IBM 3270 PC/G Troubleshooting Guide

This guide documents fault diagnosis for the IBM 3270 PC/G (machine type 5371). POST inherits the standard IBM PC XT 1xx–19xx codes plus the 28xx and 32xx codes from the IBM 3270 PC; PC/G-specific failures present in the IBM 5278 Display Attachment Unit, the IBM 5279 14" Color Display, or in the Graphics Control Program (GCP) layer above the standard 3270 PC Control Program.

Reference Documents

• IBM GA33-3141-0 — Introducing the IBM 3270 Personal Computer/G and /GX Workstations (May 1984).
• IBM SA38-0037-00 — Personal Computer Family Service Information Manual (July 1989), Chapter 10.
• IBM 1502336 — PC 3278/79 Emulation Adapter Technical Reference (October 1983). 28xx code reference.
• HelpPC diagnostic codes — 28xx and 32xx ranges.

Initial Diagnosis Workflow

A working PC/G boots:

1. Standard IBM PC XT POST (no error, single short beep).
2. Keyboard Adapter exposes the Display Adapter's video BIOS.
3. Standard 3270 PC Display Adapter video appears on the 5279 (via the 5278 attachment unit and 75-pin cable).
4. IBM PC DOS 2.0 / 2.1 boots.
5. Graphics Control Program (GCP) loads.
6. GCP initialises the 5278 attachment unit.
7. Mainframe sessions + DOS session + notepads appear in a windowed display with graphics-capable mainframe sessions.

If any of these does not happen, stop and diagnose at that stage.

POST Codes

Same code ranges as the IBM 3270 PC Troubleshooting Guide:

• 1xx–19xx — standard XT chassis (planar, memory, keyboard, floppy, hard disk).
• 28xx — 3278/79 Emulation Adapter.
• 3201–3250 — Display Adapter.
• 3261–3279 — PSS card (where fitted).
• 3280–3289 — APA card.

The PC/G's hardware adds the 5278 attachment unit, which is not detected by POST (it operates downstream of the Display Adapter's output). 5278 faults manifest as GCP initialisation errors or garbled graphics on the 5279 rather than as POST codes.

5278 Attachment Unit Diagnosis

The 5278 is the most distinctive (and most failure-prone after 40 years) component on a PC/G. Common faults:

• No display on 5279 but POST passes — 75-pin cable between 5278 and 5279 disconnected, miscabled, or with pin damage. Reseat both ends; inspect every pin under a magnifier.
• 5278 PSU dead — 5278 has its own mains lead and internal PSU. Verify the mains lead is connected and the rear-panel power switch is on.
• CGA-compatible 320 × 200 / 640 × 200 mode works but 720 × 512 APA mode does not — the 5278's vector graphics processor or frame buffer has failed; the CGA-compatible mode comes through the Display Adapter directly without using the 5278's active graphics circuit.
• Mainframe graphics output garbled but text correct — vector graphics data path within the 5278 is faulty; rerouting the same input through a 3270 PC (no /G hardware) shows correct text but no graphics — characteristic of a 5278 fault.

5279 Display Diagnosis

• No raster on 5279 — flyback or horizontal output transistor failure. Check for HV at the anode (HV probe only).
• Raster but no characters / graphics — video signal not arriving from the 5278; check the 75-pin cable.
• Dim raster — cathode emission loss; CRT end-of-life.
• Graphics smearing / colour separation — RGB DAC or driver failure.

Graphics Control Program (GCP) Diagnosis

GCP is the PC/G's software layer above the standard Control Program. Common GCP-specific failures:

• GCP will not load — verify the GCP version matches the system's installed Display Adapter / APA card configuration.
• GCP loads but 5278 not detected — 5278's signalling to the system unit Display Adapter has failed; check the cable between the 5371 and the 5278.
• Local pan / zoom not working — GCP's interaction with the 5278's local processor has failed; reseat the Display Adapter card in the 5371.
• GDDM applications display but don't accept input — IBM 5277 mouse cable / connector issue; or GCP's input handlers misconfigured.

