Service manual

- 18 volts at 20 amps referenced to 0VA

+ 48 volts at 0.7 amps referenced to 0VA

+15 volts at 10 amps referenced to 0VF

- 15 volts at 10 amps referenced to 0VF

+ 5 volts at 10 amps referenced to 0VD

All rails in the power supply are floating until it is connected to the console which connects the

chassis and signal zero volt references as described previously. The power supply provides a

SAFETY connection to EARTH for the console via its mains connection.

THE POWER SUPPLY SHOULD NEVER BE OPERATED WITH THE MAINS EARTH

DISCONNECTED.

Please note that the power supply contains LETHAL VOLTAGES greatly in excess of the

mains voltage and that its rails can produce extremely large currents which could burn out

equipment and wiring if shorted. All testing and servicing should ONLY be carried out by

qualified engineers.

All power supply rails are generated by two switch mode blocks inside the case. There are

no user serviceable parts within these; if a failure is traced to one of these blocks it should

be replaced. No attempt should be made to repair a switch mode block. Repair is only

possible by returning the block to MIDAS ENGLAND.

All rails are filtered to remove switching noise and connected to the console via diodes. This

allows two power supplies to be connected in parallel to one console; in the event that one supply

fails the other will continue to operate normally.

The circuit configuration is similar for all rails except for the polarity of some components. The

switch mode block output is first shunted by a differential filter capacitor network and then fed to

a common mode choke. The choke feeds a further capacitor network which filters common mode

noise to the chassis. This is followed by a differential choke, a smoothing capacitor network and

the power diode. The 48 volt supply does not use a differential choke; a resistor is used in place

of this part.

The power supply monitors the voltage on the mains supply at all times. If the mains is interrupted

the power supply gives the console automation system advanced warning that the rails will col-

lapse. This ensures that the automation is in reset before the 5 volt rail has become to low to

operate the system correctly. The mains is rectified by diodes D11, D12, D13 and D14 which feed

opto-isolator IC1. The output is smoothed by C87 and inverted by invertor IC2. Resistor R8 and

capacitor C88 provide a delayed turn on and diode D15 ensures fast turn off. The signal is again

inverted by the remaining sections of IC2 which are paralleled to provide high drive capability.

The power on line is fed by diodes D16 to D20 and pulled logic low by resistor R9; this allows for

power supplies to be connected in parallel.