The ZX81 has the older RAM chip (just one!) that required an extra -5V bias voltage. A failure mode is that either the non-switch-mode power brick and/or the i

The ZX81 has the older RAM chip (just one!) that required an extra -5V bias voltage. A failure mode is that either the non-switch-mode power brick and/or the internal voltage regulator can fail and deliver 9V or more into this pin, with very unhappy results.