Author Topic: 755A Trunk/Local Lights  (Read 1240 times)

Offline ESalter

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755A Trunk/Local Lights
« on: August 15, 2016, 09:57:12 PM »
Hi Everyone-

Sorry I haven't been around for a while, I'll try to make an effort to stay a little more frequent on here.

Last week we finally picked up a WE 466 to go with our 755A.  Until this point all we had hooked up to it was a converted 460 and several various types of single line phones.  Now that we have a 466, I'm a bit confused.  The phone has lamps for all 4 trunk buttons as well as the intercom button.  There's a wire for it in the cord and the BSP for the phone shows it going back to a terminal on the PBX called "L1 (LOC)".  I can't for the life of me find any such terminal on the PBX.  My dad has a document written by another collector(I forget who) that mentions two disadvantages of a 755A are the hold feature doesn't work on intercom calls and there is no illumination on the intercom button.  I find that very hard to believe because the lamp is present, there's a wire for it in the cord, and it gives a terminal name on the schematic for the phone.  Also, imagine there is an incoming call from another station, so your phone rings.  If the intercom button doesn't flash, how do you know what button to depress to answer the call?  Assume it's a local call because none of the trunk lamps are flashing? 

So, does anyone know for sure if the 755A does/doesn't have local lamp outputs for the stations, and if so, where do I find them?

Thanks!
---Eric

Offline ESalter

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Re: 755A Trunk/Local Lights
« Reply #1 on: August 18, 2016, 04:57:57 PM »
So after quite a bit of searching I've discovered a couple things about this.  Apparently the 755A doesn't come equipped with local lamp outputs.  I found a BSP that says that functionality can be added with the addition of a relay and some wiring changes.  The set of schematics we got with the 755 does show a LL relay that operates "line lamps" which appear to be what I'm after. 

It looks like the solution is going to be to make a strip of relays to add to the PBX and make the necessary wiring changes to make them operate as intended.  My dad and I are going to try to clip-lead this up for one station to make sure it works, then go from there.  Not the solution I was hoping to find, but we'll manage.  I'll try to post photos and details of our progress just in case anyone is interested.

---Eric

Offline AE_Collector

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Re: 755A Trunk/Local Lights
« Reply #2 on: August 22, 2016, 01:58:10 PM »
Hi Eric:

I am not familiar with the 755 PBX specifically and initially wondered what the purpose of the key set was. On most of the PBX's that I worked on in the old days all trunks came to the PBX operators who forwarded the calls to the PBX stations which were mainly single line phones. We did occasionally have key sets/systems behind PBX stations for certain departments but in most cases the line keys had PBX Stations on them, not trunks.

What you are setting up would presumably be for a department within a larger company that effectively has Direct in Dial since they have key sets with direct CO lines on them and then the 5th line key has a PBX station on it. So the PBX is really just a big intercom system though this set up would also give the ability for a customer to call the main published number for the company rather than the departments specific direct numbers and then the PBX operator would put the call through to the department on the intercom/PBX station.

I am thinking that the PBX was somewhat designed around this method of operation since a 466 doesn't sound (to me) like a normal 400 series key set, maybe it is a custom key set to work behind this PBX? Presumably 1A1 or 1A2 key equipment is needed for the CO lines to operate on the first 4 line buttons. If so there is no reason that the PBX station on line 5 key couldn't also be run through a key system line circuit as well which would then provide Hold and Lamp functions as well.

If the PBX has an option to operate a line lamp for any of its stations this would potentially be an equipment cost saving over needing to use a full key system circuit but a key system line circuit fir the PBX station would likely give better functionality.

Just thinking out loud trying to picture this sort of arrangement as I rarely ever  saw this set up with PBX's here in the old days. Of course once PBX's became electronic with proprietary multi line phones available many different arrangements became much more commonplace.

How many stations can a 755 PBX support? What us the station numbering?

Do you know if all stations can operate separate line lamps if the PBX is optioned for it?

Terry

Offline poplar1

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Re: 755A Trunk/Local Lights
« Reply #3 on: August 22, 2016, 03:10:47 PM »


I am thinking that the PBX was somewhat designed around this method of operation since a 466 doesn't sound (to me) like a normal 400 series key set, maybe it is a custom key set to work behind this PBX? Presumably 1A1 or 1A2 key equipment is needed for the CO lines to operate on the first 4 line buttons. If so there is no reason that the PBX station on line 5 key couldn't also be run through a key system line circuit as well which would then provide Hold and Lamp functions as well.

Just thinking out loud trying to picture this sort of arrangement as I rarely ever  saw this set up with PBX's here in the old days. Of course once PBX's became electronic with proprietary multi line phones available many different arrangements became much more commonplace.


