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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.
[[File:IBM XT 370 board.jpg|thumb|320px|The PC/370-P processor card of an IBM PC XT/370. (Image: Wikimedia Commons)]]


== Important Caveat ==
The '''[[IBM PC XT/370]]''' is an [[IBM PC XT (5160)|IBM PC XT]] with System/370 cards added, sold as 5160 models 568, 588 and 589.<ref name="sim">IBM, ''Personal Computer Family Service Information Manual'', SA38-0037-00, First Edition, January 1989, hosted as [[:File:IBM Personal Computer Family Service Information Manual SA38-0037-00.pdf]]. Chapter 6: 5160 model feature chart and power supply description (p. 6-2); "5160 XT/370" (pp. 6-17 to 6-19). Chapter 18: XT/370 parts (p. 18-19).</ref> The system board and power supply are the XT's, so the recapping work is on the [[IBM PC XT Capacitor Replacement Guide]]: sixteen tantalums on the board, thirteen of them on +5 V, C56 on +12 V, and 10 µF 16 V as the replacement part.<ref name="mz5v">[https://www.minuszerodegrees.net/5150_5160/MDC/tantalum_short_plus_5volts.htm Short-circuit on the +5 volt line of an IBM 5150/5155/5160 motherboard], minuszerodegrees.net.</ref><ref name="mz12v">[https://www.minuszerodegrees.net/5150_5160/MDC/tantalum_short_plus_12volts.htm Short-circuit on the +12 volt line of an IBM 5150/5155/5160 motherboard], minuszerodegrees.net.</ref><ref name="mz2leg">[https://www.minuszerodegrees.net/failure/IBM%2051xx%20motherboards%20-%202-legged%20tantalum%20capacitors.htm IBM 51xx motherboards/cards – types wired to use 2-legged tantalum capacitors], minuszerodegrees.net.</ref> The XT/370 models use the 64/256 KB system board, for which IBM's service manual gives the same memory decoding as the XT and 5155.<ref name="sim4">IBM, ''Personal Computer Family Service Information Manual'', SA38-0037-00 (1989), chapter 4, pp. 4-3 to 4-6.</ref>


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.
== The XT/370 cards ==
 
== 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%;"
{| class="wikitable styled-table" style="width:100%;"
|+'''370PC-P card capacitor inspection (representative; verify in situ)'''
|+'''XT/370 cards (IBM service manual pp. 6-17 to 6-19 and 18-19)'''
! Value !! Voltage !! Type !! Position
! Card !! IBM FRU !! Slot !! IBM's description
|-
|-
| 10 µF || 16 V || Tantalum bypass || Around the 68000 packages (multiple positions)
| PC/370-P processor card || 8654372 || 4 || "the center of the S/370 operations for the PC XT/370"; contains the microprocessors and control circuits that execute S/370 instructions
|-
|-
| 10 µF || 16 V || Tantalum bypass || Around the modified 8087
| PC/370-M memory card || 8654371 || 3 || 512 KB of parity-checked RAM, used by the PC/370-P or by the XT
|-
|-
| 47 µF || 16 V || Aluminium electrolytic || Card-edge +5 V bulk filter
| PC/370-P-M cable || 8654374 || || Joins the two cards
|-
|-
| 22 µF || 16 V || Tantalum bypass || Glue-logic ICs
| PC/3277-EM card || 8654373 || || 3277-2 emulation when linked to a host (models 568 and 588)
|}
|}
Source: IBM service manual.<ref name="sim" />


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:
IBM's service documents treat each card as a single field-replaceable unit. They give no parts list, capacitor positions or schematic for the PC/370-P, PC/370-M or PC/3277-EM.<ref name="sim" />


# Multimeter on diode test.
== Supply loading ==
# 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 ==
IBM rates the 5160 supply for continuous operation at 130 W. The system board takes about 2 to 4 A of the 15 A available on +5 V, leaving about 11 A for the cards in the slots.<ref name="tr">IBM, ''Personal Computer XT and Portable Personal Computer Technical Reference'', part 6139821, March 1986, pp. 3-3 to 3-5, hosted as [[:File:IBM PC XT 5155 5160 Technical Reference Mar86.pdf]].</ref> On an XT/370 that 11 A is shared by the full-length processor and memory cards, the emulation card, and the diskette, fixed disk and display adapters.


