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)

2026-05-23T21:32:20Z

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 capacitor diagnosis and replacement for the '''[[IBM 3270 PC/G]]''' (machine type 5371, July 1984). The PC/G's chassis is the 5371 system unit (XT-derived) and its capacitor topology is therefore '''identical to the [[IBM 3270 PC]]''' for system-unit work. PC/G-specific cap work covers the '''IBM 5278 Display Attachment Unit''' (separate enclosure with its own PSU) and the '''IBM 5279 14" Color Display''' deflection board.

== Important Caveat ==

Per-board exact cap parts lists for the 5371 system unit, the 5278 attachment unit, and the 5279 monitor are '''not transcribed in any publicly accessible source'''. This guide therefore documents the '''typical early-1980s practice''' for capacitor recap on each subsystem; verify printed values in situ before ordering replacements.

== Safety Warning ==

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

# '''5371 system unit PSU''' — standard IBM 130 W switching supply. Mains-rectified bulk caps hold lethal charge.
# '''5278 Display Attachment Unit''' — separate mains lead, separate internal linear PSU. Independent discharge required.
# '''5279 Color Display''' — separate mains lead, CRT anode at 12–15 kV.

Before any work:

# Power off and unplug '''every''' mains lead.
# Wait 30 seconds.
# Discharge each PSU's bulk caps through a 1 kΩ / 5 W resistor.
# '''Discharge the 5279 CRT anode to chassis ground via a high-voltage probe.'''
# Verify each PSU and the CRT anode with a multimeter.

== System Unit (5371) Capacitors ==

Identical to the [[IBM 3270 PC Capacitor Replacement Guide]]:

* '''RIFA-branded X / Y mains-suppression capacitors''' in the 130 W IBM PSU — '''replace immediately as a preventive measure'''. They vent over time, producing smoke / fish odour.
* '''Standard XT PSU bulk filter and secondary electrolytics''' — 30+ years old.
* '''Planar through-hole tantalum bypass caps''' — diode-test each in-circuit.
* '''Per-card decoupling''' on Keyboard Adapter, Display Adapter, APA, PSS, 3278/79 Emulation Adapter — see [[IBM 3270 PC Capacitor Replacement Guide]].

== IBM 5278 Display Attachment Unit Capacitors ==

The 5278 is the rarest and most failure-prone component of the PC/G after 40 years. Its internal PSU plus vector graphics circuit carry substantial capacitance.

=== 5278 Internal PSU ===

Linear PSU topology similar to the [[IBM 5120]] PSU. Representative cap values:

{| class="wikitable styled-table" style="width:100%;"
|+'''5278 PSU capacitor inspection (representative)'''
! Value !! Voltage !! Type !! Position
|-
| 2200–4700 µF || 16 V || Low-ESR aluminium electrolytic, 105 °C || +5 V bulk
|-
| 1000–2200 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
|-
| 47 µF || 50 V || Aluminium electrolytic, 105 °C || Primary startup
|-
| 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'''
|}

=== 5278 Vector Graphics Board ===

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

== IBM 5279 14" Color Display Capacitors ==

The 5279 has its own internal PSU and deflection / flyback board. Treat as a standard early-1980s 14" colour CRT:

* '''PSU primary side''' — X2 mains suppression cap (replace if RIFA), 220 µF / 200 V primary bulk.
* '''PSU secondary''' — 2200 µF / 25 V rail bulk, 470 µF / 35 V +12 V smoothing, 100 µF / 16 V feedback / bypass.
* '''Deflection board''' — 1–10 µF oscillator bypass, 22–47 µF / 35 V vertical driver, 100–470 µF / 35 V deflection rail bulk, 0.01–0.1 µF / 1.6–2 kV HV ceramic snubber on the flyback collector.

The 5279 phosphor is prone to '''burn-in''' from static graphics images displayed too long. A burned-in 5279 requires a donor CRT — no field repair is practical.

== 5277 Mouse Capacitors ==

The IBM 5277 mouse has no electrolytic capacitors of concern — just small bypass ceramics on the encoder logic. No recap required.

== Recommended Modern Replacements ==

Same guidance as for the [[IBM 3270 PC 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, 5278, 5279) and the 5279 CRT anode.
# '''Service each subsystem separately''' — do not try to do system unit, attachment unit, and display in a single bench session unless you have ample space and labelled workspace zones.
# For each unit, photograph the board(s) from both sides at high resolution before any cap removal.
# 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 ==

Verify each subsystem independently before reconnecting them:

# '''5371 system unit''' — bench-test with a known-good PSU dummy load; verify all rails.
# '''5278 attachment unit''' — bench-test PSU rails; verify the 75-pin output is responsive to test signals.
# '''5279 monitor''' — bench-test PSU rails; verify HV at the anode; verify raster appears.

Then reconnect everything and verify GCP loads cleanly, mainframe sessions appear, and graphics modes render correctly on the 5279.

== When Not to Recap ==

If the PC/G chassis POSTs cleanly, the 5278 produces valid video output, the 5279 displays cleanly, GCP loads, mainframe graphics render correctly, 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 5278, the 5279, or any of the three PSUs.
* PSU smoke, fishy odour, or audible whine from any unit.
* '''5278 output garbled or absent''' — particularly common cause is the 5278's PSU caps.
* System unstable when warm but stable when cold.

== Related Pages ==

* [[IBM 3270 PC/G]]
* [[IBM 3270 PC/G Maintenance Guide]]
* [[IBM 3270 PC/G Troubleshooting Guide]]
* [[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/G 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)