Editing Sinclair ZX Spectrum Troubleshooting Guide (section)

== Power Supply & Voltage Checks ==

Always verify the PSU '''before''' suspecting mainboard faults.

{| class="wikitable" style="text-align:left"
! Test Point !! Expected Voltage !! Purpose / Notes
|-
| Regulator input (7812/7805 tab) || +12 V DC || Main supply rail (16K/48K)
|-
| Regulator output (7805) || +5 V DC (±5%) || Logic rail for CPU, ULA, ROM
|-
| Lower RAM pin 1 || -5 V DC || Only on 4116 RAM (16K/48K)
|-
| Lower RAM pin 8 || +12 V DC || Only on 4116 RAM (16K/48K)
|-
| CPU Vcc (pin 11) || +5 V DC || Z80A supply
|-
| ULA Vcc (pin 40) || +5 V DC || ULA supply
|-
| RESET (edge connector pin 16) || Low → High || Must release to 5 V after power-up
|}

=== Common PSU Faults ===
* '''5 V missing/high''' → no boot or chip damage.
* '''12 V missing''' → no video, no sound, or RAM errors.
* '''-5 V missing''' → lower RAM (4116) failure, often black screen or stripes.
* '''Intermittent drop-outs''' due to cracked power jack solder or faulty regulator.
* '''Buzzing/hum bars''' = dried-out capacitors or PSU ripple.

Never run a Spectrum on an unstable PSU; modern regulated adapters are recommended.