https://wiki.retrotechcollection.com/index.php?action=history&feed=atom&title=IBM_3270_PC%2FGX_Maintenance_GuideIBM 3270 PC/GX Maintenance Guide - Revision history2026-07-16T23:33:28ZRevision history for this page on the wikiMediaWiki 1.45.3https://wiki.retrotechcollection.com/index.php?title=IBM_3270_PC/GX_Maintenance_Guide&diff=11081&oldid=prevJosh: 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)2026-05-23T21:32:25Z<p>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)</p>
<p><b>New page</b></p><div>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 [[IBM 3270 PC Maintenance Guide|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'''.<br />
<br />
== Safety Warning ==<br />
<br />
The 3270 PC/GX has '''three independent PSUs to discharge''' before service:<br />
<br />
# '''5371 system unit PSU''' — standard IBM 130 W switching supply.<br />
# '''5378 Display Attachment Unit''' — separate mains lead, separate internal linear PSU.<br />
# '''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.<br />
<br />
Before any work:<br />
<br />
# Power off and unplug '''every''' mains lead.<br />
# Wait at least 30 seconds.<br />
# Discharge each PSU's bulk capacitor through a 1 kΩ / 5 W resistor.<br />
# '''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.<br />
# Verify each with a multimeter.<br />
<br />
== Documentation Set ==<br />
<br />
* '''IBM GA33-3141-0''' — ''Introducing the IBM 3270 Personal Computer/G and /GX Workstations'' (May 1984). Primary reference; covers the /GX in detail.<ref>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</ref><br />
* '''IBM SA38-0037-00''' — ''Personal Computer Family Service Information Manual'' (July 1989), Chapter 10.<br />
* '''IBM 3270 PC GDDM Documentation''' — GCP / GDDM interaction.<br />
<br />
== Opening the System Unit ==<br />
<br />
Identical to the [[IBM 3270 PC/G Maintenance Guide]] — the 5371 system unit is shared between the /G and /GX:<br />
<br />
# Power off and unplug all cables.<br />
# Five Philips #2 screws on the rear panel.<br />
# Slide cover forward.<br />
<br />
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.<br />
<br />
== Opening the IBM 5378 Display Attachment Unit ==<br />
<br />
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:<br />
<br />
* '''Linear power supply''' with its own mains cord.<br />
* '''Larger vector graphics processor''' than the 5278 — drives the 5379 at 1024 × 1024 APA.<br />
* '''More frame buffer RAM''' than the 5278.<br />
* '''75-pin connector''' to the 5379.<br />
* '''Input connector''' from the 5371 system unit's Display Adapter card.<br />
<br />
Opening procedure same as the 5278 — discharge mains first, remove perimeter screws, lift cover.<br />
<br />
== Inspecting the System ==<br />
<br />
In service order:<br />
<br />
# '''Standard 3270 PC chassis inspection''' — see [[IBM 3270 PC Maintenance Guide]].<br />
# '''5378 PSU bulk caps''' — visual check; the larger PSU has correspondingly larger caps and more of them.<br />
# '''5378 vector graphics processor and frame buffer RAM''' — reseat any unsocketed parts; visual inspection of the board.<br />
# '''75-pin cable to 5379''' — clean both connectors with deoxidising contact cleaner; verify every pin is straight.<br />
# '''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).<br />
<br />
== Regular Cleaning ==<br />
<br />
* System unit cleaning identical to the [[IBM 3270 PC Maintenance Guide]].<br />
* '''5378 attachment unit''' — soft brush and low-pressure compressed air.<br />
* '''5379 19" monitor''' — exterior cleaning only; the 19" CRT is large and heavy and best left to specialist CRT technicians for internal cleaning.<br />
* '''IBM 5277 mouse''' — periodic ball and roller cleaning.<br />
<br />
== PSU Voltage Checks ==<br />
<br />
=== System Unit (5371) PSU ===<br />
<br />
Same as the 3270 PC / standard XT — see [[IBM 3270 PC Maintenance Guide]].<br />
<br />
=== 5378 PSU ===<br />
<br />
The 5378 PSU is larger than the 5278's because of the higher-resolution graphics. Rails are similar in nature but at higher current:<br />
<br />
* '''+5 V''' for the larger vector graphics processor and expanded frame buffer.<br />
* '''+12 V''' for support logic.<br />
* High-current output drive rail for the 75-pin connector to the 5379.<br />
<br />
Refer to the 5378's internal service label for exact rail tolerances.<br />
<br />
=== 5379 CRT ===<br />
<br />
19" CRT anode voltage is typically '''22–25 kV''' — higher than the 14" 5279. Measure only with an HV probe rated for ≥30 kV.<br />
<br />
== Phosphor Burn-In on the 5379 ==<br />
<br />
The 19" 5379 is particularly susceptible to phosphor burn-in because:<br />
<br />
* CAD applications often display the same outline / wireframe for hours at a time.<br />
* High-luminance phosphor regions wear faster.<br />
* A burned-in 5379 cannot be field-repaired — replacement requires a donor CRT, and 19" donor CRTs are very scarce.<br />
<br />
When operating a 5379, '''always enable any screensaver or "blank after N minutes" feature available''' in the host GCP or the mainframe application.<br />
<br />
== Keyboard and Mouse ==<br />
<br />
Same as the [[IBM 3270 PC/G Maintenance Guide]] — 122-key 5271 Keyboard Element + IBM 5277 3-button mouse.<br />
<br />
== Recommended Tools ==<br />
<br />
* Philips #2 and T15 / T20 Torx screwdrivers.<br />
* Anti-static strap.<br />
* Digital multimeter with HV-rated probe (≥30 kV rating recommended for 5379 work).<br />
* High-voltage probe for CRT discharge.<br />
* IPA + foam swabs.<br />
* Soldering iron (fine tip) + solder wick.<br />
* Deoxidising contact cleaner — particularly for the 5378-to-5379 75-pin cable.<br />
* IBM keycap removal tool.<br />
* GA33-3141-0 PDF.<br />
* SA38-0037-00 PDF (chapter 10).<br />
<br />
== Related Pages ==<br />
<br />
* [[IBM 3270 PC/GX]]<br />
* [[IBM 3270 PC/GX Troubleshooting Guide]]<br />
* [[IBM 3270 PC/GX Capacitor Replacement Guide]]<br />
* [[IBM 3270 PC/G Maintenance Guide]] — smaller-display sibling<br />
* [[IBM 3270 PC Maintenance Guide]] — shared chassis procedures<br />
* [[Recommended Tools]]<br />
<br />
== References ==<br />
<br />
* [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)].<br />
* [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.<br />
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