The 512 KB dual-ported memory card has more capacitance per board than the P card because of the DRAM array. Typical cap inventory:
The supply switches all its outputs off while a DC overcurrent or overvoltage exists, so a shorted tantalum on the system board or on any card makes the whole machine look dead.<ref name="tr" /> To find it:


{| class="wikitable styled-table" style="width:100%;"
# Start from the minimum diagnostic configuration: system board, supply and speaker only.<ref name="mzmdc">[https://www.minuszerodegrees.net/5150_5160/MDC/minimum_diag_config.htm IBM 5150/5155/5160 – Minimum Diagnostic Configuration], minuszerodegrees.net.</ref>
|+'''370PC-M card capacitor inspection (representative)'''
# If the supply still shuts down, the short is on the system board; follow [[IBM PC XT Capacitor Replacement Guide#Finding a short-circuit tantalum|the XT procedure]].
! Value !! Voltage !! Type !! Position !! Quantity (approx)
# If the board runs, refit the cards one at a time, switching off between each. Keep the PC/370-P and PC/370-M together with their cable. The card that makes the supply shut down has the short.
|-
| 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.
minuszerodegrees.net reports tantalum failures on IBM 51xx cards as well as system boards, mostly at switch-on and mostly on equipment that has not been powered for years.<ref name="mzfail">[https://www.minuszerodegrees.net/failure/failure.htm Commonly Failing Electronic Components (in vintage computers)], minuszerodegrees.net.</ref> IBM's 51xx cards use the same three-hole tantalum footprint as the system boards, wired for two or three legs; the positive leg goes in the hole marked +.<ref name="mztant">[https://www.minuszerodegrees.net/failure/IBM%2051xx%20motherboards%20-%20tantalum%20capacitors.htm Tantalum capacitors on IBM 51xx motherboards/cards], minuszerodegrees.net.</ref> Read the value and polarity off each part on the card before removing it.


== PC3277-EM Card Capacitors ==
== Voltage checks ==


The 3277 Emulation Adapter is the smallest of the three cards and carries the BNC twinax interface to the host controller. Typical capacitor inventory:
Use IBM's limits at the system board connectors, and the Technical Reference sense levels:<ref name="sim4" /><ref name="tr" />


{| class="wikitable styled-table" style="width:100%;"
{| class="wikitable styled-table" style="width:80%; text-align:center;"
|+'''PC3277-EM card capacitor inspection (representative)'''
|+'''IBM 5160 power supply limits'''
! Value !! Voltage !! Type !! Position
! Rail !! Service limits !! Sense level for power good
|-
| +5 V || +4.8 to +5.25 V || +4.5 V
|-
|-
| 10 µF || 16 V || Tantalum bypass || IC bypass throughout
| −5 V || −4.6 to −5.5 V || −4.3 V
|-
|-
| 47 µF || 16 V || Aluminium electrolytic || Card-edge +5 V bulk
| +12 V || +11.5 to +12.6 V || +10.8 V
|-
|-
| 22 µF || 16 V || Tantalum bypass || BNC interface circuit
| −12 V || −10.9 to −13.2 V || −10.2 V
|}
|}


== Recommended Modern Replacements ==
If the rails are in limits with the XT/370 cards out and fall out of limits with them in, refit the cards one at a time to find which one pulls the rail down.
 
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).
A 20 MB XT/370 has a second 130 W supply in its 5161 Expansion Unit, which needs the same checks.<ref name="sim7">IBM, ''Personal Computer Family Service Information Manual'', SA38-0037-00 (1989), chapter 7, p. 7-1.</ref>
* 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 ==
== Related pages ==


* [[IBM PC XT/370]]
* [[IBM PC XT/370]] · [[IBM PC XT/370 Maintenance Guide]] · [[IBM PC XT/370 Troubleshooting Guide]]
* [[IBM PC XT/370 Maintenance Guide]]
* [[IBM PC XT Capacitor Replacement Guide]]
* [[IBM PC XT/370 Troubleshooting Guide]]
* [[IBM PC XT (5160)]] · [[IBM Personal Computer Family Service Information Manual]]
* [[IBM PC XT (5160) Capacitor Replacement Guide]] — chassis-level cap work
* [[Capacitor Failure Symptoms]]
* [[Capacitor Failure Symptoms]]


== References ==
== 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.
<references />
* [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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Latest revision as of 20:57, 30 September 2026

The PC/370-P processor card of an IBM PC XT/370. (Image: Wikimedia Commons)

The IBM PC XT/370 is an IBM PC XT with System/370 cards added, sold as 5160 models 568, 588 and 589.[1] The system board and power supply are the XT's, so the recapping work is on the IBM PC XT Capacitor Replacement Guide: sixteen tantalums on the board, thirteen of them on +5 V, C56 on +12 V, and 10 µF 16 V as the replacement part.[2][3][4] The XT/370 models use the 64/256 KB system board, for which IBM's service manual gives the same memory decoding as the XT and 5155.[5]

