atcomsystems.ca/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems 1A2 diagrams

Can anyone point me toward schematics for the old 1A2 key systems?

We had equivalent multi-line systems here in England and I'm sure the switching elements are very similar, but it would be interesting to compare in detail as I've never had a chance to study 1A2.

The only drawings I'm aware of are the ones in the 1A2 manuals, which you can find on the Internet, but they are pretty pricey. I don't remember a schematic drawing only for them like there was for 1A1, but could be wrong.

Bill,

Apologies for the late response; I hadn't set reply notification.

I don't think it's worth me trying to track down full manuals and order in that case. Perhaps somebody could give me a quick overview of the basic 1A2 wiring for reference.

Obviously each line requires tip/ring pair, and I think I've gleaned from other posts that a second pair labeled A/A1 is used as the holding circuit through an extra set of hookswitch contacts. Presumably a 3rd pair for the line lamp, although something about lamp returns being commoned in some cases?

From a phone standpoint, each line needs Tip, Ring, A and Lamp. In fact, in 10-button sets, the grounds come in on the first line(a1,LG) and then the rest of the lines get only T,R,A and L.
When we wired for 10 button sets, we only had to use two-pair, instead of 3-pair after the first line. Geeze, no wonder I still have that 3-pair cross connect wire in the warehouse :-)

There's a pretty good reference to 1A2 with diagrams at https://www.kadiak.org/tel/1a2.html

Carl

Check this out it was provided by usroute12 in the topic of 620A2 ksu's, he found a site to get the individual practices, still pricey but not near as much as the full manual.

Excellent, thank you! I printed out some of those diagrams and studied them last night.

I take it that the line cards went through different versions over time (e.g. the 400E appears to dispense with the A relay of the 400D) and that third-party vendors offered their own versions which were directly compatible.

A few of the options pose some questions. The 400D KTU diagram shows a 30-second ring timeout option by omitting any resistor from the 1/2/3 terminals. Was this just a consequence of giving maximum flexibility by having a very high value resistance on the card, or a deliberate design feature to allow such a long delay? 30 seconds seems awfully long given a standard 2 on/4 off ring cycle. The only place I can think of where one might want such a long timeout would be if the C.O. line was on a manual board with non-machine ringing which might be of variable timing.

I see the BR link options to have the L relay operate from normal ringing on the C.O. line, but when/where were the remote A.C. and remote D.C. ringing options used?

Paul -

It's been a real long time, but if I remeber correctly the remote options were used if you wanted to close a relay to operate some other type of ringing device. I'll try to dig up some prints at home over the weekend and check.

Paul:

AT&T/Western Electric's line cards were 400A, B, D, G and H. The G version had a blue handle and an "in-use" LED. Also, options were changed using yellow plastic jumpers instead of wire jumpers. The H version was a 100% solid state version of the G, as in having no relays or other moving parts.

The ring time out options were to accommodate the various central office switch types or PBX's that may be providing the line. The long time out option was eliminated with the introduction of the G version in the late 1970's since most older offices, PBX's and cord boards were being phased out by that time.

Generally speaking, the 400E was exclusively ITT's nomencature, and F was San/Bar's. All card brands were interchangeable between systems with a few minor exceptions.

Card slots could be equipped with a manual intercom (401A or B) card. This basically provided a simple coupled transformer arrangement to create a talk path. These were used for button/buzzer type intercom systems. The problem is that they required filtered -24VDC "talk or 'A' battery on pins 3 and 18. Since ITT's and San/Bar's cards had music on hold capability through these same pins, care had to be taken to ensure that talk battery wasn't present on these pins in the slot. Failure to do so surely resulted in the MOH circuit being blown very quickly. Most AT&T/Western Electric systems had these pins factory-wired to the "A" battery output of the power supply, but ITT and San/Bar kept them isolated for field connection if needed.

1A2 permitted the ringer in the telephone set(s) to operate directly from the CO line ring voltage, or in larger systems, ringing could be generated internally and follow a slightly faster cadence. This was accommodated through the use of the relay contact, the motorized interrupter and a local ringing generator (they referred to them as frequency generators). I was told that the reason for this faster cadence was to give employees the perception that the phone line was ringing longer than it really was. I have never verified this to be fact.

I can't figure out the logic behind the J1 jumper option on the San/Bar schematic (Model 4200A with music-on-hold) at all. Pulling the BR (Bridged Ringing) jumper appears to leave the C.O. tip (pin 14) completely isolated from everything except the holding resistance and MOH circuit; the ringing section of the L relay via its associated rectifier (and blocking capacitor if A.C. ringing option selected) is then just connected across station side tip and ring. :confused:

I've browsed through some of the older threads in this section and seen some references to that. Was this just one of the lines to each phone assigned to the internal intercom so that anyone could pick up the intercom by just hitting the appropriate line button?

I also saw mention of a dial intercom card. I'm assuming that this was a similar card which provided not only talk battery for the intercom line but would also recognize DTMF tones (or even pulse?) to sound the appropriate buzzer for the required station.

I note the quite versatile signaling arrangements possible, either a low-voltage audible or conventional A.C. ringing with diode matrix to provide intercom isolation and/or split the ringing for phones not provisioned with all C.O. lines.

Presumably the M jumper to pin 18 on the 400E line card is for MOH. The DR option on the same card appears to apply -24V to pin 3 only when the line is in use/held or ringing (i.e. L relay operated). I'm still trying to work out possibilities for the RU and TU options!

The British Post Office supposedly adopted the double-burst ring {*} which is standard here because it resulted in subscribers answering more quickly. Many PBX systems, and some of the small public exchanges had single-burst ring though, generally where the timing came from an interacting relay set rather than a ringing machine.

{*} Standard cadence is 0.4 on, 0.2 off, 0.4 on, 2 secs. off.

Man, Paul! You definitely have this stuff under control. I'll do my best to answer with what I know:

As for your first question about the 4200A card, I really don't know for sure, but I'll take a guess. I don't have any of their schematics left to review. Bridged ringing and grounded ringing were options when party lines were still around. They were definitely present when 1A2 was popular. I am assuming that this option was to select between the tip side of the line or ground for ringing voltage reference. Again, that's a guess.

As for question #2 about the manual intercom: Yes, this was just a common open talk path that appeared on a line key through traditional 3-pair cross-connects. These were generally set up as private talk paths between an executive and a secretary, but anybody with an appearance of this "line" could barge in. Very frequently, the locking screw of an unused line key could be removed to become the pushbutton to buzz the other station. A momentary ground (A1) was simply placed upon the line key's A lead while pressed which closed the circuit to the buzzer at the other station.

In cases where multiple stations were involved, "side cars" of buttons were added to the phones and spare leads in the line cord were used.

Yes, there was a dial intercom card made by ITT and San/Bar that fit the standard 1A2 "sized" card slot. This card, while similar in size, could not be installed in regular slots; it had to be a dedicated slot. Western Electric had a 424C and later 494A card for dial intercom, but it was an 8" card as opposed to the traditional 4".

Yes, the ITT and San/Bar cards had option jumpers to select MOH. Their factory setting was no MOH, so one would have to change them and hopefully during this process, think about checking for the presence of A battery.