Introduction

The picture on the left shows the old equipment that has become somewhat unreliable and has had to be replaced. The picture on the right shows the two replacement 60 watt PA amplifiers. One is in use, the other is a standby.

The present Public Address amplifiers use three microphone inputs. Two are used with CB Radio type microphones with shielded cable running to the equipment. However there is a distinctly short limit to the length of microphone cable that can be used. When, in the past, a requirement came to be able to use the PA from a new and distant platform, it was decided to provide a telephone as the microphone and get the telephone speech back to the PA in normal cable.

This required a piece of relay equipment that would provide line current to operate the telephone and produce the necessary signals to connect to the PA amplifier and select the correct loudspeaker group. This equipment has also been completely replaced during this work.

The final circuit has provided for five such remote telephones to be connected to the PA input. A linefinder is used to search for a calling line, connect it to the PA input and to switch the announcement to the correct speaker group.

A similar circuit allows certain telephones to dial 595 and gain access to the PA. To prevent any telephone from dialling 595 and successfully obtaining access to the PA, authorised phones, and the direct access phones, are supplied with a button that completes the switch on process.

A music channel and an output from the fire warning system also have access to the amplifiers via the telephone access equipment.

The Access Relay Equipment

The right hand relay set has 3000 type calling relays to provide line current to distant teles and to start the uniselector which is a linefinder. The relays also act as audio transformers to deliver the speech to the amplifier. The small relays on the veroboard are operated by the linefinder and switch through the appropriate speech channel to the audio switching relay set on the left.

The linefinder also extends a condition into the audio switching relay set to operate the correct input and output relays. The matrix has diodes connected to select the correct microphone input and loud speaker group output for the calling line.

The small relays in the equipment are operated from a 24 volt supply provided by a mains unit. This 24 volt supply also circulates via switches on the microphones to operate the appropriate switching relays. The large choke and capacitors had to be added to further smooth the output of the 24 volt unit as the ripple was inducing considerable hum on the microphone inputs.

The two black boxes beneath the power unit and mains socket strip are microphone pre-amplifiers. The microphone switching is completed after the signals have been boosted in the pre-amps.

Miscellaneous Equipment

A CD player can be connected to the PA equipment for use on those days when Thomas or Father Christmas visit us. We also have a fire warning system which connects a warbling tone to the PA if set off.

The equipment has been designed so that various callers have differing priorities. eg :
The music channel has the lowest priority and is over ridden by any other caller.
Telephones connecting to the PA have the next level of priority ie they will switch off the music during an announcement.
The Fire warning system has high priority and will over ride the music or any telephone caller.
but it can be over ridden by a microphone so that information can be disseminated even though the fire warning is attempting to sound.

The above diagram is difficult to read but better copies are available below.

The following diagrams are available in PDF form :

Overall circuit diagram 120Kb
PA Circuit Elements 210Kb
Telephone Access Relay Set 118Kb
Audio Switching Relay Set 98Kb
IDF Terminations 37Kb
Norchard Speaker Arrangements 38Kb
Music Channel Equipment 87Kb
PA Tele Access R/S U Points 82Kb
PA Audio Switching R/S U Points 84Kb
24V Supply 47Kb
Microphone Connections 139Kb
Common Service Blocks 52Kb

Page provided by John Bathgate This page was last updated on 20th January 2015