The XT/370 cards

[edit | edit source]
XT/370 cards (IBM service manual pp. 6-17 to 6-19 and 18-19)
Card IBM FRU Slot IBM's description
PC/370-P processor card 8654372 4 "the center of the S/370 operations for the PC XT/370"; contains the microprocessors and control circuits that execute S/370 instructions
PC/370-M memory card 8654371 3 512 KB of parity-checked RAM, used by the PC/370-P or by the XT
PC/370-P-M cable 8654374 Joins the two cards
PC/3277-EM card 8654373 3277-2 emulation when linked to a host (models 568 and 588)

Source: IBM service manual.[1]

IBM's service documents treat each card as a single field-replaceable unit. They give no parts list, capacitor positions or schematic for the PC/370-P, PC/370-M or PC/3277-EM.[1]

Supply loading

[edit | edit source]

IBM rates the 5160 supply for continuous operation at 130 W. The system board takes about 2 to 4 A of the 15 A available on +5 V, leaving about 11 A for the cards in the slots.[6] On an XT/370 that 11 A is shared by the full-length processor and memory cards, the emulation card, and the diskette, fixed disk and display adapters.

The supply switches all its outputs off while a DC overcurrent or overvoltage exists, so a shorted tantalum on the system board or on any card makes the whole machine look dead.[6] To find it:

  1. Start from the minimum diagnostic configuration: system board, supply and speaker only.[7]
  2. If the supply still shuts down, the short is on the system board; follow the XT procedure.
  3. If the board runs, refit the cards one at a time, switching off between each. Keep the PC/370-P and PC/370-M together with their cable. The card that makes the supply shut down has the short.

minuszerodegrees.net reports tantalum failures on IBM 51xx cards as well as system boards, mostly at switch-on and mostly on equipment that has not been powered for years.[8] IBM's 51xx cards use the same three-hole tantalum footprint as the system boards, wired for two or three legs; the positive leg goes in the hole marked +.[9] Read the value and polarity off each part on the card before removing it.

Voltage checks

[edit | edit source]

Use IBM's limits at the system board connectors, and the Technical Reference sense levels:[5][6]

IBM 5160 power supply limits
Rail Service limits Sense level for power good
+5 V +4.8 to +5.25 V +4.5 V
−5 V −4.6 to −5.5 V −4.3 V
+12 V +11.5 to +12.6 V +10.8 V
−12 V −10.9 to −13.2 V −10.2 V

If the rails are in limits with the XT/370 cards out and fall out of limits with them in, refit the cards one at a time to find which one pulls the rail down.

A 20 MB XT/370 has a second 130 W supply in its 5161 Expansion Unit, which needs the same checks.[10]

[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.0 1.1 1.2 IBM, Personal Computer Family Service Information Manual, SA38-0037-00, First Edition, January 1989, hosted as File:IBM Personal Computer Family Service Information Manual SA38-0037-00.pdf. Chapter 6: 5160 model feature chart and power supply description (p. 6-2); "5160 XT/370" (pp. 6-17 to 6-19). Chapter 18: XT/370 parts (p. 18-19).
  2. ↑ Short-circuit on the +5 volt line of an IBM 5150/5155/5160 motherboard, minuszerodegrees.net.
  3. ↑ Short-circuit on the +12 volt line of an IBM 5150/5155/5160 motherboard, minuszerodegrees.net.
  4. ↑ IBM 51xx motherboards/cards – types wired to use 2-legged tantalum capacitors, minuszerodegrees.net.
  5. ↑ 5.0 5.1 IBM, Personal Computer Family Service Information Manual, SA38-0037-00 (1989), chapter 4, pp. 4-3 to 4-6.
  6. ↑ 6.0 6.1 6.2 IBM, Personal Computer XT and Portable Personal Computer Technical Reference, part 6139821, March 1986, pp. 3-3 to 3-5, hosted as File:IBM PC XT 5155 5160 Technical Reference Mar86.pdf.
  7. ↑ IBM 5150/5155/5160 – Minimum Diagnostic Configuration, minuszerodegrees.net.
  8. ↑ Commonly Failing Electronic Components (in vintage computers), minuszerodegrees.net.
  9. ↑ Tantalum capacitors on IBM 51xx motherboards/cards, minuszerodegrees.net.
  10. ↑ IBM, Personal Computer Family Service Information Manual, SA38-0037-00 (1989), chapter 7, p. 7-1.


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