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)

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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)

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This guide documents capacitor diagnosis and replacement for the '''[[IBM 3270 PC/GX]]''' (machine type 5371). System-unit cap work is identical to the [[IBM 3270 PC Capacitor Replacement Guide|3270 PC]]; PC/GX-specific cap work covers the '''IBM 5378 Display Attachment Unit''' (larger and more capable than the /G's 5278) and the '''IBM 5379 19" Color Display''' (higher anode voltage and more capacitance than the /G's 14" 5279).

== Important Caveat ==

Per-board exact cap parts lists for the 5371 system unit, the 5378 attachment unit, and the 5379 monitor are '''not transcribed in any publicly accessible source'''. This guide documents typical early-1980s practice; verify printed values before ordering replacements.

== Safety Warning ==

The PC/GX has '''three independent PSUs and a particularly high-voltage CRT''' 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. Larger than the /G's 5278.
# '''5379 Color Display''' — separate mains lead. '''19" CRT anode voltage is typically 22–25 kV''' — higher than the /G's 14" 5279, and stores significantly more energy.

Before any work:

# Power off and unplug '''every''' mains lead.
# Wait 30 seconds (longer for the 5379 — large 19" tubes recover residual charge for several minutes).
# Discharge each PSU's bulk caps.
# '''Discharge the 5379 CRT anode to chassis ground via a high-voltage probe rated for at least 30 kV.''' Repeat the discharge after a few minutes' wait.
# Verify each PSU and the CRT anode with a multimeter.

== System Unit (5371) Capacitors ==

Identical to the [[IBM 3270 PC Capacitor Replacement Guide]] and the [[IBM 3270 PC/G Capacitor Replacement Guide|3270 PC/G]] system unit. The 5371 is the same machine type for both /G and /GX.

* '''RIFA-branded X / Y mains-suppression capacitors''' in the 130 W IBM PSU — preventive replacement.
* Standard XT PSU bulk filter and secondary electrolytics.
* Planar through-hole tantalum bypass caps.
* Per-card decoupling on Keyboard Adapter, Display Adapter, APA, PSS, 3278/79 Emulation Adapter.

== IBM 5378 Display Attachment Unit Capacitors ==

The 5378 is the rarest single component on a PC/GX after 40 years, and its internal PSU plus higher-resolution vector graphics circuit carry substantial capacitance.

=== 5378 Internal PSU ===

Larger than the 5278's PSU. Representative cap values:

{| class="wikitable styled-table" style="width:100%;"
|+'''5378 PSU capacitor inspection (representative)'''
! Value !! Voltage !! Type !! Position
|-
| 4700–10000 µF || 16 V || Low-ESR aluminium electrolytic, 105 °C || +5 V bulk × 2 (higher current than 5278)
|-
| 2200–4700 µF || 35 V || Aluminium electrolytic, 105 °C || +12 V bulk
|-
| 220 µF || 35 V || Aluminium electrolytic, 105 °C || −12 V bulk
|-
| 100 µF || 50 V || Aluminium electrolytic, 105 °C || Primary auxiliary × 2
|-
| 47 µF || 50 V || Aluminium electrolytic, 105 °C || Primary startup
|-
| 470 µF || 200 V || Aluminium electrolytic, 105 °C || Primary bulk (lethal-charge)
|-
| 0.1 µF || 275 VAC || X2 class || Mains suppression — '''replace if RIFA-branded'''
|}

=== 5378 Vector Graphics Board ===

The 5378's vector graphics board is larger than the 5278's because of the higher resolution and expanded frame buffer. Representative caps:

{| class="wikitable styled-table" style="width:100%;"
|+'''5378 vector graphics board capacitor inspection (representative)'''
! Value !! Voltage !! Type !! Position
|-
| 10 µF || 16 V || Tantalum bypass || IC bypass throughout (more ICs than the 5278)
|-
| 22 µF || 16 V || Tantalum bypass || Frame buffer RAM bypass × multiple
|-
| 47 µF || 16 V || Aluminium electrolytic || Local +5 V power filter
|-
| 100 µF || 16 V || Aluminium electrolytic || Output drive stage filter (75-pin output)
|-
| 220 µF || 16 V || Aluminium electrolytic || High-current output stage filter
|}

== IBM 5379 19" Color Display Capacitors ==

The 5379 has a larger internal PSU and a more powerful deflection board than the 5279, reflecting the 19" tube's higher anode voltage and larger deflection requirements:

