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IBM 3270 PC Capacitor Replacement Guide

This guide documents capacitor diagnosis and replacement for the IBM 3270 PC (machine type 5271). The 3270 PC chassis is mechanically an IBM PC XT (5160), so chassis-level capacitor work is identical to the standard XT — the RIFA mains-suppression caps and through-hole tantalum bypass caps are the primary failure targets. The 3270-specific cards (Keyboard Adapter, 3270 PC Display Adapter, APA, PSS, 3278/79 Emulation Adapter) each carry their own electrolytic and tantalum decoupling that should be inspected separately. The IBM 5272 colour display and IBM 3295 plasma monitor carry significant additional capacitance on their own boards.

Important Caveat

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Per-card exact capacitor parts lists for the 3270 PC's specialist cards are not transcribed in any publicly accessible source. The cards' service documentation describes FRU-level replacement only — IBM expected service engineers to swap whole cards rather than re-component them. This guide therefore documents the typical early-1980s ISA card practice for capacitor recap; verify printed values on each cap before ordering replacements.

Safety Warning

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The 3270 PC uses the standard IBM 130 W switching PSU (Astec / Zenith) with mains-rectified bulk capacitors that hold a lethal charge after power-off. Before any PSU work:

  1. Power off and unplug the mains lead.
  2. Wait at least 30 seconds.
  3. Discharge each bulk capacitor through a 1 kΩ / 5 W resistor.
  4. Verify with a multimeter.

The IBM 5272 Color Display and IBM 3295 Plasma Monitor both carry lethal high voltage on their internal PSUs and high-voltage circuits. Discharge the CRT anode to chassis ground via a high-voltage probe on the 5272 before any work on its deflection / flyback board. The 3295 plasma uses a high-voltage AC drive for the plasma panel which is similarly dangerous.

Chassis-Level Capacitor Work

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Identical to the IBM PC XT (5160) Capacitor Replacement Guide:

  • RIFA-branded X / Y class mains-suppression capacitors in the PSU. The canonical failure target on any IBM PC family system of this era — replace immediately as a preventive measure regardless of visible condition. They vent over time, producing smoke, a fishy odour, and on rare occasions a small fire.
  • PSU bulk filter electrolytics — 30+ years old; replace with 105 °C low-ESR equivalents.
  • PSU secondary-side electrolytics — same.
  • Planar through-hole tantalum bypass caps — diode-test each in-circuit; remove any showing close to 0 Ω.

For the full XT PSU and planar cap list refer to IBM PC XT (5160) Capacitor Replacement Guide.

A specific 3270 PC failure that seasip.info documents is "a blown capacitor took out my 5271 motherboard" — the 3270 PC's full-load PSU operation makes cap failures more aggressive than on a lightly-loaded stock XT.

Keyboard Adapter Card

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Small full-length 8-bit ISA card carrying:

  • The keyboard scan-code translation logic (8048 microcontroller or equivalent).
  • The video BIOS ROM for the 3270 PC Display Adapter — ROM 6323581 (simple variant) or ROM 6323582 (complicated variant with NMI + serial port).
  • Decoupling caps on each major IC.
Keyboard Adapter card capacitor inspection (representative)
Value Voltage Type Position
10 µF 16 V Tantalum bypass IC bypass on each major chip
47 µF 16 V Aluminium electrolytic Card-edge +5 V bulk
22 µF 16 V Tantalum bypass ROM socket bypass

3270 PC Display Adapter

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The largest of the 3270 PC's specialist cards. Carries:

  • SCN2672 PVTC (Programmable Video Timing Controller) at I/O 0180h–019Bh.
  • 8 KB framebuffer (4 KB 3270 layer + 4 KB CGA/MDA layer).
  • Video DAC for RGB output to the 5272.
3270 PC Display Adapter capacitor inspection (representative)
Value Voltage Type Position
10 µF 16 V Tantalum bypass SCN2672 bypass × 2
22 µF 16 V Tantalum bypass Framebuffer RAM array bypass × 2
47 µF 16 V Aluminium electrolytic Card-edge +5 V bulk
100 µF 16 V Aluminium electrolytic Video output stage bulk
0.1 µF 25 V Ceramic decoupling Per-IC throughout (rarely fails)

