Editing IBM PS/2 Model 50 Troubleshooting Guide

[[File:IBM PS-2 Model 50 (photo).jpg|thumb|right|300px|IBM PS/2 Model 50. Source: Wikimedia Commons.]]
This guide covers diagnostic procedures and the complete IBM PS/2 numeric POST error code reference applicable to the '''[[IBM PS/2 Model 50]]''' (machine type 8550, both the original 8550-021 and the 8550-031 / 061 Model 50 Z).

== Audio Beep Codes ==

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Model 50 power-on audio codes'''
! Pattern !! Meaning
|-
| 1 short || POST OK
|-
| 2 short || Numeric error displayed on screen
|-
| 1 long + 1 short || Display or display adapter problem
|-
| 1 long + 2 short || Display or display adapter problem (alt path)
|-
| 1 long + 3 short || EGA / advanced display problem
|-
| Continuous short beeps || Keyboard problem &mdash; check fuse, cable, 8042
|-
| Continuous solid beep || System board failure
|-
| Click then no beep || PSU started but Power Good not asserted in 150 ms &mdash; shorted planar tantalum
|-
| No display, no sound || PSU fold-back from a planar short
|}

== Numeric POST Error Codes ==

The Model 50 uses the standard PS/2 Major/Minor numeric error format. MCA-specific codes (especially 165 card-ID-mismatch) are critical for this generation.

=== 1xx — System Board ===

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Major code 1xx — system board'''
! Code !! Meaning
|-
| 101 || Interrupt failure (8259A PIC)
|-
| 102 || Timer 0 failure / BIOS ROM checksum error
|-
| 103 || Timer interrupt failure or 2 KB CMOS RAM test
|-
| 104 || 80286 protected mode failure
|-
| 105 || Last 8042 keyboard controller command not accepted
|-
| 106 || Converting logic test
|-
| 107 || NMI test (parity check)
|-
| 108 || Timer bus test (8253/8254)
|-
| 109 || DMA (8237) test error
|-
| 110 || Planar parity error
|-
| 111 || I/O parity error (MCA adapter)
|-
| 112 || Watchdog timeout (MCA adapter failure)
|-
| 113 || DMA arbitration timeout (MCA adapter failure)
|-
| 114 || External ROM checksum error
|-
| 121 || Unexpected hardware interrupt
|-
| 151 || Bad battery
|-
| 152 || RTC or CMOS error
|-
| 160 || Planar ID not recognised &mdash; loose MCA riser
|-
| 161 || CMOS empty &mdash; '''replace Dallas DS12887 RTC'''
|-
| 162 || CMOS checksum mismatch
|-
| 163 || Time and date not set
|-
| 164 || Memory size error &mdash; run SETUP
|-
| 165 || '''Card ID mismatch''' &mdash; run Auto Configuration from Reference Diskette
|-
| 166 || MCA card busy error
|-
| 167 || RTC not updating
|-
| 199 || User-indicated configuration not correct
|}

==== 165 card ID mismatch ====

The single most common POST error on a Model 50 with any installed MCA cards. Occurs when:

* A new MCA card has been added since the last Auto Configuration run.
* An existing card has been removed.
* An ADF (Adapter Definition File) is missing from the working Reference Diskette.
* An MCA card's edge fingers are dirty and the card is intermittently reporting a different ID.

Resolution: boot the Reference Diskette, select '''Backup the Reference Diskette to a working diskette''' if you do not have a working copy, then '''Set Configuration → Run Auto Configuration'''. Insert each option diskette as prompted to load missing ADFs.

==== 161 / 162 / 163 cluster ====

Dead Dallas DS12887 RTC battery. Replace per [[IBM PS/2 Model 50 Maintenance Guide]] and re-run SETUP from the Reference Diskette.

=== 2xx — Memory ===

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Major code 2xx — memory'''
! Code !! Meaning
|-
| 201 || Memory test failed. The hex digits preceding "201" identify the failing 64 KB bank and bit
|-
| 202 / 203 || Memory address line failure (0–15 / 16–23)
|-
| 207 || ROM failure
|-
| 211 || Base 64 K on I/O channel failed (planar SIMMs)
|-
| 215 / 216 || Base 64 K on daughter card failed (MCA memory adapter)
|-
| 221 || ROM-to-RAM mapping error (latches until fixed)
|-
| 225 || Wrong / defective / slow memory &mdash; check 125-ns vs 85-ns SIMM
|}

A 225 error on a Model 50 that previously worked usually means the wrong-speed SIMM was fitted (85-ns where 125-ns belongs, or vice versa, or a non-PS/2-pinout SIMM).

