Power Circuit (ZX Spectrum 128)

The ZX Spectrum 128 gets three power rails directly from the external power supply.

There are no internal power regulators.

The power brick should supply +5V, +12V and -12V to the system.

Start by checking with a multimeter that the power supply pins output those voltages, and when inserted it into the console, those voltages remain.

If the voltages drop once connected, check on the console for a short.