Editing Acorn Archimedes A4000 Capacitor Guide (section)

== Safety: mains PSU — risk of fatal shock ==

The A4000's power supply runs directly from the mains. Its primary side, the reservoir (bulk) capacitor and the mains-suppression capacitors '''can hold a lethal charge even after the machine is unplugged''', and a shock from the primary side '''can kill you'''. '''Always disconnect the mains lead before opening the case,''' and never work on the supply while it is plugged in.

Recapping the PSU is a legitimate part of restoring these machines. Acorn's original service manual told dealers to exchange the whole PSU module rather than repair it, but that advice assumed replacement modules were on the shelf; none have been made for decades, so for restoration the supply is '''recapped, not discarded'''. Only attempt PSU work if you are competent with mains switch-mode supplies. Before touching the primary side, '''discharge the reservoir capacitor through a suitable bleeder resistor and confirm 0&nbsp;V with a meter''', and treat the X/Y mains-suppression capacitors as charged until proven otherwise. After any internal work, re-apply the earth-continuity and DC-insulation (class&nbsp;1) safety tests before returning the machine to service.<ref name="sm">[[Acorn A3010, A3020 and A4000 Service Manual]], Acorn Computers &mdash; hosted on this wiki. Module-level service manual: it treats the PSU as an exchange unit, specifies the earth-continuity and DC-insulation safety tests, and gives the +5&nbsp;V/+12&nbsp;V/&minus;5&nbsp;V rails. The exchange-only guidance dates from when spare PSU modules were stocked.</ref>