Editing IBM PC XT/370 Maintenance Guide

This guide documents preventive maintenance for the '''[[IBM PC XT/370]]''' (1983) — 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 ==

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:

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

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

== Documentation Set ==

* '''IBM SA38-0037-00''' — ''Personal Computer Family Service Information Manual'' (July 1989). The single most useful service document; Chapter 6 specifically covers the XT/370. Available on Bitsavers.<ref>http://bitsavers.org/pdf/ibm/pc/SA38-0037-00_Personal_Computer_Family_Service_Information_Manual_Jul89.pdf</ref>
* '''IBM 6137739''' — ''Virtual Machine/Personal Computer User's Guide'' (December 1984).
* '''Kozuh, F. P., Livingston, B., Spillman, R. J.''' — "System/370 capability in a desktop computer," ''IBM Systems Journal'' 23(3):245, 1984. Engineering reference.
* '''Standard IBM PC XT documents''' — SY27-2522 (XT Technical Reference, 1986) — same chassis service procedures apply.

== Opening the System Unit ==

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

# Power off and unplug all cables (mains, keyboard, video, printer, 3270 host-connect coax).
# Five screws on the rear panel (Philips #2).
# 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 ==

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) ===

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

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

* 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 × 1 DRAM packages of the era; failed DRAMs are replaceable if the failure is identified to a specific chip.

=== PC3277-EM (3270 Emulation Adapter) ===

* '''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]]).<ref>SA38-0037-00 §6-17.</ref>
* Connects to the IBM 3174 or 3274 cluster controller via BNC twinax.

== Card-Backplane Connector ==

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 — handle with care.

== PSU Voltage Checks ==

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

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''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
|-
| &minus;5 V || &minus;4.75 V to &minus;5.25 V
|-
| &minus;12 V || &minus;11.4 V to &minus;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 — see [[IBM PC XT/370 Capacitor Replacement Guide]].

== Floppy and Hard Drive Maintenance ==

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 ==

* '''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 EBCDIC↔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 — upgrade if you have v1 only.

== Keyboard ==

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

== Recommended Tools ==

* 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).

== Related Pages ==

* [[IBM PC XT/370]]
* [[IBM PC XT/370 Troubleshooting Guide]]
* [[IBM PC XT/370 Capacitor Replacement Guide]]
* [[IBM PC XT (5160) Maintenance Guide]] — shared chassis procedures
* [[Recommended Tools]]

== References ==

* [http://bitsavers.org/pdf/ibm/pc/SA38-0037-00_Personal_Computer_Family_Service_Information_Manual_Jul89.pdf IBM SA38-0037-00 PC Family Service Information Manual (July 1989)]. Authoritative XT/370 service manual.
* Kozuh, Livingston, Spillman (1984), ''IBM Systems Journal''.
* [https://en.wikipedia.org/wiki/PC-based_IBM_mainframe-compatible_systems PC-based IBM mainframe-compatible systems — Wikipedia].

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