Linear Power Supply VT 25514
This power supply can be identified by its black case and disctintive circular cutout for the smoothing capacitor. All linear power supplies in machines produced by ICL are of this design, with just a few exceptions which use the M5093 model. It was also used by Cleartone between serial numbers 106000 and 109500 approximately.
The power supply provides +5V at 2.5A and -5v at 100mA. The +5V line powers the bulk of the computer, the -5V line is mainly used in the sections which employ analogue op-amp circuitry such as the audio amplifier and the cassette interface.
A compact toroidal transformer is employed. This has two secondary windings connected to two rectifier and reservoir sets to provide the two outputs. Both outputs are electronically regulated. The low current capacity of the -5V supply allows a simple circuit to be used whose output is fixed.
The +5V arrangement employs heavier duty components including a TIP42 power transistor as the pass element. The output voltage of the +5V circuit is adjustable using a small preset resistor. Since the regulator does not function without a load a 10R 10W resistor must be connected across the +5V output while testing the power unit when it is disconnected from the computer.
To set up the output, disconnect all the power supply leads from the microcomputer PCB and connect the 10R resistor across any one of the +5V connections (coloured red and black). Switch on the power and adjust the output to +5V, +/-50mV. The preset control must work smoothly, if the voltage changes erratically there is a fault which must be corrected before the unit is used again. Leave the power applied for 10 minutes and check the adjustment again. Re-fit the power unit to the computer and ensure that the output voltage does not exceed +5V when the power is turned on. Do not attempt to adjust the output voltage when the power unit is connected to the computer!