Editing Atari 1200XL Troubleshooting Guide (section)

=== Clock Signal Chain ===
{| class="wikitable styled-table" style="width:90%; text-align:center;"
|+ '''Clock Signal Verification'''
|-
! Signal !! Frequency !! Source !! Test Point !! Expected Waveform
|-
| Master oscillator || 14.31818 MHz (NTSC colour burst × 4) || Y1 crystal via GTIA (U19) || U19 pin 28 (OSC) || Sine/square, 14.3 MHz
|-
| Fast Phase 0 (FØ0) || 1.79 MHz || ANTIC (U20) pin 35 || U20 pin 35 || Square wave, 1.79 MHz
|-
| Phase 0 (Ø0) || 1.79 MHz || ANTIC (U20) pin 34 || U22 pin 37 (CPU Ø0 input) || Square wave, 1.79 MHz
|-
| Phase 2 (Ø2) || 1.79 MHz || CPU (U22) pin 39 || U22 pin 39 || Square wave, 1.79 MHz
|}

'''If no clock at CPU pin 37:'''
# Check Y1 crystal for damage or cold solder joints.
# Verify GTIA (U19) has +5V on pin 27 (Vcc) and ground on pin 3 (Vss).
# Check ANTIC (U20) has +5V on pin 21 (Vcc) and is generating Ø0 at pin 34.
# If GTIA oscillator output (pin 28) is present but ANTIC Ø0 output (pin 34) is absent, suspect ANTIC (U20).
# If neither signal is present, suspect GTIA (U19) or Y1.