Editing Atari 130XE Troubleshooting Guide (section)

=== "Black Screen" (No Boot) Flowchart ===
# Confirm +5V at mainboard and at multiple IC Vcc pins.
# Check for a brief click or beep at power-on (indicates CPU is executing code).
# Verify clock signal — probe 6502C pin 39 (Φ0 output) with oscilloscope; expect ~1.79 MHz (NTSC) or ~1.77 MHz (PAL).
# Verify RESET line — 6502C pin 40 should pulse LOW at power-on then go HIGH (+5V). If stuck LOW, check reset circuit (R/C network).
# If clock present but CPU not running: swap/test FREDDIE (CO61991) — it generates the master clock to the CPU.
# Reseat or substitute OS ROM (CO61598B). A blank or corrupted ROM will prevent any boot.
# Test base 64K RAM (first bank) — see RAM Faults section.
# Check MMU (CO61618) — address decoding failures prevent all chip selects.