Ken Shirriff wrote a great article describing the repair process of the vintage IBM 1401 mainframe computer:
The problem started when the machine was powered up at the same time someone shut down the main power, apparently causing some sort of destructive power transient. The computer’s core memory completely stopped working, making the computer unusable. To fix this we had to delve into the depths of the computer’s core memory circuitry and the power supplies.
See the full post on his blog.