=== 3xx — Keyboard ===

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Major code 3xx — keyboard'''
! Code !! Meaning
|-
| 301 || Keyboard did not respond / stuck key. The XX value preceding "301" is the scan code. Also occurs if keyboard plugged into mouse port
|-
| 302 || User-indicated keyboard test failure
|-
| 303 || Keyboard or 8042 controller error
|-
| 304 || Planar (8042) error / CMOS does not match keyboard
|-
| 305 || +5 V keyboard fuse blown — replace fuse on planar
|}

=== 4xx / 5xx — Integrated VGA ===

* 401 — Planar VGA error (monochrome path).
* 501 — Planar VGA error (colour path).
* 2401 — Diagnostic VGA error.
* 2408 — VGA ground circuit fault.

=== 6xx — Diskette ===

Standard PS/2 6xx codes. The 6xx codes cover the planar floppy controller and the 1.44 MB drive.

* 601 — Drive or controller test failed.
* 602 — Boot record invalid.
* 606–626 — Verify / format / read / write / NEC / DMA / seek / CRC / address-mark errors.
* 650 — Drive speed error.
* 651–656 — Format / verify / read / write / controller / drive failures.
* 657–660 — Write-protect or change-line signal stuck.

=== 7xx — 80287 Math Coprocessor ===

7xx errors raised only if software invokes the 80287. POST does not run a 80287 test unless explicitly enabled.

=== 17xx — ST-506 Fixed Disk ===

For Model 50 (8550-021) and Model 50 Z 30 MB (8550-031) with the ST-506 Fixed Disk Adapter/A:

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Major code 17xx — ST-506 fixed disk'''
! Code !! Meaning
|-
| 1701 || Controller POST error
|-
| 1702 || Controller timeout
|-
| 1703 || Drive seek failure
|-
| 1780 || Drive 0 timeout at IPL (drive not up to speed / stiction / cable off)
|-
| 1790 / 1791 || Format not recognised / data cable off
|}

=== 104xx — ESDI Fixed Disk ===

For Model 50 Z 60 MB (8550-061) with the ESDI Fixed Disk Adapter/A:

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Major code 104xx — ESDI'''
! Code !! Meaning
|-
| 10450 || Replace drive (read-write test failed)
|-
| 10451 || Replace drive (read-verify test failed)
|-
| 10452 || Replace drive (seek test failed)
|-
| 10454 || Controller (sector buffer test failed)
|-
| 10455 || Controller failure
|-
| 10463 || Drive (read/write sector error)
|-
| 10465 / 10466 || Controller (ECC error)
|-
| 10480 / 10481 || Drive 0 / 1 seek failure
|-
| 10490 / 10491 || Drive 0 / 1 read failure
|}

=== 8602 — Pointing device ===

* 8601 / 8603 / 8604 — mouse / 8042 errors. Most commonly a mouse plugged into the keyboard port.

== Reference Diskette and Advanced Diagnostics ==

The Model 50 has '''no built-in SETUP'''. All configuration through the Model 50 Reference Diskette (version 1.07 final). Key procedures:

# Always '''cold-boot''' the Reference Diskette (insert before powering on). Warm-booting may report false errors.
# '''Backup the Reference Diskette''' to a working diskette on first use. The original is read-only.
# '''Set Configuration → View configuration''' shows the current state.
# '''Set Configuration → Run Auto Configuration''' runs MCA card detection and ADF lookup.
# '''Test the computer → Diagnostics''' runs the full hardware test suite.
# '''Press Ctrl-A''' from the main menu to enter '''Advanced Diagnostics''' (FRU-level testing with minor NN codes).

ADF files for MCA cards are supplied on the option diskette that ships with each card. Copy each ADF onto your working Reference Diskette before fitting the card.

== Dead System Isolation ==

If the Model 50 shows no signs of life:

# Verify mains power and the PSU fan turns.
# Open the chassis.
# Verify PSU rails at the planar power connector with a multimeter.
# Pull all MCA cards. Try to boot with planar + riser + SIMM(s) + floppy only.
# Listen for a beep. If no beep, suspect bank-0 SIMM or planar power short.
# Re-add MCA cards one at a time. The failing card is the one that re-introduces the failure.

