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OB31 Siemens magneto-telephone

Started by dsk, November 01, 2021, 12:32:13 PM

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I bought my-selves a problem  ;D
I did need the handset, and did not now about this phone, but guessed that it was a post war railroad telephone, but no!
This was a model designed in 1931, and the next came in 33.  And my interest woke up. Not easy to find an easy readable diagram so I made one.
The magneto is definitely defect, the coil was burned. And the crank handle was missing.  I had a handle from a field telephone, and wit a little work it was possible to just call my field exchange by sending DC so I just added a power of 24V DC, do when I turn the handle the line gets 24V and the field exchange signal drops.

The 2 ringer coils has a resistance 150 ohms each, so a total of 300 ohms is pretty low!


Nice find, what do you use to make your schematics?


Cool phone!
Someone must have been crankin on that mag to a dead short to burn the coil?
Looks like maybe a field phone could provide a doner?
Can't see too well from pics.
Curious about the 24volts to drop the door??
Was that a CB exchange???
And the 300 ohm ringer???
Seems like that low would hold the line open???
Could a 1K resistor in line solve that?
Hee Hee, is good to hear from you again friend!
Can't wait to hear your comments!!!
Class is in session!


To make the diagram, I used pencil to make a lot of sketches during measuring with an ohm meter. Later I found documentation on net, but most of those are not detailed enough. The best I found is the lower one in the triple sketch. 
This phone does not work on CB, and as you see it is no capacitor in series with the ringer.  If I did put in a capacitor I am pretty sure it would work, but maybe with a REN load equal to 5 ( 5 * WE 500 phones)
Not sure how that generator was fried, but the phone has originally been used to communicate between 2 power-plants.  The german field phone generators are slightly to big to fit inside the housing, I may try the LME generator I used when I made my DIY exchange.  There I may put in another larger generaor. e.g. one from a German FF-33 field phone.


Magnetos are still available as surplus material for a reasonable price. Not sure about shipment to Norway though. Later types are very compact, it might be possible to fit them into the OB31.
To fry a magneto it would be required to hook it to an external voltage I think?


I did not think it was a CB phone.
But my limited experience with LB phones here, I see from 3 to about 5 volts dc doing a great job powering them.
I do know the operating range is quite broad, but just would not expect to see 24 volts. Still learning!


24 volts was just to get voltage enough to trig the signal on the field exchange, the a considerable power loss in a series resistor to the transmitter. Just to get it possible to use. If I find a better generator, I'l be using 3V I guess.



Now, I got a small enough generator, in lack of professional tools it looks as it looks inside, but it works.
Actually it is stationed in our kitchen with a line down to the SB-22 in the man-cave.


Quote from: dsk on November 01, 2021, 12:42:57 PM
I bought my-selves a little challenge:

NOK 100 or approx $ 10 for a phone is not horrible, but I n the start I was actually most interested in the handset, but it was actually a pretty rare phone. 

I struggled to identify it but here is the best info I have found:

and now, I found a small enough generator, and it works nicely.

Doug Rose

Very nice is a very cool phone. Glad you found a mag to fit....Doug


If someone here may evaluate the circuit... The extremely low ohm ringer made for magneto-ringing.  300 ohms only, and I could really get a nice "ding" and "dong" by using a 3V battery an the line terminals, just turning a button cell forth and back.