Terry

Terry. both the 750 and 755 PBXs use multi-line phones that have a common talk path for all lines. They don't have all the lines extended as pairs to each phone. The 205 telephone set used with the 750 has only 4 pairs: 2 pairs between the phone and the subset for the talk path (same as a 202), and 2 pairs for telling the equipment which line to pick up: 1, 2, 3 and a common ground. (Intercom is accessed when none of the 1, 2, 3 line leads is grounded). 755 is similar but I think also has a lead for intercom, also has 4 trunks instead of 3 (though a 4th trunk can be added to a 750A, if I recall correctly). So a key system (1A, 1A1 or 1A2) is not needed with phones on a 750 or 755 PBX, since the holding, intercom path, lamps (separately mounted in the case of 205 sets), etc. are accomplished in the PBX.
« Last Edit: August 22, 2016, 03:27:19 PM by poplar1 »
"C'est pas une restauration, c'est une rénovation."--François Martin.

Offline ESalter

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Re: 755A Trunk/Local Lights
« Reply #4 on: August 22, 2016, 07:52:33 PM »
That's exactly right.  The 755 has 4 trunks and 20 stations.  Picture it being like a key system except the intercom/local link is private.  On key systems anyone that depresses the intercom button is connected to that link.  On a 755 those links are direct and private(it dose support conferencing though).  It's capable of 3 local links at once.  The stations are numbered 20-30.  The key sets do have a separate lead coming back to select local/intercom.   I'll try to take a video demonstrating how it works tomorrow.

---Eric

Offline AE_Collector

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Re: 755A Trunk/Local Lights
« Reply #5 on: August 23, 2016, 08:30:12 AM »
The 755 has 4 trunks and 20 stations.  Picture it being like a key system except the intercom/local link is private. The stations are numbered 20-30.

---Eric

Oh okay, so quite a hybrid system. It is a much smaller system than I envisioned at only 20 stations and 4 trunks. In theory every station could be one of the 466 sets giving direct access to all 4 trunks but I think you mentioned that single line phones can be used as well? If so, they may just be intercom stations presumably. Being such a small system there likely is no dial 9 for a trunk capability?

Stations are probably all of the 2x's and 3x's? Maybe 21-20 and 31-30?

How easy is it to find a 466 phone for this system?

Anyway, thanks for all the info....very interesting! Looking forward to the You-Tube!

Terry
« Last Edit: August 23, 2016, 08:34:21 AM by AE_Collector »

Offline ESalter

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Re: 755A Trunk/Local Lights
« Reply #6 on: August 23, 2016, 09:45:42 AM »
I guess you could consider it a hybrid.  That private dial intercom is what makes it a PBX and not a key system.  I was tired yesterday when I gave the station numbers, they are 20-29 and 30-39.  755s apparently could come with either one crossbar switch(only stations 20-29) or with both crossbar switches so it had stations 20-29 and 30-39.

It is designed for all of the telephones to be the 462, 466, or 566 type key sets, which have access to all 4 trunks as well as the local links.  If you use single line phones they need to have A-lead.  Most of the ones we are using don't have the extra hookswitch contacts, so my dad hid a strip of relays in the battery tray to function like the extra hookswitch contacts.  All of our single line phones are hooked to the intercom so they don't have easy access to trunk lines unless a key strip was added.  It was apparently designed for non-key stations to access trunks by means of calling(on the intercom) a key station with a control key and having them connect your station to a vacant trunk with the control key. 

462(non-illuminated) and 466(illuminated) sets are fairly rare compared to 464 types.  We have had the 755 since late 2000 and I have been searching for a 466 to go with it since probably 2006.  Other than this one that I bought, I know of one other one I missed out on(on ebay, I mean), so they are pretty few and far between.

---Eric

Offline ESalter

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Re: 755A Trunk/Local Lights
« Reply #7 on: September 28, 2016, 12:45:25 PM »
I posted this link on another thread, but figured I should post it here too.  This is a demo video I shot of the 755 yesterday.

https://www.youtube.com/watch?v=4kPVZttaqUM

---Eric

Offline AE_Collector

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Re: 755A Trunk/Local Lights
« Reply #8 on: September 28, 2016, 03:30:10 PM »
Thanks Eric, well done. I just love hearing all those relays in operation. Fortunately my first 15 years at the Telco (1975-1990) were spent in CO's and at PBX sites, mostly installing electronic replacements. The entire time we were there the old equipment was working in the background until cut-over day and then we were off yo the next site. And since we were the Telco's COE installation department (not a contractor there just to install the new switch) we wound up working on the old equipment quite often as well including line adds to old offices that weren't scheduled for replacement yet.

What are all the rest of the horizontal trunk/link positions on the crossbar switches used for if the system only has three links? Is it three links per 10 stations or three links if equipped for ten or twenty stations?

Terry

Offline ESalter

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Re: 755A Trunk/Local Lights
« Reply #9 on: September 28, 2016, 06:34:50 PM »
You're very lucky to have memories of being in an active electromechanical CO.  I've seen strowger equipment still in place at an office, but no longer being used.  That's as close as I will ever get, I bet.

As for the horizontals on the crossbar, there are a total of 5 bars, making for 10 possible selections.  From bottom to top they are:

Trunk 1
Trunk 2
Trunk 3
Trunk 4
Link 1 Completing End
Link 1 Originating End
Link 2 Completing End
Link 2 Originating End
Link 3 Completing End
Link 3 Originating End

It has 3 links total, regardless of the number of stations the system is equipped for.

---Eric