Sinclair Research produced several official utilities, some of which were originally restricted to authorized service technicians. Sinclair ZX Spectrum Test ROM (1983):
Most test programs output a grid. On a 48K Spectrum, there are 8 RAM chips (IC3 to IC10). A typical result: zx spectrum test program link
In a world of emulators and FPGA clones, the original hardware is becoming a rarity. The test program link ensures that the heart of the 8-bit era keeps beating, one diagnostic loop at a time. Sinclair Research produced several official utilities