Editing IBM PC XT/370 Troubleshooting Guide

This guide documents fault diagnosis for the '''[[IBM PC XT/370]]'''. POST is the standard IBM PC XT (5160) POST in the 1xx–19xx range; XT/370-specific failures present as VM/PC boot errors or as cards-not-detected by the XT host BIOS.

== Reference Documents ==

* '''IBM SA38-0037-00''' — ''Personal Computer Family Service Information Manual'' (July 1989), Chapter 6 (XT/370). Authoritative service manual.<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).
* IBM PC XT documents — POST codes, BIOS, hardware troubleshooting carry over directly.

== Initial Diagnosis ==

Power-on sequence on a working XT/370:

# IBM PC XT POST runs to completion (no error message; single short beep).
# PC DOS 2.10 boots from the C: drive.
# AUTOEXEC.BAT calls the VM/PC loader.
# VM/PC initialises the 370PC-P card via the ISA bus.
# VM/PC contacts the IBM 3174 or 3274 mainframe controller via the PC3277-EM card.
# VM/PC downloads CMS system files from the host mainframe to local minidisks (first run only).
# CMS prompt appears.

If any of these does not happen, stop and diagnose at that stage.

=== Stage 1 — XT Host POST Fails ===

Standard IBM PC XT POST codes apply. The 1xx–19xx code series identifies the failed FRU at the chassis level. Cross-reference with [[IBM PC XT (5160) Troubleshooting Guide]].

Common POST codes:

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Selected XT POST codes that apply to the XT/370'''
! Code !! Meaning !! First action
|-
| 1xx || Planar / system board || Reseat / replace planar
|-
| 2xx || Memory error || Identify failing 64 KB block from on-screen address
|-
| 301 / 302 || Keyboard fault || Verify keyboard connected; check for stuck keys
|-
| 6xx || Floppy drive fault || Cable, drive, controller
|-
| 17xx || Hard disk fault || Cable, drive, fixed-disk adapter
|}

=== Stage 2 — XT POSTs OK but VM/PC Won't Load ===

* '''DOS boots normally''' — XT chassis is healthy, problem is in the 370PC subsystem.
* '''VM/PC initialisation message appears but hangs''' — 370PC-P card not responding. Reseat the 370PC-P card. Reseat the 370PC-P to 370PC-M backplane connector.
* '''VM/PC reports "memory error" or "card not present"''' — 370PC-M card not detected. Reseat M card; clean backplane connector with deoxidising contact cleaner.
* '''VM/PC reports "host connection failed"''' — PC3277-EM card or the cable to the 3174 / 3274 controller. Check the BNC coax cable, terminator, and controller-side configuration.

=== Stage 3 — VM/PC Loads but CMS Crashes ===

* '''S/370 Program Check''' — usually means a guest CMS program has executed an opcode the modified 68000 does not implement. The instruction trap then fails because the unmodified 68000's interpreter does not recognise the opcode either. Resolution: update CMS / VM/PC to a known-good level; or restrict the guest program to instructions documented as supported by the XT/370 emulator.
* '''Spurious memory errors''' — 370PC-M DRAM failure. Run the VM/PC built-in memory test to identify the failing chip.
* '''Disk I/O errors''' — XT 10 MB ST-412 drive failure (more likely than a card fault).

== 370PC Card Diagnostic Workflow ==

Because the XT host BIOS does not know about the 370PC cards, the cards do not produce POST codes — they are detected only when VM/PC loads. Diagnostic workflow:

# Verify the XT chassis POSTs cleanly with the cards installed.
# Verify DOS boots from C:.
# Run a small DOS program from C: to confirm RAM is intact (e.g. CHKDSK).
# Load VM/PC and observe the initialisation messages.
# If VM/PC reports a 370PC-P or 370PC-M error, power off and reseat the cards and the backplane connector.
# If the error persists, swap with a known-good card set.

'''There is no in-system diagnostic for the modified 68000s or the modified 8087.''' If a card fails, the verdict is replacement — and replacement cards are unobtainium.

== 3270 Host Connection ==

The PC3277-EM card (or the standard 3278/79 Emulation Adapter on late-production units) connects to the IBM 3174 or 3274 cluster controller via BNC twinax. Common faults:

* '''No carrier on the host link''' — BNC cable, terminator, or 3174/3274 port.
* '''Host connection times out''' — controller-side configuration. The XT/370's host LU (Logical Unit) must be defined on the controller as a 3278 or 3279 emulator.
* '''VM/PC sees the host but cannot download CMS''' — VM CP (Control Program) configuration on the host mainframe; the user ID must have appropriate permissions and minidisk allocations.

== VM/PC Operating System Faults ==

* '''VM/PC boots but CMS commands not recognised''' — minidisks not formatted. Format using the VM/PC FORMAT command (formats a DOS file as a CMS minidisk).
* '''CMS files do not export properly to DOS''' — character set conversion issue. Use the EXPORT command with explicit EBCDIC-to-ASCII translation.
* '''CMS application crashes with "no such opcode"''' — guest software executed an instruction outside the XT/370 emulator's supported subset. Replace with a different version of the application or a CMS-included tool.

