Editing Atari 400 Troubleshooting Guide (section)

== Preliminary & Power-up Checks ==

Begin by confirming the '''power supply''' and system voltages before suspecting mainboard faults.

=== Power Checks ===
* Verify the external PSU outputs '''+5 V DC''' (4.85–5.15 V) under load.
* Inspect the '''power jack''' and internal fuse for corrosion or breakage.
* Confirm the power switch moves freely and makes firm contact.

{| class="wikitable styled-table"
! Test Point!!Expected Voltage!!Notes
|-
|Mainboard edge connector (pin 1 = +5 V, pin 2 = GND)||+5 V DC||Primary logic rail
|-
|Cartridge slot pin 12 (+5 V)||+5 V DC||Cartridge power
|-
|CPU (6502) pin 8 (Vcc)||+5 V DC||Core supply
|-
|RESET line (CPU pin 40)||Low → High||Should pulse low at power-on, then rise to 5 V
|}

=== Visual Inspection ===
* Remove the top cover; check for burnt components, loose wires, or corrosion—especially near the power input and voltage regulator.
* Inspect for '''bulging/leaking capacitors''' and reflow any suspect solder joints.