Author Topic: German OB05 magneto desk phone  (Read 887 times)

Offline countryman

  • **
  • Posts: 704
German OB05 magneto desk phone
« on: April 05, 2020, 04:38:55 PM »
The OB05 was designed 1905 but remained in production for a long time. I'm just dealing with two sets from 1926 / 27. One is in original condition but extremely filthy, and the handset is missing parts. The other is fairly clean but parts of the wiring loom and the induction coil are cut off and missing :-(
Once again I had problems with the confusing original wiring diagrams (like reproduced here.)
I tried to draw a basic, simplified version. Please correct me if there is anything wrong.
The original diagram mentions 2 modifications:
1) a 2 F capacitor between Lb and C for automatic clearing signal [edit: ring-off signal] (now obsolete) and
2) a 2 F capacitor between MB and Lb for CB service. A local battery will still be used in this mode, and there will be DC current through the receiver.

I plan to restore set no. 1 for LB use as original. Set 2 might be restored for true CB use with a more modern induction coil (incoming calls only).
« Last Edit: April 06, 2020, 03:44:24 AM by countryman »

Offline dsk

  • **
  • Posts: 4377
Re: German OB05 magneto desk phone
« Reply #1 on: April 05, 2020, 05:24:59 PM »
Interesting!Looks right schematic. 
For CB use I would try out this like enclosed diagram.
My zbsa11 has a similar ringer, and that works with 0.47 uF capacitor, but not loud.
The resistor on the line will reduce the stress of the transmitter, but still be OK for the signal.  Shorting the line terminals are important!
Then it will work OK on CB without battery.
dsk

Offline Jack Ryan

  • **
  • Posts: 1621
Re: German OB05 magneto desk phone
« Reply #2 on: April 05, 2020, 08:34:36 PM »
The circuit looks correct but why is it so?

Why is MB connected to Lb? The transmitter circuit does not need to connect to any other part of the circuit - was this an attempt to save on line cord wires?

If saving wires, why is C brought out to the terminal block?

What are the details of "automatic clearing"?

Some of the handsets had "press to hear better" buttons. How common were they?

Thanks
Jack

Offline dsk

  • **
  • Posts: 4377
Re: German OB05 magneto desk phone
« Reply #3 on: April 06, 2020, 02:16:31 AM »
The circuit looks correct but why is it so?
We have to remember this is an early phone, and the design should try to be better than the last...
Why is MB connected to Lb? The transmitter circuit does not need to connect to any other part of the circuit - was this an attempt to save on line cord wires?
No, but it was a way to make it easy to put in a capacitor if needed. The capacitors of those days was huge and might have need of a battery box sized cabinet.

If saving wires, why is C brought out to the terminal block?
I believe the idea of saving wires not was actual, but the C wire could just as well be called line B

What are the details of "automatic clearing"?
Automatic clearing was about opposite of the DC circuit in CB systems.  The exchange did put on a dc voltage on the line during the conversation, and this was blocked in a capacitor in the speech circuit, but when going on hook the DC flow trough the low ohm ringer  (300 ohms) and activated a ring off signal.
Some of the handsets had "press to hear better" buttons. How common were they?
Those handsets was not extremely common, but many magneto telephones had a button to short out the induction coil so you just had the receiver element across the line to get stronger signal and block all sidetone. During this the battery current in the transmitter circuit still ran as normal.

Thanks
Jack

dsk

Offline Jack Ryan

  • **
  • Posts: 1621
Re: German OB05 magneto desk phone
« Reply #4 on: April 06, 2020, 04:20:32 AM »
Thanks Dsk but it doesn't gel for me.

Quote
We have to remember this is an early phone, and the design should try to be better than the last

That's true but in 1905 local battery telephones were well known and, at least initially, they were not made with thoughts of conversion to CB - that was an afterthought.


Quote
...No, but it was a way to make it easy to put in a capacitor if needed. The capacitors of those days was huge and might have need of a battery box sized cabinet.

The capacitor was needed to DC isolate the bell. It is only mixed up with the transmitter circuit because of the original unusual circuit design.


Quote
I believe the idea of saving wires not was actual, but the C wire could just as well be called line B

That is true but again, because of the unusual circuit, MB, C and W are all brought out and all connected together.


Quote
Automatic clearing was about opposite of the DC circuit in CB systems.  The exchange did put on a dc voltage on the line during the conversation, and this was blocked in a capacitor in the speech circuit, but when going on hook the DC flow trough the low ohm ringer  (300 ohms) and activated a ring off signal.

There are two capacitors, one for automatic clearing (1) and one to block DC in the bell circuit (2). I am asking about (1).

(a) Does automatic clearing apply to the original LB circuit or to the CB modification?
(b) What exactly is automatic clearing?


Quote
Those handsets was not extremely common,

Thanks, it must be a collector preference - many of the refurbished versions I have seen had one.


Quote
but many magneto telephones had a button to short out the induction coil so you just had the receiver element across the line to get stronger signal and block all sidetone. During this the battery current in the transmitter circuit still ran as normal. dsk

I haven't seen that on these telephones or, for that matter on other German LB telephones. Perhaps I haven't seen enough German LB telephones.

