The SB Zusatz 33 was a cradle attachment that fit on the lid of a Feldfernsprecher 33 (FF33) field telephone. The handset of the FF33 was intended to fit on the cradle. The SB Zusatz 33 essentially turned the Ortsbatterie Betrieb (OB) FF33 into an Schlusszeichen Betrieb (SB) telephone that could be used on SB telephone exchanges (i.e. A große feldklappenschrank with an Untersatz C base unit).
The SB Zusatz 33 allowed direct current to flow through it from the SB exchange when the handset was placed in the cradle. This direct current was detected by the SB exchange and it indicated that the call has ended. SB exchanges were abandoned mid-war.
Referring the the above electrical schematic… The purpose of capacitor C is to keep the electronics of the OB FF33 away from the SB direct current. The choke coil Dr is intended to prevent the incoming ringing alternating current from being short-circuited via the switch U when the handset is hung up in the cradle. (Switch U is closed when the handset is hung up in the cradle.) The incoming ringing alternating current encounters high inductive resistance in the choke coil Dr, so its flow is blocked. It instead flows through capacitor C to the ringer of the FF33. When the handset is in the cradle and the magneto of the FF33 is cranked, the outgoing ringing alternating current is blocked by choke coil Dr and flows via the capacitor C to the remote station.
If the handset is picked up to answer a call, the direct current loop La-U-Dr-Lb is interrupted by contact U. When speaking, the alternating current generated by the handset microphones flows through the capacitor C between the FF33 and the remote station. After the call is finished, the handset is hung up again, and the La-U-Dr-Lb loop forms the path for conducting direct current from the SB exchange.
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WW2 German Field Telephone Equipment 