Author Topic: Swedish Televerket / Ericsson BC 311 extension phone  (Read 636 times)

Offline countryman

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #15 on: November 12, 2019, 02:48:27 AM »
with that circuit I broke my own rule and copied&pasted it right away. Here's the link: http://www.erel.de/INH/20/2040311/index.htm
click on the flag "Schaltplan". Thanks for reminding me of that missing reference.
I'll try to fix the image issue in my earlier posts.
« Last Edit: November 12, 2019, 03:07:19 AM by countryman »

Offline Jack Ryan

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #16 on: November 12, 2019, 06:28:44 AM »
with that circuit I broke my own rule and copied&pasted it right away. Here's the link: http://www.erel.de/INH/20/2040311/index.htm
click on the flag "Schaltplan". Thanks for reminding me of that missing reference.
I'll try to fix the image issue in my earlier posts.

Thanks, I see it now.

Regards
Jack

Offline countryman

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #17 on: November 12, 2019, 07:15:48 AM »
Looking at the diagram once more. What type of "slave" set (extension) would be used here? A dial phone would work, but could not call the "master". A magneto phone would send ringing voltage down the office line when cranked with the master switched in bypass, the position shown in the diagram. That was never a safe practice on automated exchanges, let alone VOIP routers...
I'm still wondering about the original setup.

Offline ma_xyz

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #18 on: November 12, 2019, 12:48:53 PM »
That's a cool instrument! Looking forward to restoration progress pics :-)

Looking at the diagram once more. What type of "slave" set (extension) would be used here? A dial phone would work, but could not call the "master". A magneto phone would send ringing voltage down the office line when cranked with the master switched in bypass, the position shown in the diagram.
On erel.de it is mentioned that the extension would also use a magneto: "Die Endstelle wird per Kurbelinduktor gerufen, diese verfügt ebenfalls über einen Induktor zum Anruf der Zwischenstelle." ("The end station is called by magneto, the end station uses also a magneto to call the intermediate station.").
I'm not sure about that but probably some choke-inductors/highpass circuit would protect the exchange side, also mostly the power would be consumed by the additional ringer in the loop shown on the diagram at right?

Offline Jack Ryan

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #19 on: November 12, 2019, 06:00:02 PM »
It was pretty hard to kill an exchange - they were quite robust.

If you look up the Bellset No 20 (N520) on Bob Freshwater's WEB site it will give you lots of connection information. Follow the N diagram links to get the full picture.

The extension telephone is basically a normal common battery telephone with a magneto added. The images I have posted are of an intermediate extension used by the PMG in Australia and its circuit diagram.

Intermediate systems made by (Swedish) Ericsson and also made by German manufacturers worked on the same principal.

Regards
Jack


Offline Jack Ryan

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #20 on: November 12, 2019, 06:15:40 PM »
Here is what looks like another intermediate set and an intermediate switch.

Jack


Offline countryman

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #21 on: November 13, 2019, 01:02:58 PM »
Thanks for the assistance! I managed to fix the dial meanwhile. The ring with the cam rotates on the shaft after loosening the screw. I also needed to adjust the finger stop, which limits the travel of the finger wheel in the normal position. Now both contacts do what they should.
Did this dial design have equivalents on the international markets?
I can see sending ringing voltage back to a mechanical exchange wouldn't do much harm besides a possible "surprise" for a lineman. But how about electronic PBX?

Offline Jack Ryan

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #22 on: November 13, 2019, 05:15:25 PM »
Did this dial design have equivalents on the international markets?

Do you mean was it used on the international market? Yes

Do you mean was it licensed on the international market? Yes, although one licensee, EB, apparently didn't like it and made some modifications.


Quote
I can see sending ringing voltage back to a mechanical exchange wouldn't do much harm besides a possible "surprise" for a lineman.

No more so that if the line happened to ring while the lineman was stripping insulation with his teeth.


Quote
But how about electronic PBX?

I don't think their usage overlapped - in any case, I don't remember ever coming across a case where an intermediate phone was used on a PAX or PABX although one could be used as a secretarial set.

There were alternative intermediate telephone systems that used battery ringing (a button and a buzzer) - the PMG used both. There were also specially designed secretarial sets that were functionally similar for use in an office environment.

Jack

Offline countryman

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #23 on: November 14, 2019, 07:33:05 AM »
The dial seems to be referred to as "RG 112" in the Ericsson documents I found, but I don't get any further than that...

In fact it would not make sense to operate that setup behind a PBX of any kind, but I think of doing so for demonstration.
« Last Edit: November 14, 2019, 07:44:38 AM by countryman »

Offline dsk

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #24 on: November 14, 2019, 08:06:59 AM »
I have looked at the Elektrisk bureau copy some years ago, the phone was used on the POTS here.  The problem with the dial was the cham did not always release in reverce and sometimes it did break the line.  At that time it was repaired by cleaning and oiling.

dsk

Offline countryman

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #25 on: December 18, 2019, 04:33:58 AM »
I got the dial eventually fixed. The ring with the cam needed adjustment, and also the finger stop. It's underside doubles as a stop defining the finger wheel normal position, and that was off, too.
I have not worked on the phone's missing internal wiring yet.
Thinking it over, this setup would be great to demonstrate magneto phones and connect them to POTS or VOIP lines. The magneto extension could call this intermediate set, where an "operator" would dial the number and make the connection.
Still there is the issue with magneto voltage possibly traveling down the line.
I wonder how serious this issue is? Is there any way to prevent possible damage to modern electronics in a PBX or VOIP interface?
These deal with 48V DC and ringing voltage themselves, so might even have enough internal protection?
Has anybody ever killed such a device with external voltage and cares to report?

Offline RB

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #26 on: December 18, 2019, 02:11:47 PM »
That would depend entirely on the device.
If it had protection built in, no prob.
If no protection, poof...
If protection switched in and out, all bets off.
IMHO, of course ;)

Offline dsk

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #27 on: December 18, 2019, 02:21:17 PM »
It is made for equipment without electronics, so I would have configured it different, e.g. a third wire for the intercom signalling.

dsk

Offline countryman

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Re: Swedish Televerket / Ericsson BC 311 extension phone
« Reply #28 on: January 11, 2020, 03:50:33 PM »
I was lucky and found a fragment of a similar phone for parts. I now can replace the terminal block, the ringer and the contact for the push button on top. Also, the damaged base can be replaced. The old one had the ringer mounting lugs and the threaded spacers for the gong and the terminal block ripped off.

The red fragment is a funny thing, it resembles other old metal phones that were "refurbished" for the antique market as I mentioned here.




This shell has not only been painted red with industrial grade paint (hard to remove), but also must have been sandblasted before. It has a cradle with a fake, functionless gong. Inside the shell there only was an induction coil with 2 windings dated 1940. I wonder who did this...
I plan to take the cradle off and use this shell with the damaged base part as a subset, containing the second ringer and a battery box. I also think about a protective circuit to allow the operation on a VOIP terminal. Then this intermediate set could act as a stylish interface to demonstrate magneto phones...