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IBM PC XT/370 Maintenance Guide

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This guide documents preventive maintenance for the IBM PC XT/370 (1983) โ€” IBM's hardware-emulation S/370 workstation built on the IBM PC XT (5160) chassis with three large 370PC emulation cards. The chassis-level maintenance is identical to the standard XT; XT/370-specific maintenance is centred on the three emulation cards (370PC-P, 370PC-M, PC3277-EM) and on the VM/PC operating system.

Safety Warning

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The XT/370 PSU is the standard IBM 130 W switching supply used in the 5160 XT, containing mains-rectified bulk capacitors that hold a lethal charge after power-off. Before any work inside the chassis:

  1. Power off and unplug the mains lead.
  2. Wait at least 30 seconds.
  3. Discharge each PSU bulk filter capacitor through a 1 kฮฉ / 5 W resistor.
  4. Verify with a multimeter.

Display: standard CRT (IBM 5151 monochrome or 5153 CGA) โ€” discharge the CRT anode separately before any work on the deflection / flyback board.

Documentation Set

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  • IBM SA38-0037-00 โ€” Personal Computer Family Service Information Manual (July 1989). The single most useful service document; Chapter 6 specifically covers the XT/370. Available on Bitsavers.[1]
  • IBM 6137739 โ€” Virtual Machine/Personal Computer User's Guide (December 1984).
  • Kozuh, F. P., Livingston, B., Spillman, R. J. โ€” "System/370 capability in a desktop computer," IBM Systems Journal 23(3):245, 1984. Engineering reference.
  • Standard IBM PC XT documents โ€” SY27-2522 (XT Technical Reference, 1986) โ€” same chassis service procedures apply.

Opening the System Unit

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The XT/370 chassis opens the same way as a standard IBM PC XT (5160):

  1. Power off and unplug all cables (mains, keyboard, video, printer, 3270 host-connect coax).
  2. Five screws on the rear panel (Philips #2).
  3. Slide the cover forward to release.

Inside, the three 370PC cards occupy three contiguous full-length 8-bit ISA slots. The remaining slots typically carry:

  • Standard IBM XT Floppy / Hard Disk Adapter (in the slot reserved for it).
  • IBM Monochrome Display & Printer Adapter or IBM CGA card.
  • Optional Game Adapter, Async Adapter, etc.

The Three 370PC Cards

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The cards must be removed and re-inserted as a matched set. They communicate over a proprietary card-to-card backplane connector on the rear edge of the cards, in addition to the standard ISA bus.

370PC-P (Processor Card)

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  • Carries the two modified Motorola 68000s (Micro/370) and the modified Intel 8087.
  • Heatsink-mounted on the 68000 packages โ€” verify thermal paste integrity at each service interval.
  • Card cannot be repaired if a Micro/370 or modified 8087 fails โ€” these are custom IBM/Motorola/Intel parts.

370PC-M (Memory Card)

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  • Carries 512 KB dual-ported DRAM.
  • Communicates with the 370PC-P via the dedicated rear-edge connector.
  • Adds 384 KB to the XT host's 256 KB to give 640 KB total host RAM.
  • DRAMs on the M card are standard 64K ร— 1 DRAM packages of the era; failed DRAMs are replaceable if the failure is identified to a specific chip.

PC3277-EM (3270 Emulation Adapter)

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  • Earlier-production XT/370s carry the dedicated PC3277-EM card.
  • Later-production XT/370s replaced it with the standard 3278/79 Emulation Adapter (same card as on the IBM 3270 PC).[2]
  • Connects to the IBM 3174 or 3274 cluster controller via BNC twinax.

Card-Backplane Connector

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The rear-edge connector between the 370PC-P and 370PC-M cards is a known service item:

  • Clean the connector with deoxidising contact cleaner at every service interval.
  • If the system fails to boot VM/PC but the XT host POSTs normally, reseating the 370PC-P / 370PC-M backplane connector is the first diagnostic step.
  • The connector is mechanically delicate โ€” handle with care.

PSU Voltage Checks

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Standard IBM PC XT PSU (130 W switching). Rail tolerances:

IBM PC XT PSU rail tolerances
Rail Acceptable range
+5 V +4.75 V to +5.25 V
+12 V +11.4 V to +12.6 V
−5 V −4.75 V to −5.25 V
−12 V −11.4 V to −12.6 V

The XT/370 with all three cards installed plus a 10 MB ST-412 drive draws close to the PSU's 130 W capacity. Rail sag under load indicates aged PSU electrolytics โ€” see IBM PC XT/370 Capacitor Replacement Guide.

Floppy and Hard Drive Maintenance

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The XT/370 uses the standard XT 5.25" 360 KB DSDD floppy and 10 MB ST-412 hard drive. Maintenance is identical to the IBM PC XT (5160) Maintenance Guide:

  • Clean the floppy head with IPA on a foam swab.
  • Replace the drive belt if intermittent.
  • The 10 MB ST-412 drive is end-of-life on most surviving units; data migration to a modern XT-IDE / CompactFlash adapter is the practical restoration path.

VM/PC Maintenance

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  • VM/PC system diskettes are increasingly rare. If you have a working VM/PC installation, image the system files immediately.
  • CMS minidisks live as DOS files in the format USER.NNN (user name + minidisk number). They are accessible from DOS once VM/PC has been booted at least once and a minidisk has been formatted.
  • EXPORT / IMPORT commands transfer between CMS and PC DOS with automatic EBCDICโ†”ASCII conversion. Use these to move files to/from a modern host before any extended use.
  • VM/PC version 2 (November 1985) added page-cache support โ€” upgrade if you have v1 only.

Keyboard

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Standard IBM PC XT 83-key Model F keyboard. Maintenance is identical to the IBM PC XT (5160) Maintenance Guide โ€” buckling-spring mechanism, IBM tool for keycap removal.

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  • Philips #2 screwdriver, T15 / T20 Torx.
  • Anti-static strap.
  • Digital multimeter.
  • IPA + foam swabs.
  • Soldering iron (fine tip) + solder wick.
  • Deoxidising contact cleaner for the card-to-card backplane connector.
  • Spare CR2032 (where the planar uses a battery-backed Dallas RTC; not all XT/370 planars do).
  • SA38-0037-00 PDF.
  • CMS Reference Manual (mainframe-CMS-compatible).
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References

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  1. โ†‘ http://bitsavers.org/pdf/ibm/pc/SA38-0037-00_Personal_Computer_Family_Service_Information_Manual_Jul89.pdf
  2. โ†‘ SA38-0037-00 ยง6-17.
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