Section 4 is intended as a guide to fault diagnosis and repair. It is written for repair staff who are familiar with standard fault-finding procedures and who have the requisite test equipment available. The table below lists the minimum required test equipment and materials.
| Equipment | Specification / Manufacturer |
|---|---|
| Frequency Meter | General Purpose |
| Oscilloscope | Rise time 100 nanosec |
| Digital Multimeter | General purpose |
| Known good LED display unit | Cambridge Computer |
| Known good keyboard unit | Cambridge Computer |
| Mains Adaptor | Cambridge Computer |
| Superglue | Open market |
Three types of board are in circulation, Issue 3, 4 and 5. Only a small number of Issue 3 boards are in existence. They differ from Issue 4 boards in that certain components have been manually wired in. In Issue 4 boards, the printed circuit has been amended to accommodate these components. The relevant components are listed below:
| Resistors | R80, R81, R82. In addition R23 (l00k) has been relocated in parallel with D9. |
| Capacitors | C31, C32. |
| Transistors | T25. |
| Diodes | D28, D29, D30. |
| Preset potentiometers | RV2. |
On Issue 3 boards, resistors R52 and R75 have been removed from the circuit and leave gaps. There is also a gap where R23 has been removed in order to be relocated in parallel with D9.
The issue 5 boards have been designed for foreign electromagnetic standard compliance. The differences have to be developped.