Doing that is a bit strange really because the receiver would no longer be matched to the line so there would not be optimum power transfer.

Thanks again Dsk.

Jack

Offline dsk

  • **
  • Posts: 4377
Re: German OB05 magneto desk phone
« Reply #5 on: April 06, 2020, 05:17:44 AM »
At least regarding that automatic ring off, it was still in use during WWii  and depends on this phone where to connect the wire.

(upper right corner) 


Depending on if the capacitor should operate in series with the ringer or in series with the phone circuit. 
dsk

Offline dsk

  • **
  • Posts: 4377
Re: German OB05 magneto desk phone
« Reply #6 on: April 06, 2020, 05:22:09 AM »

Offline countryman

  • **
  • Posts: 704
Re: German OB05 magneto desk phone
« Reply #7 on: April 06, 2020, 07:10:44 AM »
The "push to hear louder" button also appeared on other LB phones. It's on my Swedish Televerket BC311 model and on a Danish "Jydsk" phone, too - both presumably influenced by Ericsson. BTW also the Siemens & Halske OB05 followed an Ericsson design. On later OB05 -like mine- the button disappeared.


from this thread.

Contacts 9 and 10 short out the transmitter, while 7 and 8 mute the ringer when cranking the magneto. Here the button is not in the handset but on top of the housing and has a dual function. As it is not labelled, I wondered about it's purpose for quite a while...

What the ring off signal is concerned, the OB05 with the capacitor between b and C would be referred to as "positives Schlusszeichen" in the German literature. When the subscriber hangs up, there will be a DC current through the 300 Ohm ringer, activating an indicator in the manual exchange. Erel.de

@dsk, thanks for that very interesting e-book link!
« Last Edit: April 06, 2020, 07:18:45 AM by countryman »

Offline Jack Ryan

  • **
  • Posts: 1621
Re: German OB05 magneto desk phone
« Reply #8 on: April 06, 2020, 08:50:00 PM »
The "push to hear louder" button also appeared on other LB phones. It's on my Swedish Televerket BC311 model and on a Danish "Jydsk" phone, too - both presumably influenced by Ericsson. BTW also the Siemens & Halske OB05 followed an Ericsson design. On later OB05 -like mine- the button disappeared.

Thanks for that. There are several variations of buttons related to talking on telephones. The most common are the press-to-talk type that simply supply power to the transmitter. Others mute the transmitter to reduce the pickup of local noise and also, because sidetone is reduced, cause the speaker to speak louder. It is interesting to see the solutions to similar problems that different jurisdictions used.


Quote
Contacts 9 and 10 short out the transmitter, while 7 and 8 mute the ringer when cranking the magneto. Here the button is not in the handset but on top of the housing and has a dual function. As it is not labelled, I wondered about it's purpose for quite a while...

The circuit is complicated by the fact that it is an intermediate telephone but even so, what a strange circuit.

The transmitter and receiver are in series across the line and the induction coil windings are across the elements and in series across the line. Contacts 9 and 10 short the transmitter and the primary of the IC. Also, the resistance ratio of the IC is abnormal - usually the transmitter and IC primary are in series but not here.

It requires a bit more thinking to determine what the designer had in mind.


Quote
What the ring off signal is concerned, the OB05 with the capacitor between b and C would be referred to as "positives Schlusszeichen" in the German literature. When the subscriber hangs up, there will be a DC current through the 300 Ohm ringer, activating an indicator in the manual exchange.

Even better described on page 18 here:

Thanks for the explanations and references - it is clear now. I had not paid any attention to that before and I don't think it existed in the UK, US or here in Australia.

Jack

Offline dsk

  • **
  • Posts: 4377
Re: German OB05 magneto desk phone
« Reply #9 on: April 08, 2020, 04:30:20 AM »
I have been looking more on German war phones, and even found a bad design. I  guess it may not be the reason for loosing the war, but this diagram ....
Indicates that a 1400 ohms ringer is all to seize the line in CB modus. This is a version of a phone who originally had a 360 ohms ringer, and that will work.  Strapping the capacitor  C2 solves the problem :-)

dsk
PS Source: https://www.nachrichtentruppe.de/?id=132:tischfernsprecher-38-schaltplan&catid=29  DS

Offline dsk

  • **
  • Posts: 4377
Re: German OB05 magneto desk phone
« Reply #10 on: April 08, 2020, 05:07:16 AM »
Loo bit a little bit closer on the last one I posted, a 1400 ohms ringer with 2 coils in series gives 350 ohms with the coils in parallel.
Thank you for making me thinking  8) by my selves.  The diagram still states a 1400 ohms ringer!

dsk

Offline countryman

  • **
  • Posts: 704
Re: German OB05 magneto desk phone
« Reply #11 on: April 05, 2021, 11:58:48 AM »
The circuit looks correct but why is it so?

Why is MB connected to Lb? The transmitter circuit does not need to connect to any other part of the circuit - was this an attempt to save on line cord wires?

If saving wires, why is C brought out to the terminal block?

(...)

Thinking it over again, the complicated wiring might have to do with the fact that this setup neither rings its own bell when calling, nor a possible extra bell. This would be connected between W and MB, replacing the jumper in the terminal block.