Editing IBM PC XT/370 Capacitor Replacement Guide

This guide documents capacitor diagnosis and replacement for the '''[[IBM PC XT/370]]'''. Because the XT/370 is mechanically an [[IBM PC XT (5160)]] chassis with three add-in 370PC cards, the capacitor topology is '''identical to a stock XT''' for chassis-level work — the canonical XT failure points (RIFA mains-suppression caps in the PSU and through-hole tantalum bypass caps on the planar) are the primary targets. The three 370PC cards each carry their own through-hole electrolytics that should be inspected separately.

== Important Caveat ==

Per-board exact capacitor parts lists for the 370PC-P, 370PC-M, and PC3277-EM cards are '''not transcribed in any publicly accessible source''' that this guide author has located. The XT/370 cards are unique IBM products and their service documentation (SA38-0037-00 chapter 6) describes FRU-level replacement only — IBM expected service engineers to replace whole cards rather than re-component them. This guide therefore documents the '''typical late-1970s / early-1980s ISA card practice''' for capacitor recap; verify printed values on each cap before ordering replacements.

== Safety Warning ==

The XT/370 uses the standard '''IBM 130 W switching PSU''' (Astec / Zenith) with mains-rectified bulk capacitors that hold a lethal charge after power-off. Before any PSU work:

# Power off and unplug the mains lead.
# Wait at least 30 seconds.
# Discharge each bulk capacitor through a 1 kΩ / 5 W resistor.
# Verify with a multimeter.

The integrated 5151 monochrome or 5153 CGA display (if a 5151-style cabinet is fitted) carries high voltage on the flyback transformer. Discharge the CRT anode to chassis ground via a high-voltage probe before any work on the deflection / flyback board.

== Chassis-Level Capacitor Work ==

Identical to the [[IBM PC XT (5160) Capacitor Replacement Guide]]:

* '''RIFA-branded X / Y class mains-suppression capacitors''' in the PSU. These are the canonical failure target on any IBM PC family system of this era — '''replace immediately as a preventive measure''' regardless of visible condition. They vent over time, producing smoke, a fishy odour, and on rare occasions a small fire.
* '''PSU bulk filter electrolytics''' — 30+ years old; replace with 105 °C low-ESR equivalents.
* '''PSU secondary-side electrolytics''' — same.
* '''Planar through-hole tantalum bypass caps''' — diode-test each in-circuit; remove any showing close to 0 Ω.

For the full XT PSU and planar cap list refer to [[IBM PC XT (5160) Capacitor Replacement Guide]].

== 370PC-P (Processor Card) Capacitors ==

The processor card carries the modified Motorola 68000 pair and the modified Intel 8087, plus glue logic. Typical capacitor types:

{| class="wikitable styled-table" style="width:100%;"
|+'''370PC-P card capacitor inspection (representative; verify in situ)'''
! Value !! Voltage !! Type !! Position
|-
| 10 µF || 16 V || Tantalum bypass || Around the 68000 packages (multiple positions)
|-
| 10 µF || 16 V || Tantalum bypass || Around the modified 8087
|-
| 47 µF || 16 V || Aluminium electrolytic || Card-edge +5 V bulk filter
|-
| 22 µF || 16 V || Tantalum bypass || Glue-logic ICs
|}

Failure mode of concern: '''tantalum short circuit'''. A shorted tantalum near the 68000 will pull down the +5 V rail and prevent the card from being detected by VM/PC. Diagnostic procedure:

# Multimeter on diode test.
# Probe each tantalum in-circuit: black to ground, red to +5 V rail.
# Good cap reads open / high resistance; shorted cap reads close to 0 Ω.
# Remove the shorted cap to confirm.
# Replace with a fresh tantalum or low-ESR ceramic of equal value, equal or higher voltage rating.

== 370PC-M (Memory Card) Capacitors ==

The 512 KB dual-ported memory card has more capacitance per board than the P card because of the DRAM array. Typical cap inventory:

{| class="wikitable styled-table" style="width:100%;"
|+'''370PC-M card capacitor inspection (representative)'''
! Value !! Voltage !! Type !! Position !! Quantity (approx)
|-
| 0.1 µF || 25 V || Ceramic decoupling || One per DRAM (32 positions for 32-chip 512 KB) || ~32
|-
| 10–47 µF || 16 V || Tantalum or aluminium electrolytic || Bulk DRAM array decoupling || 4–6
|-
| 100 µF || 16 V || Aluminium electrolytic || Card-edge +5 V bulk || 2
|-
| 22 µF || 16 V || Tantalum bypass || Glue-logic ICs || 4–6
|}

The per-DRAM ceramic caps rarely fail. The card-edge bulk caps and tantalum bypass caps are the more likely failure points after 40+ years.

