The Receiver Carrier WB400

External Photographs

The Warning receiver is the part of the system most visible to the general public, it was located in Post Offices, Shops, Pubs, Council Offices, Fire Stations and Police stations to name a few. The ROC posts were also fitted with one. The receiver would relay the Police messages for attack and fallout warnings, the recipient would warn the local people by the use of hand sirens, whistles, maroons and gongs.

The WB400 receiver was powered by a six volt dry battery in the base and when correctly adjusted would emit the quiet tick of the confidence tone sent by Carrier Control Point. When a calling signal or speech message was transmitted, the volume increased automatically.

In peacetime the receiver was normally switched off and was turned on for tests only. To change the battery required the removal of the base plate. The red knob on the front is a combined On/Off and Volume control. Below that is a small pull out draw containing an instruction card. The text on the Instruction Card may be seen in a cross referenced sub-topic.

On many occasions the receiver would be accidentally left on and flatten the battery or the user would forget to switch it on for the six monthly test message. When the replacement WB1400 was designed, these shortcomings were overcome by feeding the power from the telephone exchange and automatically turning the receiver on when required.

The instruction card drawer extended, the text on the card is reproduced on a subtopic in the cross reference at the bottom of the page.

Internal View

This the internal view of the Receiver Carrier WB400 (Diagram WB 28013). To protect the electronics, the chassis plate has a cork seal, a plastic membrane covers the rear of the speaker grill. A desiccater unit is fitted to the chassis plate which may be viewed from the battery compartment to check it is still Blue, if damp gets in the colour changes to Pink.

The four square cans are on the RF board which amplifies and detects the audio on the 72kHz carrier. The second board is the audio board containing a squelch circuit to mute the output when the confidence tick tone is being received. Next to the audio board in the output transformer with two transistors mounted on its tags.

Circuitry

There was no EMP protection in the circuitry which employed Germanium Junction Transistors. The RF board has three CV7005 (OC71) transistors, the audio board one more CV7005 more with two CV7006 (OC72) output transistors and two CV7004's (OC45) in the squelch.

The circuit is very simple for this 72kHz AM receiver

A much larger version (2130Wx900H) of the diagram may be seen by clicking the link in the cross reference box below.