pair cables can support an almost unlimited number of stations.
With that said, someone wants to reproduce the functions of a 5 line + hold rotary set.
This would be for the San Antonio Fire Museum.
They also want a 500 set in black with a blank apparatus plate, auto ringdown.
The rest, getting the Gamewell system going is easy, that is a very simple telegraph system.
Thank you.


I still have nightmares over the com-key..... :o
Cowthief the one you are referring to is the Com-Key 416 it had two masters (acting as the KSU for 2 lines each equaling 4) and slave telephone sets.
a fifth line could be added with an additional module that resembled the 1a box in external appearance.

There was also a larger comkey system that had modified 10 20 and 30 button sets as well as a attendant console and busy lamp field (for you younger telephonistas a BLF)