ZX Spectrum 48K Issue 2No picture, won't boot

The 5 V rail

Measure at the output of the 7805 regulator: between 4.75 and 5.25 V. The regulator runs very hot in a Spectrum, which is normal; if the 5 V is low, switch off and look for a short.

The +12 V rail

The DC-DC converter (the two transistors and the coil next to them) makes the +12 V and −5 V the 4116 memory chips need. Measure the +12 V here.

The −5 V rail

If −5 V is missing while +12 V is present, the 4116s die. Always check it, and before powering a machine with new memory chips.

Reset

At power-on, the Z80’s _RESET rises after a moment, while a capacitor charges. If it stays stuck low, the Z80 never starts.

The clock

The ULA derives the Z80’s 3.5 MHz clock from the 14 MHz crystal. Without a clock nothing runs: check the crystal and the ULA.

The Z80 at work

_M1 pulses every time the Z80 fetches an instruction. If it pulses, the processor is alive; if not, go back to reset and the clock.

The ROM

The ROM holds BASIC and the start-up code. If the Z80 is working but the Sinclair message never appears, suspect the ROM.