Mainframe Connection Diagnosis

Same as the IBM 3270 PC Troubleshooting Guide — BNC twinax to the 3174 / 3274 cluster controller. GDDM release 4+ must be configured on the mainframe to send graphics protocol data to the PC/G workstation.

Common Field Symptoms

• POST passes but 5279 shows no signal — 5278 attachment unit dead or 75-pin cable disconnected.
• POST 3201 — Display Adapter card failure (same as 3270 PC).
• POST passes, DOS boots, but GCP fails — GCP version / Display Adapter combination mismatch.
• Graphics work in DOS CGA mode but not in mainframe GDDM sessions — 5278 vector graphics processor failure.
• Random reboots when warm — PSU caps aged (system unit PSU + 5278 PSU); recap.
• Smell of fish from 5371 or 5278 — RIFA X2 mains-suppression cap venting; power off immediately.

Diagnostic Workflow Summary

1. Power on; observe POST.
2. If POST fails, treat as standard 3270 PC chassis fault (see IBM 3270 PC Troubleshooting Guide).
3. If POST passes but no 5279 display, check the 5278 (PSU, cable to 5279) before assuming a card fault.
4. If 5279 shows DOS text but no graphics, suspect 5278's vector graphics processor.
5. If GCP fails to load, verify version and card configuration.
6. If graphics output is garbled, suspect 5278 frame buffer RAM or vector processor.

⚠️ Power-supply RIFA capacitor and tantalum shorts

Two age-related failures are near-universal on this era of IBM hardware:

• RIFA mains-filter capacitors in the power supply are metallised-paper parts that crack and fail short with age, producing acrid smoke shortly after power-on. Replace them pre-emptively with modern X2-class parts.
• Tantalum capacitors on the planar (system board) and on ISA cards fail short with age. A shorted tantalum will prevent the power supply from starting (dead machine, PSU protection latched) — look for a cracked or discoloured tantalum and lift suspect ones to find the short.

IBM PC/XT switching supplies also need a minimum load to start, so a bare supply on the bench may not run without a dummy load.

⚠️ CMOS / RTC battery

This machine keeps its configuration in battery-backed CMOS, and the battery is a common failure. On AT-class boards the clock/CMOS is often a Dallas DS1287/DS12887 module with the cell sealed inside; it lasts about ten years and then dies, giving 161 / 163 CMOS and clock errors at POST (and sometimes spurious floppy-drive errors). PS/2 planars use a rechargeable barrel or pack battery that leaks and corrodes the board. Replace a dead Dallas module (or rework it with an external coin cell), and on a leaking planar battery remove it and clean the corrosion before it eats the traces.

POST beep and error codes

The IBM Power-On Self Test signals faults by beeps and, where a display works, by a numeric code:

IBM 3270 PC/G. Source: Wikimedia Commons.

IBM POST beep codes

Beeps	Meaning
1 short	Normal POST — system OK
2 short	POST error (numeric code shown on screen)
No beep	Power supply or system-board fault
Continuous / repeating short	Power supply or system board
1 long, 1 short	System-board fault
1 long, 2 short	Display-adapter fault (MDA/CGA)
1 long, 3 short	Display-adapter fault (EGA/later)

Common numeric codes include 161/163 (dead CMOS battery/clock), 201 (memory), 301 (keyboard) and 1701 (hard disc). A code ending in the family prefix identifies the failing subsystem.

Related Pages

• IBM 3270 PC/G
• IBM 3270 PC/G Maintenance Guide
• IBM 3270 PC/G Capacitor Replacement Guide
• IBM 3270 PC Troubleshooting Guide — chassis-level POST codes
• IBM 3270 PC/GX — sibling with similar diagnostics

References

• IBM SA38-0037-00 (July 1989), Chapter 10.
• IBM GA33-3141-0 (May 1984).