The Feldfernsprecher 33 (FF33), the Ortsbatterie Betrieb 43 (OB43), and the Tischfernsprecher 38 field telephones all use the same handset. The handset microphone capsules are powered by 1.5 volt batteries. As such, the microphone is active, and it sends out sound activated pulses to the passive speaker in a connected telephone.
The microphone capsule (kapsel) has carbon grains (Kohlekörner) inside it. When speaking into the microphone, sound waves vibrate the sound plate (Schallblech), and the grains strike one another creating electrical pulses.
Over the decades, the carbon grains 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 you can try the following recharging method.
I bought a couple of Czech postwar TP-25 handsets. These handsets are copies of the WW2 handsets used on German field telephones, except they are molded in light plastic. The plug on the end of the cord is also different. The microphone capsules of the TP-25 are also designed to operate on 1.5 volts and they will work in a wartime German handset. More importantly, the wartime German microphone capsules will fit inside TP-25 handsets.
I decided to use the TP-25 handset to hold sick microphone capsules during recharging. To do so, I wired into the microphone circuit via the TP-25 handset cord.
To recharge a capsule, connect the microphone circuit to the La and Lb/E terminals on a field telephone.
While holding the talk button down in the TP-25 handset, turn the magneto crank on the field telephone. A couple of turns should do it.
The capsule is recharged and should now work.
For more information about WW2 German field telephones, check out my book: https://www.lulu.com/en/us/shop/rotwang-manteuffel/world-war-2-german-field-telephone-equipment-a-basic-guide-for-reenactors-and-historians/paperback/product-976w9q.html?page=1&pageSize=4
WW2 German Field Telephone Equipment 