== Common Field Symptoms ==

* '''XT POSTs, DOS boots, VM/PC won't load''' — 370PC card subsystem. Reseat in order of likelihood: backplane connector → 370PC-P → 370PC-M → PC3277-EM.
* '''VM/PC loads, CMS prompt appears, but every command hangs''' — 370PC-P custom 68000 microcode issue, or 370PC-M memory failure.
* '''VM/PC loads, host connection fails''' — coax cable, terminator, controller-side configuration.
* '''Random reboots during VM/PC operation''' — PSU at capacity. Recap PSU; see [[IBM PC XT/370 Capacitor Replacement Guide]].
* '''Smell of fish from PSU''' — RIFA X2 mains-suppression cap is venting. Power off immediately and replace.

== Diagnostic Workflow Summary ==

# Power on; observe POST. If POST fails, treat as standard XT chassis fault.
# If POST passes, observe DOS boot. If DOS fails, suspect XT hard drive or floppy.
# If DOS boots, load VM/PC. If VM/PC fails, reseat the 370PC cards.
# If VM/PC loads, attempt host connection. If host connect fails, check the BNC cable and the 3174/3274 controller.
# If host connect succeeds, attempt a CMS command. If CMS commands hang, suspect 370PC-P custom 68000 failure.

== ⚠️ Power-supply RIFA capacitor and tantalum shorts ==

Two age-related failures are near-universal on this era of IBM hardware:

* '''RIFA mains-filter capacitors''' in the power supply are metallised-paper parts that crack and fail '''short''' with age, producing acrid smoke shortly after power-on. Replace them pre-emptively with modern X2-class parts.<ref name="ibm_rifa">[https://www.minuszerodegrees.net/failure.htm minuszerodegrees.net — IBM failure symptoms]; [https://www.classic-computers.org.nz/blog/2010-11-04-restoring-an-IBM-xt.htm Repairing and Restoring an IBM XT]; and [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ Adam's Vintage Computer Restorations]. Source for the RIFA mains-filter capacitor failing short (smoke) and the tantalum capacitors failing short and preventing the PSU from firing.</ref>
* '''Tantalum capacitors''' on the planar (system board) and on ISA cards fail short with age. A shorted tantalum will '''prevent the power supply from starting''' (dead machine, PSU protection latched) &mdash; look for a cracked or discoloured tantalum and lift suspect ones to find the short.<ref name="ibm_rifa">[https://www.minuszerodegrees.net/failure.htm minuszerodegrees.net — IBM failure symptoms]; [https://www.classic-computers.org.nz/blog/2010-11-04-restoring-an-IBM-xt.htm Repairing and Restoring an IBM XT]; and [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ Adam's Vintage Computer Restorations]. Source for the RIFA mains-filter capacitor failing short (smoke) and the tantalum capacitors failing short and preventing the PSU from firing.</ref>

IBM PC/XT switching supplies also need a '''minimum load''' to start, so a bare supply on the bench may not run without a dummy load.<ref name="ibm_rifa">[https://www.minuszerodegrees.net/failure.htm minuszerodegrees.net — IBM failure symptoms]; [https://www.classic-computers.org.nz/blog/2010-11-04-restoring-an-IBM-xt.htm Repairing and Restoring an IBM XT]; and [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ Adam's Vintage Computer Restorations]. Source for the RIFA mains-filter capacitor failing short (smoke) and the tantalum capacitors failing short and preventing the PSU from firing.</ref>

== POST beep and error codes ==

The IBM Power-On Self Test signals faults by beeps and, where a display works, by a numeric code:

<templatestyles src="Template:StyledTable/styles.css" />
[[File:IBM PC XT-370 (photo).jpg|thumb|right|300px|IBM PC XT/370. Source: Wikimedia Commons.]]
{| class="wikitable styled-table" style="width:85%; text-align:left;"
|+'''IBM POST beep codes'''
! Beeps !! Meaning
|-
| 1 short || Normal POST &mdash; system OK
|-
| 2 short || POST error (numeric code shown on screen)
|-
| No beep || Power supply or system-board fault
|-
| Continuous / repeating short || Power supply or system board
|-
| 1 long, 1 short || System-board fault
|-
| 1 long, 2 short || Display-adapter fault (MDA/CGA)
|-
| 1 long, 3 short || Display-adapter fault (EGA/later)
|}

Common numeric codes include '''161/163''' (dead CMOS battery/clock), '''201''' (memory), '''301''' (keyboard) and '''1701''' (hard disc). A code ending in the family prefix identifies the failing subsystem.<ref name="ibm_post">[http://o-r-b.nl/modificaties/beeb_codes.htm Standard Original IBM POST Error Codes]; and [https://www.ibm.com/support/pages/troubleshooting-post-beep-and-no-beep-errors IBM — POST beep errors]. Source for the IBM POST beep and numeric error codes.</ref>

== References ==

<references />

== Related Pages ==

* [[IBM PC XT/370]]
* [[IBM PC XT/370 Maintenance Guide]]
* [[IBM PC XT/370 Capacitor Replacement Guide]]
* [[IBM PC XT (5160) Troubleshooting Guide]] — chassis-level POST codes

== 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.
* Kozuh, Livingston, Spillman (1984), ''IBM Systems Journal'' 23(3):245.
* [https://www.stanislavs.org/helppc/diagnostic_codes.html HelpPC — IBM PC diagnostic codes].

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