Kasten Pz. Nr. 23a

Bordsprechverstärker Kst. Pz. Nr. 23 und Pz. Nr. 23a
(Pak und Artillerie auf Selbstfahrlafetten, Panzer Spähwagen)

The boxes 23 and 23a were intended for use with the Fusprech.a, d, and f transceivers. Box 23a was different from box 23 by having a built-in power supply with a vibrating inverter to provide the anode voltage for the vacuum tube. (Box 23 obtained its voltage from the SEU Umformer which also supplied power to the transceiver.) Both boxes provide for the direct connection (sockets) for the microphone and headphones of the radio operator (Funker) who is probably also the vehicle commander, and another member of the crew (the driver ?). With the microphone switch disengaged, the radio operator (Funker) was connected to the intercom system. With the microphone switch engaged, the radio operator (Funker) was connected to the transmitting part of the transceiver through a relay contact. Here, through the dual-purpose RL12P10 power amplifier tube (HF final driver when transmitting, and LF final stage when receiving) approximately 4 Watts of LF power allows the speaker LS(Fu)a to be connected, through which the other members of the crew can hear the radio communications.

It is accepted, that both amplifiers 23 and 23a were introduced and manufactured together in 1942/43. The documented shortcomings of the 23a box (12 V feedback over the microphone connection cable to the transceiver) made very early modifications necessary. In this modified version (mid 1943 ?) connection to the transceivers (FuSprech.) "a", "d" and "f" was possible. With the first version, the documentation stated that it could connect to the FuSprech.a, FuSprech.d, FuSprech.c, FuSprech.e, and FuSprech.f. We can assume that the FuSprech.c, FuSprech.e versions were planned for production but were ultimately abandoned.

The Kasten Pz. Nr. 23 shares power from the SEU Umformer with the transceiver. The SEU delivers around 275 volts of anode voltage, and in contrast to amplifier boxes 11, 20 and 24 (Ua ~ 130 V) inserts this voltage through the resistance of the anodes and screen grid paths. Externally, the Kasten 23 has two flip switches like the Kasten 20, however it has an additional speaking (intercom) transformer Ü3. This transformer is required for the adaptation (adjustment) on the lower output resistor of the receive component (5 Ohm) with the FuSprech.a and FuSprech.d (up-transforming at about 1700 Ohm is possible, with the parallel connection of the receiver output with the output of the amplifier 23).

The independent generation of anode voltage in the Kasten 23a (230V, 7mA) allows it to exist in an intercom system without a transceiver. The design incorporates a very compact built-in vibrating inverter section next to the amplifier. Both are within the metal container and are shielded from each other. The microphone input transformer of the amplifier is shielded, to avoid the magnetic interference of the vibrating inverter W.Gl.12e which vibrates at 150 Hz. The radiation of the radio interference from the 12V cable and in the anode voltage line can be significantly reduced by shielding the Kasten 23a well.

Whereas in Kasten 23 the switching for operation with the FuSprech.a,d,f (c,e) is accomplished with the outside flip switch, this same function is achieved internally in the Kasten 23a with the sliding connectors.

From the Kasten 23a there are two versions that are known, which differ by how they switch on the 12 Volt power source. With the (probable) first version, the 12V operating voltage passes through the uni-pole "Ein/Aus" switch of the volume control knob. After the following fuse with "potential 2" marked +12V, passes over the HF choke D2 to the vibrating inverter, and then to the filament of the RV12P2000 and to the microphone voltage supply. Potential 2 passes also through the HF choke 1 with a middle contact of Bu1 "z. Sender Mikr". Bu1 is connected with a 3-wire rubber cable with the microphone connector plug of the transceiver. When this connection is made, the +12V passes over the middle wire of the Kasten 23a, and heats the vacuum tube even when the Kasten 23a is switched off "Aus" and lets the vibrating inverter to operate. This inadequacy drains current unnecessarily. In the modified version of the Kasten 23a the +12 Volt feed is fixed with a double-pole on/off switch "Aus/Ein". The possible reverse feedback of the switched-off transceiver over the connection cable with the switched-on Kasten 23a is avoided at least with the Fusprech.f through the function of the two-pole on/off switch "Ein/Aus".

It is possible that Kasten 23 also had two versions. It is clear in the schematic that also here in setting "Ein" +12 V passes through the middle contact of the connector "z. Sender Mikrofon". However, only one version has been encountered.

In the publication Fu 88738 v. September 1943 of the Infantry Signals School in Halle (Fusprech.f and onboard intercom system of self-propelled artillery (Sf) - the original vibrating inverters W.Gl.12a and b were no longer used at the end of August 1943, and were replaced with the W.Gl.12e which was designed for higher anode voltages of 300V.