== PC3277-EM Card Capacitors ==

The 3277 Emulation Adapter is the smallest of the three cards and carries the BNC twinax interface to the host controller. Typical capacitor inventory:

{| class="wikitable styled-table" style="width:100%;"
|+'''PC3277-EM card capacitor inspection (representative)'''
! Value !! Voltage !! Type !! Position
|-
| 10 µF || 16 V || Tantalum bypass || IC bypass throughout
|-
| 47 µF || 16 V || Aluminium electrolytic || Card-edge +5 V bulk
|-
| 22 µF || 16 V || Tantalum bypass || BNC interface circuit
|}

== Recommended Modern Replacements ==

Same as for any IBM PC era restoration:

* Manufacturer: Panasonic FR / FM / FC, Nichicon HE / HZ (post-2007 date codes), Rubycon ZLH / ZLJ / YXJ, United Chemi-Con KZH / KZE.
* '''Avoid''' generic Chinese-brand electrolytics.
* 105 °C rated.
* Voltage equal or higher than original.
* Capacitance equal to original.

For tantalums:

* Vishay 593D / 595D or KEMET T491 / T494 — modern high-reliability tantalum.
* OR substitute with a '''low-ESR ceramic''' (X7R or X5R) of the same value — ceramics do not fail short and are a safer long-term choice for low-current bypass positions.

== Recap Procedure ==

# Discharge the PSU bulk capacitors.
# Remove the chassis cover (5 rear screws, slide cover forward).
# '''Remove the three 370PC cards as a matched set''' — the backplane connector between 370PC-P and 370PC-M is sensitive; do not force.
# Photograph each card from both sides at high resolution.
# Mark each cap's polarity before desoldering.
# Desolder with solder wick. Limit each cycle to 5–7 seconds at no more than 350 °C.
# Fit replacements matching the silkscreen polarity.
# Solder both leads; inspect for clean fillets; trim flush.
# Reinsert cards as a matched set, ensuring the rear-edge backplane connector is fully seated.

== Post-Recap Verification ==

# Power on with the original (uncapped) PSU first to verify chassis still POSTs cleanly.
# Once XT POST passes, reseat the three 370PC cards.
# Boot PC DOS 2.10.
# Load VM/PC.
# Verify VM/PC initialises the 370PC-P card (initialisation message).
# Verify VM/PC sees the 370PC-M card's full 512 KB (initialisation message).
# Connect to host mainframe; verify CMS prompt appears.

If any test fails after recap, re-inspect the polarity of every replaced cap before suspecting another fault.

== Polarity Reference ==

[[File:IBM 5150 tantalum polarity reference.jpg|center|thumb|640px|Polarity reference for IBM motherboard and add-in card tantalum capacitors. Match the silkscreen "+" to the cap "+" side. (Image: minuszerodegrees.net)]]

== When Not to Recap ==

If the XT/370 POSTs cleanly, DOS boots, VM/PC initialises the cards, host connection succeeds and CMS runs reliably with no instability, the caps are within tolerance.

Always recap if:

* '''RIFA-branded X / Y mains-suppression cap present in the PSU''' (preventive replacement).
* Visible cap failure on the planar, any 370PC card, or the PSU.
* PSU smoke, fishy odour or audible whine.
* System unstable when warm but stable when cold.
* 370PC card fails to be detected by VM/PC and reseating did not help (suspect a shorted tantalum on the card).

== Related Pages ==

* [[IBM PC XT/370]]
* [[IBM PC XT/370 Maintenance Guide]]
* [[IBM PC XT/370 Troubleshooting Guide]]
* [[IBM PC XT (5160) Capacitor Replacement Guide]] — chassis-level 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 6.
* [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ Retro Repairs and Refurbs — 1983 IBM PC 5160 (XT) Power Supply Rebuild]. PSU rebuild reference applicable to the XT/370.
* [https://www.minuszerodegrees.net/failure/failure.htm Commonly Failing Electronic Components, minuszerodegrees.net].

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[[Category:Capacitor Replacement Guides]]