The Feldfernsprecher 33 (FF33) field telephone contains a crank magneto which generates about 90 volts of ringing current.
The magneto can be tested without hooking the FF33 up. Testing is done by placing a jumper wire in between terminals La and Lb/E, pressing the white test button (Prüftaste), and turning the magneto crank. If all is in order, the bell on the FF33 should ring.
To understand how this works, lets look at an electric schematic of the magneto, test button, and bell…
The schematic above shows the FF33 at rest. In this state, voice signals will pass through the La and Lb/E terminals and the magneto induction coil of the FF33. When turned, the magneto crankshaft moves out towards the crank. When it moves out, the crankshaft causes an electrical contact on the magneto to change state…
In the schematic above, when the magneto crankshaft contact changes state, the magneto’s induction coil is connected to terminals La and Lb/E. The FF33 will put out a ringing current. If another field telephone is connected to terminals La and Lb/E, its bell will ring. Let’s disconnect the FF33 from the other phone, install a jumper, and try out the test button…
With the test button pushed, a jumper wire across terminals La and Lb/E, and the magneto crankshaft turned, the magneto is connected in series with the bell in the FF33, and the bell rings. The magneto crankshaft contact is located at the top of the magneto end bell.
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WW2 German Field Telephone Equipment 