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IBM PC XT/370 Capacitor Replacement Guide

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IBM XT 370 board
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

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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 manual treats each card as a single field-replaceable unit and gives no parts list, capacitor positions or schematic for the PC/370-P, PC/370-M or PC/3277-EM.[1]

Supply loading

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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

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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]

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References

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  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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