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Recap guide

Recap the IBM PC, XT and AT

The classic fixes are RIFA line-filter caps and shorted tantalum beads, on the board and on ISA cards, rather than a blanket recap.

Open the interactive checklist Tick off each capacitor as you replace it. It saves in your browser.

Worth a recap. Reports say it is a common and useful fix on this machine.

Which board do I have?

Boards changed during production. These notes come from the linked sources, and many are only partly documented, so check the board in front of you.

  • IBM XT supply, Astec AA12151 (IBM 1501438) (Medium confidence). How to tell: Astec 130W supply, IBM part number on the label. Capacitors: Original RIFA metallised paper mains filter caps are the problem part; output side electrolytics were also replaced. No value list in the write-up. Capacitor list differs from other revisions: not known. Source
  • XT 64-256KB system board (Medium confidence). How to tell: Board described as the 64-256KB system board, one dated 3 Feb 1984. Capacitors: All tantalum beads were replaced plus a 5-30pF trimmer at C1; counts and values not itemised. Capacitor list differs from other revisions: not known. Source

Where to start

Clues from write-ups and forum threads, not a diagnosis. Low confidence means one thread or one repair; high means several write-ups agree.

  • Power supply (Medium confidence). A 5160 power supply rebuild warns about RIFA caps and output electrolytics. It ends by swapping in an ATX board and gives no cap list. Source
  • Shorting tantalum beads (Medium confidence). One XT restoration replaced all the tantalums on the motherboard and ISA cards, and a forum thread discusses electrolytic or polymer substitutes. Source
  • 5150 supply (Low confidence). A 5150 revival thread found power supply cap failures and advises replacing all similar caps at once. Source

Tips, tales and maybes

Found in write-ups and forum threads. These are other people's reports, not tested by us. Check part references and values against your own board.

  • Watch out: Rifa mains caps before first power-up (Medium confidence). A restoration write-up says the XT supply has metallised-paper mains filter caps that tend to fail short and smoke, so they should be replaced before powering on. The author instead swapped in a modern TFX supply and cautions to keep the -5V rail if your ISA cards need it. Source
  • Watch out: XT with 2V on +12V: the tantalums were loading it (Medium confidence). A dead XT 5160 with +12V at 2V turned out to have tantalum caps shorting the rail. A reply notes old PC supplies want at least 1A on +5V to give true readings. The board used 3-pin tantalums with the two outer pins tied, so orientation is forgiving, and posters call recapping the +/-12V and +/-5V tantalums standard. Source
  • Story: Tantalums blow near ISA slots too (Low confidence). In a general VOGONS thread a poster blew a tantalum on an XT again, this time in an unusual spot next to an ISA slot, and said they usually fail near the power connector or floppy area. Another planned to hunt for replacements after his XT blew up. Source
  • Tip: Replace all tantalums on a card that fireballs (Low confidence). After a ET4000 VLB card's C13 tantalum went with a small fireball, one poster said to replace all the tantalums on the board, with 10uF 16V parts, not just the one that blew. Others debated whether electrolytics are a fair substitute because inductance and leakage differ. Source
  • Maybe: A failed tantalum can stop shorting (Low confidence). On an AOpen ISA board a posters explained that once a shorted tantalum has fully failed it can stop shorting, so the board works but with poorer smoothing; the owner fitted a 10uF 16V electrolytic. This is one person's explanation. Source

Before you start

  • Boards changed during production. Check your board number against the guides before you order parts.

Guides we used

This page and the interactive bench are built from these. They have the photos and the full detail, so please read them and support their authors.

The capacitor list

We have not checked a capacitor list for this machine yet, so none is shown. Follow the guide above for your board, then add the capacitors in the interactive checklist and it builds a shopping list for you.

Skip the sourcing

While you are in there

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Steps, tools and safety

The steps, the tools in top, middle and budget picks, and the safety list are the same for every machine, so they live on one page: How to recap, step by step.

Values are shown as published in the guides above, and boards changed during production. Check the board in front of you. A recap involves hot tools and, on some machines, dangerous voltages. You do it at your own risk.

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