VGA Controller – Timing
Horizontal timing on the left half of the board: A 25MHZ crystal clocks a 12bit ripple counter, counter output goes to 3 separate 4-to-16 decoders, a collection of AND gates and inverters switch a pair of flip-flops on and off at the right clock counts to produce the horizontal sync.
Vertical timing on the right half of the board, almost identical to the horizontal timing, just clocked by the horizontal sync rather than the crystal.
To test the timing a grid is drawn by turning on the green output, controlled by the vertical and horizontal count decoders. The (approximately) correct voltage is achieved by using a resistor and diode so that the green output voltage is one diode drop.
- Pump brake pedal until it goes hard
- Remove spring clip from bottom of linkage pin and remove pin
- Remove split-pin and spring
- Syphon out brake fluid from master cylinder reservoir
- Disconnect brake lines from master cylinder
- Disconnect hydraulic pipes from servo
- Remove 4 bolts holding servo in place (loose nuts on back)
- Remove servo & master cylinder
- Place servo in a vice and remove the two bolts holding the master cylinder to the servo