A common problem afflicting WW2 German field telephones is the handsets are dead. A dead handset may be easy to fix.
The Feldfernsprecher 33 (FF33) and Tischfernsprecher 38 WW2 German field telephones use the same handset. The handset microphone is powered by a 1.5 volt battery. As such, the microphone is active, and it sends out sound activated pulses to the passive speaker in a connected telephone.
Suppose two FF33s are wired together. If the handset of one FF33 appears to be dead when pressing the talk button and speaking into the handset of the other FF33, check the battery in the other FF33. If the battery is okay, then the problem may lie in the handset itself.
Try cranking the magneto on the phone with the inoperative handset. After cranking for a few seconds, the microphone may come back to life. If not, remove the microphone capsule from the handset.
The microphone capsule has carbon granules inside it. When speaking into the microphone, the granules strike one another creating electrical pulses. Over the decades, these granules may have gotten compacted. To loosen them, aggressively tap the capsule on a hard surface. If the microphone does not come back to life, then there may be a problem in the handset wiring.
Place the capsules back in the handset. On the handset plug, pins 1 and 5 are wired to the microphone and push to talk button. Pins 2 and 4 are wired to the speaker. Hook an ohm meter up to pins 1 and 5. When the talk button is not pressed, it should read 0 ohms. When the talk button is pressed, the meter should read between 100 and 150 ohms if the handset is wired correctly. If the meter is connected between pins 2 and 4, the meter should read between 1800 and 2000 ohms.
If the meter fails to register when the talk button is pressed, then there is most likely a wiring problem in the handset. This could be due to a broken wire in the handset, cord, or plug. If a capsule ohm reading is outside the range, then the capsule probably has to be replaced. Keep in mind that a dead handset is usually not due to wiring problems in the body of the phone.
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WW2 German Field Telephone Equipment 