You can find a number of problems related to this device at the Powerbook 520 troubleshooting guide.
Identification and Background
Released in May 1994, the Powerbook 520 features a 9.5" gray-scale display, light-touch keyboard, and a multi-use trackpad. Released along with models 520c, 540, and 550, the Powerbook 520 was the base model of the newly released series. Although the bargain model, it offered 25 MHz of power. The main difference between the Powerbook 520 and 520c was that the 520c included a color display.