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.