Editing Atari 400 Capacitor Replacement Guide (section)

== What the rails actually are ==

Atari is explicit about this, and it matters because it tells you what each capacitor is working against:

<blockquote>"The Power Supply PCB receives 9 VAC from an external power adaptor (transformer) and provides +5 Vdc, +12 Vdc, and −5 Vdc for the Console system."<ref name="fsm2">Atari, Inc., ''ATARI Home Computer Field Service Manual 400/800'', June 1982, page 1-6, "Power Supply". Source of the 9 V AC input figure and the four-rail breakdown.</ref></blockquote>

{| class="wikitable styled-table" style="width:80%;"
|+ Atari 400 supply rails
! Rail !! Purpose
|-
| +5 V A || Supply voltage for the logic PCBs
|-
| +5 V B || Specially filtered supply for the video circuitry
|-
| +12 V || Supply for the dynamic RAM chips (and SIO pin 12)
|-
| −5 V || Supply for the dynamic RAM chips
|}

The 16K RAM card is populated with eight DRAMs at Z505–Z512 that need all three of +12 V, +5 V and −5 V; that is why the 400 has rails an XL-series machine does not.<ref name="fsm" />

The regulators are '''A202 (5 V)''' and '''A201 (12 V)''' on the power supply PCB.<ref name="fsm" /> Console5 identifies these as a 7805 and a 7812 respectively.<ref name="c5" />