IBM 3270 PC/G Maintenance Guide

This guide documents preventive maintenance for the IBM 3270 PC/G (machine type 5371, July 1984 graphics variant of the IBM 3270 PC). Chassis-level maintenance is identical to the 3270 PC; PC/G-specific maintenance covers the IBM 5278 Display Attachment Unit (the external graphics-converter box) and the IBM 5279 14" Color Display.

Safety Warning

The 3270 PC/G uses the standard IBM 130 W switching PSU (same as the 5160 XT). The IBM 5278 attachment unit has its own internal PSU. The IBM 5279 CRT carries lethal high voltage on its flyback. Before any work:

1. Power off and unplug the mains lead(s) — both the system unit and the 5278.
2. Wait at least 30 seconds.
3. Discharge each PSU bulk capacitor through a 1 kΩ / 5 W resistor.
4. Discharge the 5279 CRT anode to chassis ground via a high-voltage probe before any work on the deflection board.
5. Verify both with a multimeter.

Documentation Set

• IBM GA33-3141-0 — Introducing the IBM 3270 Personal Computer/G and /GX Workstations (May 1984). Primary reference.
• IBM SA38-0037-00 — Personal Computer Family Service Information Manual (July 1989), Chapter 10. Covers 5271, 5371, and the AT-based 5373 in detail.
• IBM 3270 PC GDDM Documentation — describes the GCP / GDDM interaction.

Opening the System Unit

The 5371 system unit opens the same way as the 3270 PC's 5271:

1. Power off and unplug all cables (mains, keyboard, 75-pin cable to 5278, BNC twinax to controller).
2. Five Philips #2 screws on the rear panel.
3. Slide cover forward.

Inside, the card complement is the same as a standard IBM 3270 PC (Keyboard Adapter, 3270 PC Display Adapter, APA, PSS, 3278/79 Emulation Adapter). The PC/G's graphics enhancement is external — the 5278 attachment box does the active vector graphics work, driven by data over the 75-pin cable from the system unit's Display Adapter.

Opening the IBM 5278 Display Attachment Unit

The 5278 is a separate enclosure roughly the size of a small lunchbox. It contains:

• Its own linear power supply (separate mains cord).
• Vector graphics processor driving the 5279 monitor.
• Frame buffer RAM.
• 75-pin connector to the 5279.
• Input connector from the 5371's Display Adapter.

To open the 5278:

1. Power off and unplug both the 5278's mains lead and the 75-pin cable to the 5279.
2. Wait 30 seconds and discharge the 5278's bulk caps.
3. Remove screws around the perimeter of the case.
4. Lift the cover.

Inspecting the System

In service order:

1. Standard 3270 PC chassis inspection — see IBM 3270 PC Maintenance Guide. RIFA caps in the IBM PSU are the canonical preventive replacement target.
2. 5278 PSU bulk caps — visual check for bulging, leaks.
3. 5278 vector graphics processor — reseat if unsocketed; visual check for damaged traces near the 75-pin connector.
4. 75-pin cable between 5278 and 5279 — high pin count means many potential intermittent contacts. Clean both connectors with deoxidising contact cleaner.
5. 5279 CRT — inspect flyback for cracked insulation; check for phosphor burn-in (static graphics images displayed too long).

Regular Cleaning

• System unit cleaning is identical to a standard 3270 PC.
• 5278 attachment unit — soft brush and low-pressure compressed air; do not blow air over any leaking electrolyte.
• 5279 monitor — exterior cleaning only unless you have HV experience.
• IBM 5277 mouse — open the bottom plate, remove the ball, wipe the rollers clean of grime build-up.

PSU Voltage Checks

System Unit (5371) PSU

Same as the IBM 3270 PC and standard XT — see IBM 3270 PC Maintenance Guide for the rail table.

5278 PSU

The 5278 has its own internal PSU. Verify rails at the appropriate test points inside the 5278 chassis. The 5278 needs:

• +5 V for the vector graphics processor and frame buffer RAM.
• +12 V for support logic.
• Possibly a higher rail for the 75-pin output drive (manufacturer-specific).

Refer to the 5278's internal service label for exact rail values.

5277 Mouse

The IBM 5277 is a 3-button mouse, originally retailing at $340. Maintenance:

• Periodically clean the ball and roller mechanism.
• The 5277 is fairly robust mechanically; replacement parts are not widely available so handle with care.
• The mouse cable can develop intermittent contact near the strain relief on the mouse side.

75-Pin Cable to 5279

The 75-pin connector between the 5278 and 5279 is the most distinctive (and most problematic) interconnect in the PC/G system. Maintenance:

• Clean both connector ends with deoxidising contact cleaner.
• Verify each pin is straight (use a magnifier).
• The cable is a proprietary IBM assembly; replacement is difficult — verify pinout against IBM service documentation if you must build a replacement.

GCP System Diskettes

The Graphics Control Program is installed from diskettes (or pre-installed on the Model 16's hard drive). Image the GCP diskettes immediately if you have an original set — they are increasingly rare and the installation procedure required mainframe sysprog assistance in period.

Keyboard

The PC/G uses the same 122-key 5271 Keyboard Element as the 3270 PC, plus the IBM 5277 mouse. Keyboard maintenance is identical to the IBM 3270 PC Maintenance Guide.

Recommended Tools

• Philips #2 and T15 / T20 Torx screwdrivers.
• Anti-static strap.
• Digital multimeter with HV-rated probe.
• High-voltage probe for CRT discharge.
• IPA + foam swabs.
• Soldering iron (fine tip) + solder wick.
• Deoxidising contact cleaner — particularly for the 75-pin cable to the 5279.
• IBM keycap removal tool.
• GA33-3141-0 PDF.
• SA38-0037-00 PDF (chapter 10).

Related Pages

• IBM 3270 PC/G
• IBM 3270 PC/G Troubleshooting Guide
• IBM 3270 PC/G Capacitor Replacement Guide
• IBM 3270 PC Maintenance Guide — shared chassis procedures
• IBM 3270 PC/GX — sibling extended-graphics workstation
• Recommended Tools

References

• IBM GA33-3141-0 (May 1984).
• IBM SA38-0037-00 (July 1989), Chapter 10.
• IBM GDDM Documentation — 3270 PC/G and 3270 PC/GX Workstations.