APA / PSS Cards

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Optional cards adding CGA-compatible graphics modes and RAM-loaded fonts respectively. Both carry similar decoupling patterns:

APA / PSS card capacitor inspection (representative)
Value Voltage Type Position
10 µF 16 V Tantalum bypass IC bypass throughout
22 µF 16 V Tantalum bypass Memory bypass
47 µF 16 V Aluminium electrolytic Card-edge +5 V bulk

3278/79 Emulation Adapter

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Card-edge interface to the IBM 3174 / 3274 cluster controller via BNC twinax. Carries the 4 KB (or larger, on revised cards) buffer plus the BNC interface circuit:

3278/79 Emulation Adapter capacitor inspection (representative)
Value Voltage Type Position
10 µF 16 V Tantalum bypass IC bypass throughout
22 µF 16 V Tantalum bypass Buffer RAM bypass
47 µF 16 V Aluminium electrolytic Card-edge +5 V bulk
100 µF 16 V Aluminium electrolytic BNC interface power filter

IBM 5272 Color Display

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The 5272 has its own internal PSU and deflection board:

  • PSU primary side: X2 mains suppression cap (potentially RIFA — replace if RIFA-branded), 220 µF / 200 V primary bulk.
  • PSU secondary: 2200 µF / 25 V rail bulk, 470 µF / 35 V +12 V smoothing, 100 µF / 16 V feedback / bypass.
  • Deflection board: 1–10 µF oscillator bypass, 22–47 µF / 35 V vertical driver, 100–470 µF / 35 V deflection rail bulk, 0.01–0.1 µF / 1.6–2 kV HV ceramic snubber on the flyback collector.

IBM 3295 Plasma Monitor

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The 3295 plasma display drives a plasma panel via a high-voltage AC signal — its PSU is markedly different from the 5272:

  • Plasma drive board — high-voltage AC drive to the panel, typically several hundred volts. The HV drive board carries large bulk filter caps and AC coupling caps. Do not work on the 3295 plasma drive board unless you have specific HV experience.
  • Logic board — standard +5 V / +12 V logic capacitors similar to a CRT display.
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Same guidance as for any IBM PC era restoration — see IBM PC XT (5160) Capacitor Replacement Guide for the full brand/grade list.

Recap Procedure

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  1. Discharge the PSU bulk capacitors.
  2. Discharge the 5272 CRT anode (or the 3295 plasma drive board) if working on a display.
  3. Remove the chassis cover (5 rear screws, slide cover forward).
  4. Remove each card individually and photograph both sides at high resolution before any cap work.
  5. Mark each cap's polarity before desoldering.
  6. Desolder with solder wick at no more than 350 °C, 5–7 seconds per cycle.
  7. Fit replacements matching the silkscreen polarity.
  8. Solder both leads; inspect for clean fillets; trim flush.

Post-Recap Verification

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  1. Verify XT chassis POSTs cleanly with no cards installed.
  2. Re-insert the keyboard adapter card; verify the Display Adapter video BIOS is exposed.
  3. Re-insert the Display Adapter; verify video appears.
  4. Re-insert each remaining card one at a time, running POST after each.
  5. Boot the Control Program; verify all configured sessions initialise.

If any test fails after recap, re-inspect the polarity of every replaced cap.

When Not to Recap

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If the 3270 PC POSTs cleanly, the keyboard adapter card properly exposes the Display Adapter's video BIOS, all cards initialise without 28xx or 32xx errors, the Control Program loads, and the system runs reliably under sustained mainframe-session load, the caps are within tolerance.

Always recap if:

  • RIFA-branded X / Y mains-suppression cap present in the PSU (preventive replacement).
  • Visible cap failure on the planar, any card, the PSU, the 5272 display, or the 3295 plasma monitor.
  • PSU smoke, fishy odour or audible whine.
  • A card has been observed to fail intermittently — community-reported as the most common 3270 PC fault.
  • System unstable when warm but stable when cold.
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References

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