IBM 3270 PC/G Capacitor Replacement Guide

This guide documents capacitor diagnosis and replacement for the IBM 3270 PC/G (machine type 5371, July 1984). The PC/G's chassis is the 5371 system unit (XT-derived) and its capacitor topology is therefore identical to the IBM 3270 PC for system-unit work. PC/G-specific cap work covers the IBM 5278 Display Attachment Unit (separate enclosure with its own PSU) and the IBM 5279 14" Color Display deflection board.

Important Caveat

Per-board exact cap parts lists for the 5371 system unit, the 5278 attachment unit, and the 5279 monitor are not transcribed in any publicly accessible source. This guide therefore documents the typical early-1980s practice for capacitor recap on each subsystem; verify printed values in situ before ordering replacements.

Safety Warning

The 3270 PC/G has three independent PSUs to discharge before service:

5371 system unit PSU — standard IBM 130 W switching supply. Mains-rectified bulk caps hold lethal charge.
5278 Display Attachment Unit — separate mains lead, separate internal linear PSU. Independent discharge required.
5279 Color Display — separate mains lead, CRT anode at 12–15 kV.

Before any work:

Power off and unplug every mains lead.
Wait 30 seconds.
Discharge each PSU's bulk caps through a 1 kΩ / 5 W resistor.
Discharge the 5279 CRT anode to chassis ground via a high-voltage probe.
Verify each PSU and the CRT anode with a multimeter.

System Unit (5371) Capacitors

Identical to the IBM 3270 PC Capacitor Replacement Guide:

RIFA-branded X / Y mains-suppression capacitors in the 130 W IBM PSU — replace immediately as a preventive measure. They vent over time, producing smoke / fish odour.
Standard XT PSU bulk filter and secondary electrolytics — 30+ years old.
Planar through-hole tantalum bypass caps — diode-test each in-circuit.
Per-card decoupling on Keyboard Adapter, Display Adapter, APA, PSS, 3278/79 Emulation Adapter — see IBM 3270 PC Capacitor Replacement Guide.

IBM 5278 Display Attachment Unit Capacitors

The 5278 is the rarest and most failure-prone component of the PC/G after 40 years. Its internal PSU plus vector graphics circuit carry substantial capacitance.

5278 Internal PSU

Linear PSU topology similar to the IBM 5120 PSU. Representative cap values:

**5278 PSU capacitor inspection (representative)**
Value	Voltage	Type	Position
2200–4700 µF	16 V	Low-ESR aluminium electrolytic, 105 °C	+5 V bulk
1000–2200 µF	35 V	Aluminium electrolytic, 105 °C	+12 V bulk
220 µF	35 V	Aluminium electrolytic, 105 °C	−12 V bulk
100 µF	50 V	Aluminium electrolytic, 105 °C	Primary auxiliary
47 µF	50 V	Aluminium electrolytic, 105 °C	Primary startup
220 µF	200 V	Aluminium electrolytic, 105 °C	Primary bulk (lethal-charge)
0.1 µF	275 VAC	X2 class	Mains suppression — replace if RIFA-branded

5278 Vector Graphics Board

**5278 vector graphics board capacitor inspection (representative)**
Value	Voltage	Type	Position
10 µF	16 V	Tantalum bypass	IC bypass throughout
22 µF	16 V	Tantalum bypass	Frame buffer RAM bypass
47 µF	16 V	Aluminium electrolytic	Local power filter
100 µF	16 V	Aluminium electrolytic	Output drive stage filter (75-pin output)

IBM 5279 14" Color Display Capacitors

The 5279 has its own internal PSU and deflection / flyback board. Treat as a standard early-1980s 14" colour CRT:

PSU primary side — X2 mains suppression cap (replace if RIFA), 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.

The 5279 phosphor is prone to burn-in from static graphics images displayed too long. A burned-in 5279 requires a donor CRT — no field repair is practical.

5277 Mouse Capacitors

The IBM 5277 mouse has no electrolytic capacitors of concern — just small bypass ceramics on the encoder logic. No recap required.

Recommended Modern Replacements

Same guidance as for the IBM 3270 PC Capacitor Replacement Guide: Panasonic FR/FM/FC, Nichicon HE/HZ (post-2007 date codes), Rubycon ZLH/ZLJ/YXJ, United Chemi-Con KZH/KZE; 105 °C; equal capacitance, equal-or-higher voltage; verify lead spacing.

Recap Procedure

Discharge all three PSUs (5371, 5278, 5279) and the 5279 CRT anode.
Service each subsystem separately — do not try to do system unit, attachment unit, and display in a single bench session unless you have ample space and labelled workspace zones.
For each unit, photograph the board(s) from both sides at high resolution before any cap removal.
Mark each cap's polarity before desoldering.
Desolder with solder wick at no more than 350 °C, 5–7 seconds per cycle.
Fit replacements matching silkscreen polarity.
Solder both leads; inspect for clean fillets; trim flush.

Post-Recap Verification

Verify each subsystem independently before reconnecting them:

5371 system unit — bench-test with a known-good PSU dummy load; verify all rails.
5278 attachment unit — bench-test PSU rails; verify the 75-pin output is responsive to test signals.
5279 monitor — bench-test PSU rails; verify HV at the anode; verify raster appears.

Then reconnect everything and verify GCP loads cleanly, mainframe sessions appear, and graphics modes render correctly on the 5279.

When Not to Recap

If the PC/G chassis POSTs cleanly, the 5278 produces valid video output, the 5279 displays cleanly, GCP loads, mainframe graphics render correctly, and the system runs reliably, the caps are within tolerance.

Always recap if:

RIFA-branded X / Y mains-suppression cap present in any of the three PSUs (preventive replacement).
Visible cap failure on the 5371 planar, any card, the 5278, the 5279, or any of the three PSUs.
PSU smoke, fishy odour, or audible whine from any unit.
5278 output garbled or absent — particularly common cause is the 5278's PSU caps.
System unstable when warm but stable when cold.

Related Pages

IBM 3270 PC/G
IBM 3270 PC/G Maintenance Guide
IBM 3270 PC/G Troubleshooting Guide
IBM 3270 PC Capacitor Replacement Guide — shared system-unit cap work
Capacitor Failure Symptoms

References