=== 5379 Internal PSU ===

{| class="wikitable styled-table" style="width:100%;"
|+'''5379 19" CRT PSU capacitor inspection (representative)'''
! Value !! Voltage !! Type !! Position
|-
| 4700 µF || 25 V || Low-ESR aluminium electrolytic, 105 °C || +12 V bulk smoothing
|-
| 2200 µF || 16 V || Low-ESR aluminium electrolytic, 105 °C || +5 V bulk smoothing
|-
| 470 µF || 35 V || Aluminium electrolytic, 105 °C || +12 V auxiliary
|-
| 220 µF || 200 V || Aluminium electrolytic, 105 °C || Primary bulk (lethal-charge)
|-
| 0.1 µF || 275 VAC || X2 class || Mains suppression — '''replace if RIFA-branded'''
|}

=== 5379 Deflection / Flyback Board ===

{| class="wikitable styled-table" style="width:100%;"
|+'''5379 deflection / flyback board capacitor inspection (representative)'''
! Value !! Voltage !! Type !! Position
|-
| 1–10 µF || 25 V || Aluminium electrolytic || Vertical / horizontal oscillator bypass
|-
| 22–100 µF || 35 V || Aluminium electrolytic || Vertical deflection driver (larger than 5279 because 19" tube needs more deflection current)
|-
| 220–680 µF || 35 V || Aluminium electrolytic || Deflection rail bulk × 2
|-
| 0.01–0.1 µF || 2 kV || Ceramic disc (HV) || Snubber on flyback collector
|-
| 0.0027–0.01 µF || 2 kV || Polypropylene HV || Horizontal deflection tuning
|}

The 19" 5379 phosphor is particularly susceptible to '''burn-in''' from sustained CAD / GDDM use — a burned-in tube cannot be field-repaired, only replaced with a donor.

== Recommended Modern Replacements ==

Same guidance as the [[IBM 3270 PC Capacitor Replacement Guide]] and [[IBM 3270 PC/G Capacitor Replacement Guide]]: Panasonic FR/FM/FC, Nichicon HE/HZ (post-2007 date codes), Rubycon ZLH/ZLJ/YXJ, United Chemi-Con KZH/KZE; 105 °C; equal capacitance, equal-or-higher voltage; verify lead spacing.

== Recap Procedure ==

# Discharge all three PSUs (5371, 5378, 5379) plus the 5379 CRT anode.
# Service each subsystem '''separately''' on different bench days if possible — the PC/GX has more capacitor count than any other PC family system on the wiki.
# Photograph each board from both sides at high resolution.
# Mark each cap's polarity before desoldering.
# Desolder with solder wick at no more than 350 °C, 5–7 seconds per cycle.
# Fit replacements matching silkscreen polarity.
# Solder both leads; inspect for clean fillets; trim flush.

== Post-Recap Verification ==

# '''5371 system unit''' — verify all rails on the bench under a 2 A resistive load.
# '''5378 attachment unit''' — verify all rails; verify the 75-pin output is responsive.
# '''5379 19" monitor''' — verify all rails; verify HV at the anode (with HV probe); verify a clean raster appears; check geometry on a known-good test pattern.
# Reassemble; boot GCP; verify mainframe graphics render at 1024 × 1024 on the 5379.

== When Not to Recap ==

If the PC/GX chassis POSTs cleanly, the 5378 produces valid 1024 × 1024 output, the 5379 displays without geometry distortion or visible burn-in, GCP loads, mainframe GDDM graphics render cleanly, and the system runs reliably, the caps are within tolerance.

Always recap if:

* '''RIFA-branded X / Y mains-suppression cap''' present in any of the three PSUs (preventive replacement).
* Visible cap failure on the 5371 planar, any card, the 5378, the 5379, or any of the three PSUs.
* PSU smoke, fishy odour or audible whine from any unit.
* '''5378 output garbled or absent''' at 1024 × 1024 — most commonly the 5378's high-current PSU caps.
* '''5379 geometry distortion or shifted raster''' — deflection-board electrolytic ageing.
* System unstable when warm but stable when cold.

== Related Pages ==

* [[IBM 3270 PC/GX]]
* [[IBM 3270 PC/GX Maintenance Guide]]
* [[IBM 3270 PC/GX Troubleshooting Guide]]
* [[IBM 3270 PC/G Capacitor Replacement Guide]] — smaller-display sibling
* [[IBM 3270 PC Capacitor Replacement Guide]] — shared system-unit cap work
* [[Capacitor Failure Symptoms]]

== 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)].
* [https://www.minuszerodegrees.net/failure/failure.htm Commonly Failing Electronic Components, minuszerodegrees.net].

{{Navbox-IBMComputers|state=collapsed}}

[[Category:IBM]]
[[Category:Capacitor Replacement Guides]]

IBM 3270 PC/GX Capacitor Replacement Guide - Revision history

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)