One of the first things I wanted to when I got the machine was have a look at the game board and power wiring. Most of this lives behind the backglass. There is a lock on the top right of the machine. Sadly this lock came with no key, and I could not locate one inside the machine. Looking at the lock, it appeared that the previous owner had also misplaced the keys as the lock itself looked like it had been attacked with a screwdriver. Using a screwdriver myself, I undid the lock.
With the lock undone, I could then slide the actual backglass up and out to reveal the bulbs, score displays and door containing the components.
The lock itself controls a long metal rail that runs across the top of the backglass. With the lock engaged, the backglass cannot be moved. With the lock open, it removes the metal rail, allowing the backglass to be slid up and out.