IBM 3270 PC/GX Maintenance Guide

This guide documents preventive maintenance for the IBM 3270 PC/GX (machine type 5371, July 1984 extended-graphics variant of the IBM 3270 PC). Chassis-level maintenance is identical to the 3270 PC; PC/GX-specific maintenance covers the IBM 5378 Display Attachment Unit (larger external graphics-converter box than the /G's 5278) and the IBM 5379 19" Color Display.

Safety Warning

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

5371 system unit PSU — standard IBM 130 W switching supply.
5378 Display Attachment Unit — separate mains lead, separate internal linear PSU.
5379 19" Color Display — separate mains lead. The 5379 19" CRT carries particularly high anode voltage (typically 22–25 kV — higher than 14" tubes) and stores significant energy.

Before any work:

Power off and unplug every mains lead.
Wait at least 30 seconds.
Discharge each PSU's bulk capacitor through a 1 kΩ / 5 W resistor.
Discharge the 5379 CRT anode to chassis ground via a high-voltage probe. Repeat the discharge if the system has been off for a long time — large 19" tubes can recover residual charge.
Verify each with a multimeter.

Documentation Set

IBM GA33-3141-0 — Introducing the IBM 3270 Personal Computer/G and /GX Workstations (May 1984). Primary reference; covers the /GX in detail.^[1]
IBM SA38-0037-00 — Personal Computer Family Service Information Manual (July 1989), Chapter 10.
IBM 3270 PC GDDM Documentation — GCP / GDDM interaction.

Opening the System Unit

Identical to the IBM 3270 PC/G Maintenance Guide — the 5371 system unit is shared between the /G and /GX:

Power off and unplug all cables.
Five Philips #2 screws on the rear panel.
Slide cover forward.

The card complement inside is the same as the IBM 3270 PC (Keyboard Adapter, 3270 PC Display Adapter, APA, PSS, 3278/79 Emulation Adapter). The 5378 attachment box is external.

Opening the IBM 5378 Display Attachment Unit

The 5378 is larger than the /G's 5278 because it must drive the higher 1024 × 1024 resolution at the same refresh rates. Internal layout:

Linear power supply with its own mains cord.
Larger vector graphics processor than the 5278 — drives the 5379 at 1024 × 1024 APA.
More frame buffer RAM than the 5278.
75-pin connector to the 5379.
Input connector from the 5371 system unit's Display Adapter card.

Opening procedure same as the 5278 — discharge mains first, remove perimeter screws, lift cover.

Inspecting the System

In service order:

Standard 3270 PC chassis inspection — see IBM 3270 PC Maintenance Guide.
5378 PSU bulk caps — visual check; the larger PSU has correspondingly larger caps and more of them.
5378 vector graphics processor and frame buffer RAM — reseat any unsocketed parts; visual inspection of the board.
75-pin cable to 5379 — clean both connectors with deoxidising contact cleaner; verify every pin is straight.
5379 19" CRT — inspect flyback (high anode voltage means flyback insulation degradation is more catastrophic than on smaller tubes); check for phosphor burn-in (CAD applications often display the same outline for hours, which burns the phosphor).

Regular Cleaning

System unit cleaning identical to the IBM 3270 PC Maintenance Guide.
5378 attachment unit — soft brush and low-pressure compressed air.
5379 19" monitor — exterior cleaning only; the 19" CRT is large and heavy and best left to specialist CRT technicians for internal cleaning.
IBM 5277 mouse — periodic ball and roller cleaning.

PSU Voltage Checks

System Unit (5371) PSU

Same as the 3270 PC / standard XT — see IBM 3270 PC Maintenance Guide.

5378 PSU

The 5378 PSU is larger than the 5278's because of the higher-resolution graphics. Rails are similar in nature but at higher current:

+5 V for the larger vector graphics processor and expanded frame buffer.
+12 V for support logic.
High-current output drive rail for the 75-pin connector to the 5379.

Refer to the 5378's internal service label for exact rail tolerances.

5379 CRT

19" CRT anode voltage is typically 22–25 kV — higher than the 14" 5279. Measure only with an HV probe rated for ≥30 kV.

Phosphor Burn-In on the 5379

The 19" 5379 is particularly susceptible to phosphor burn-in because:

CAD applications often display the same outline / wireframe for hours at a time.
High-luminance phosphor regions wear faster.
A burned-in 5379 cannot be field-repaired — replacement requires a donor CRT, and 19" donor CRTs are very scarce.

When operating a 5379, always enable any screensaver or "blank after N minutes" feature available in the host GCP or the mainframe application.

Keyboard and Mouse

Same as the IBM 3270 PC/G Maintenance Guide — 122-key 5271 Keyboard Element + IBM 5277 3-button mouse.

Recommended Tools

Philips #2 and T15 / T20 Torx screwdrivers.
Anti-static strap.
Digital multimeter with HV-rated probe (≥30 kV rating recommended for 5379 work).
High-voltage probe for CRT discharge.
IPA + foam swabs.
Soldering iron (fine tip) + solder wick.
Deoxidising contact cleaner — particularly for the 5378-to-5379 75-pin cable.
IBM keycap removal tool.
GA33-3141-0 PDF.
SA38-0037-00 PDF (chapter 10).