IBM 3270 PC/GX Troubleshooting Guide - Revision history

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Josh: Expand troubleshooting: RIFA/tantalum + Dallas/planar battery (161/163) + POST codes; cited

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 # If GCP fails to load, verify version matches /GX configuration.
 # If 5379 shows convergence drift or burn-in, suspect CRT end-of-life; donor required.
 == Related Pages ==

Josh: Deep technical XT/370 + 3270 PC family page with verified sources (IBM SA38-0037-00 Service Info Manual, IBM GA33-3141-0, IBM 1502336, Kozuh/Livingston/Spillman IBM Systems Journal 1984, seasip.info, Wikipedia)

2026-05-23T21:32:27Z

Deep technical XT/370 + 3270 PC family page with verified sources (IBM SA38-0037-00 Service Info Manual, IBM GA33-3141-0, IBM 1502336, Kozuh/Livingston/Spillman IBM Systems Journal 1984, seasip.info, Wikipedia)

New page

This guide documents fault diagnosis for the '''[[IBM 3270 PC/GX]]''' (machine type 5371). POST is identical to the [[IBM 3270 PC]] and [[IBM 3270 PC/G]] — standard XT 1xx–19xx codes plus 28xx (3278/79 Emulation Adapter) and 32xx (Display Adapter / APA / PSS). PC/GX-specific failures manifest in the '''IBM 5378''' Display Attachment Unit, the '''IBM 5379''' 19" Color Display, or the Graphics Control Program (GCP) layer.

== 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).

== Initial Diagnosis Workflow ==

A working PC/GX boots:

# Standard IBM PC XT POST.
# Keyboard Adapter exposes the Display Adapter's video BIOS.
# Standard 3270 PC Display Adapter output is routed through the 5378 to the 5379.
# IBM PC DOS 2.0 / 2.1 boots.
# Graphics Control Program (GCP) loads.
# GCP initialises the 5378.
# Mainframe sessions render on the 5379 at 1024 × 1024 APA resolution.

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

== POST Codes ==

Identical to the [[IBM 3270 PC Troubleshooting Guide]]:

* '''1xx–19xx''' — standard XT chassis.
* '''28xx''' — 3278/79 Emulation Adapter.
* '''3201–3250''' — Display Adapter.
* '''3261–3279''' — PSS card.
* '''3280–3289''' — APA card.

The 5378 is not POST-tested (it operates downstream of the Display Adapter); 5378 faults present as '''GCP errors''' or '''no display on the 5379''' rather than POST codes.

== 5378 Attachment Unit Diagnosis ==

The 5378 is larger and more complex than the /G's 5278, and is the most likely PC/GX-specific failure point:

* '''No display on 5379 but POST passes''' — 75-pin cable between 5378 and 5379 disconnected or with pin damage; or 5378 internal PSU dead.
* '''5378 PSU dead''' — 5378 has its own mains lead and internal PSU. Verify the mains lead is connected, the rear-panel power switch is on, and the 5378's status LEDs (if any) are illuminated.
* '''CGA-compatible mode works but 1024 × 1024 APA does not''' — the 5378's high-resolution vector graphics processor or extended frame buffer has failed; CGA-compatible mode comes through the Display Adapter directly.
* '''Mainframe graphics rendered correctly but very slowly''' — 5378's local processor may be running in fallback mode (e.g. host-side rendering instead of local pan / zoom).

== 5379 19" Display Diagnosis ==

* '''No raster on 5379''' — flyback failure (high anode voltage means flyback insulation is the most common end-of-life mechanism on 19" 5379s); horizontal output transistor failure; PSU dead.
* '''Raster but no characters / graphics''' — video signal from 5378 not arriving; check 75-pin cable.
* '''Visible phosphor burn-in''' — display has been used for sustained periods on static graphics. No field repair; replacement requires a 5379 donor.
* '''Convergence drift''' (colour misregistration) — adjustment via the 5379 service controls; if unfixable, replacement.
* '''Geometry distortion''' (pincushion / barrel) — deflection coil or yoke issue.

== Graphics Control Program (GCP) Diagnosis ==

Same as the [[IBM 3270 PC/G Troubleshooting Guide]] — GCP runs on PC DOS, loads after standard 3270 PC Control Program init, and manages the 5378's local processor via the system unit's Display Adapter card.

GCP-specific failures on the /GX:

* '''GCP loads but reports "Display Attachment Unit not found"''' — 5378 not detected. Verify the cable between the 5371 and 5378 (different connector from the 75-pin cable to the 5379).
* '''GDDM application crashes when sending high-resolution graphics''' — 5378 frame buffer issue; reseat the frame buffer RAM modules in the 5378.
* '''Mouse not tracked correctly''' — IBM 5277 cable issue or GCP misconfiguration.

== Common Field Symptoms ==

* '''POST passes but no 5379 display''' — 5378 PSU or 75-pin cable.
* '''POST passes, DOS boots, GCP fails''' — version mismatch (GCP for /GX is a different image from GCP for /G).
* '''Graphics work in DOS but not in mainframe sessions''' — 5378 vector graphics processor failure.
* '''Random reboots when warm''' — PSU caps aged (any of the three PSUs: 5371, 5378, 5379); recap.
* '''Smell of fish from any unit''' — RIFA X2 mains-suppression cap venting; power off immediately.
* '''Severe phosphor burn-in on 5379''' — CRT end-of-life.
* '''Display geometry distortion appears gradually''' — deflection-board electrolytic ageing; recap the 5379 deflection board.

== Diagnostic Workflow Summary ==

# Power on; observe POST.
# If POST fails, treat as standard 3270 PC chassis fault.
# If POST passes but no 5379 display, check 5378 (PSU, cable to 5379, cable from 5371).
# If 5379 shows DOS text but no high-resolution graphics, suspect 5378 vector graphics processor or expanded frame buffer.
# If GCP fails to load, verify version matches /GX configuration.
# If 5379 shows convergence drift or burn-in, suspect CRT end-of-life; donor required.

== Related Pages ==

* [[IBM 3270 PC/GX]]
* [[IBM 3270 PC/GX Maintenance Guide]]
* [[IBM 3270 PC/GX Capacitor Replacement Guide]]
* [[IBM 3270 PC/G Troubleshooting Guide]] — smaller-display sibling, very similar workflow
* [[IBM 3270 PC Troubleshooting Guide]] — chassis-level codes

== References ==

* [http://bitsavers.org/pdf/ibm/pc/SA38-0037-00_Personal_Computer_Family_Service_Information_Manual_Jul89.pdf IBM SA38-0037-00 (July 1989)], Chapter 10.
* [https://www.bitsavers.org/pdf/ibm/pc/3270/3270_PC/GA33-3141-0_Introducing_the_IBM_3270_Personal_Computer_G_and_GX_Workstations_198405.pdf IBM GA33-3141-0 (May 1984)].

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