A shorted tantalum on the planar pulls the PSU into fold-back protection (Power Good not asserted within 150 ms) and produces a "click then nothing" symptom. Inspect the planar for shorted caps with a multimeter on diode test — see [[IBM PS/2 Model 50 Capacitor Replacement Guide]].

== ESDI Drive Stiction Recovery (8550-061) ==

If a 60 MB ESDI drive does not spin up:

# Power off and remove the drive from its caddy.
# Hold horizontal and rotate gently around the spindle axis.
# Reinstall and power on.
# Frees a stuck spindle for one boot. Image the drive contents immediately and replace.

== Common Symptoms ==

* '''Click, no beep''' &mdash; shorted planar tantalum. Recap planar.
* '''161 / 162 / 163 at every boot''' &mdash; dead DS12887 RTC battery. Replace module.
* '''165 at every boot''' &mdash; run Auto Configuration from the Reference Diskette.
* '''201 with bank/bit prefix''' &mdash; identify failing SIMM by the prefix hex digits.
* '''225 after SIMM swap''' &mdash; wrong-speed SIMM (Model 50 needs 125-ns, Model 50 Z needs 85-ns) or non-PS/2-pinout SIMM.
* '''Drive not detected''' &mdash; ST-506 stiction, ESDI cable seated badly, or controller card edge contacts dirty.
* '''Random reboots under load''' &mdash; aged 94 W PSU electrolytics, often near rated load.
* '''VGA wrong colours''' &mdash; planar VGA gate array failing.
* '''Keyboard 305 / mouse 8603''' &mdash; ports swapped or keyboard fuse blown.

== ECA Recalls ==

* '''ECA 087''' — Model 50 Z planar replacement under certain serial number ranges (specific memory controller issue).
* '''ECA 092''' — ESDI Fixed Disk Adapter/A firmware update for drive compatibility (Model 50 Z 60 MB units).

The IBM HMM (S52G-9971-02, October 1994) documents the affected serial number ranges for each ECA.

== ⚠️ 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>

== ⚠️ CMOS / RTC battery ==

This machine keeps its configuration in battery-backed CMOS, and the battery is a common failure. On AT-class boards the clock/CMOS is often a '''Dallas DS1287/DS12887''' module with the cell sealed inside; it lasts about ten years and then dies, giving '''161 / 163''' CMOS and clock errors at POST (and sometimes spurious floppy-drive errors). PS/2 planars use a rechargeable barrel or pack battery that '''leaks''' and corrodes the board. Replace a dead Dallas module (or rework it with an external coin cell), and on a leaking planar battery remove it and clean the corrosion before it eats the traces.<ref name="ibm_batt">[https://www.classic-computers.org.nz/blog/2009-10-10-renovating-a-dallas-battery-chip.htm Fixing a Flat Dallas DS1287 RTC], Classic Computers; and [https://www.ardent-tool.com/misc/Dallas_Rework.html Reworking Dallas RTC Modules], Ardent Tool. Source for the Dallas DS1287/DS12887 internal-battery death (161/163 CMOS errors) and the leaking planar battery.</ref>

== References ==

<references />

== Related Pages ==

* [[IBM PS/2 Model 50]]
* [[IBM PS/2 Model 50 Maintenance Guide]]
* [[IBM PS/2 Model 50 Capacitor Replacement Guide]]
* [[IBM PS/2 Model 60 Troubleshooting Guide]] — tower sibling
* [[Capacitor Failure Symptoms]]

== References ==

* [https://www.ardent-tool.com/trouble/ps2error.html PS/2 Error Codes], Ardent Tool of Capitalism. Authoritative reference for the complete 1xx–25xx and 100xx–112xx PS/2 numeric error code listing, the Power-Good-in-150-ms PSU spec, and the dead-system isolation procedure.
* [http://museodelcomputer.org/parts/ibm/ps2_50z/docs/error_codes.htm PS/2 Error Codes], Museodelcomputer.org. Reference for audio code patterns and minor code structure.
* [https://www.dosdays.co.uk/computers/IBM%20PS2%20Model%2050/ibm_ps2_model_50.php IBM PS/2 Model 50 — DOS Days]. Reference for the ESDI controller architecture and SIMM-speed dependencies.
* IBM, ''IBM Personal System/2 Hardware Maintenance Manual'' (S52G-9971-02, October 1994). Source for ECA 087 / 092 recalls and FRU listings.
* IBM, ''IBM Personal System/2 Model 50 Reference Diskette''. Source for SETUP, Auto Configuration, Advanced Diagnostics (Ctrl-A) and the ADF-based MCA card configuration.

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