Editing IBM PS/2 Model 70 Troubleshooting Guide

[[File:IBM PS-2 Model 70 (photo).jpg|thumb|right|300px|IBM PS/2 Model 70. Source: Wikimedia Commons.]]
This guide documents fault diagnosis for the '''[[IBM PS/2 Model 70]]''' (machine type 8570, all submodels). The Model 70 shares much of its POST architecture with the [[IBM PS/2 Model 50]] / [[IBM PS/2 Model 60|60]] but has 80386DX-specific and Power Platform-specific failure modes, and is uniquely afflicted by the SMD electrolyte leakage that defines the PS/2 plague.

== Reference Diskette and Diagnostics ==

The Model 70 requires the '''Model 70-specific Reference Diskette'''. The Power Platform-equipped 8570-B21 / -B61 uses a different Reference Diskette image with BIOS extensions for the 80486 daughtercard. Reference Diskettes are not cross-compatible between PS/2 submodels.

Boot Options:

* '''F1''' — boot the Reference Diskette (Set Configuration, Set Features, Display Configuration, Copy Option Diskette, Run Auto Configuration).
* '''Ctrl-A''' from the Reference Diskette menu — Advanced Diagnostics. Advanced Diagnostics for the Model 70 includes 80386-specific tests (paging, protected mode entry/exit, 32-bit data path) not run on the Model 50 / 60.

== POST Sequence ==

The Model 70 POST runs in this order:

# Reset; CPU register check (80386 includes paging and protected mode tests).
# ROM checksum.
# CMOS / RTC battery check.
# Planar RAM count (with one beep at 640 KB).
# Onboard VGA initialisation.
# Floppy controller and drive seek.
# MCA adapter ID scan (each card returns a 16-bit ID).
# ADF lookup against CMOS configuration record.
# ESDI controller initialisation.
# Boot device selection.

A four-digit numeric code displayed top-left on the screen is a fault code from the test block currently running.

== Beep Codes ==

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Model 70 beep codes'''
! Beeps !! Meaning
|-
| 1 short || POST passed; normal boot.
|-
| 2 short || Configuration error; numeric error on screen.
|-
| 1 long, 1 short || Planar fault.
|-
| 1 long, 2 short || Display adapter fault.
|-
| Continuous || Power supply or planar fault.
|-
| None, no display || Planar or PSU fault before video init.
|}

== Numeric POST Codes ==

POST codes follow the IBM convention where the test block is the leading digit(s). The Model 70 shares the 1xx–10xxx code families with the Model 50 / 60 but adds 80386-specific tests.

=== 1xx — Planar / System Board ===

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Planar POST codes'''
! Code !! Meaning !! Notes
|-
| 101 || Interrupt failure || Planar IRQ controller fault.
|-
| 102 || Timer failure ||
|-
| 103 || Timer IRQ failure ||
|-
| 104 || Protected-mode failure || 80386 protected mode test failed. Often planar SMD cap leakage near the CPU.
|-
| 105 || Keyboard controller failure ||
|-
| 106 || Logic gate / planar logic failure ||
|-
| 107 || NMI failure ||
|-
| 108 || Timer-bus timeout ||
|-
| 109 || DMA test failure ||
|-
| 110 || Planar parity check 0 ||
|-
| 111 || I/O parity check ||
|-
| 112 || Watchdog timeout ||
|-
| 113 || DMA arbitration timeout ||
|-
| 114 || Paging test failure || 80386-specific.
|-
| 121 || Unexpected hardware interrupt ||
|-
| 151 || Real-time clock failure || RTC chip itself (not just the cell).
|-
| 161 || '''CMOS configuration empty''' || Dead RTC battery; replace DS12887 module.
|-
| 162 || '''CMOS checksum bad''' ||
|-
| 163 || '''Time and date not set''' ||
|-
| 164 || Memory size mis-match with CMOS || Run SETUP from Reference Diskette.
|-
| 165 || '''MCA adapter ID mismatch''' || Run Auto Configuration.
|-
| 166 || MCA arbitration failure ||
|-
| 167 || Real-time clock failure ||
|}

A '''104''' on a Model 70 frequently indicates SMD electrolyte leakage near the 80386 CPU socket has corrupted address or control lines. Recap before any other diagnosis.

=== 2xx — RAM ===

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Memory POST codes'''
! Code !! Meaning
|-
| 201 || Memory test failed (planar or memory adapter card)
|-
| 202 || Memory address line error
|-
| 203 || Memory address line error
|-
| 215 || Memory option-adapter failure (e.g., MCA Memory Adapter)
|-
| 216 || Memory option-adapter address conflict
|-
| 225 || Wrong memory speed for board (rare on Model 70)
|}

The 201 number is followed by the failed memory block address (8 hex digits). Use the address to identify the failing SIMM bank or MCA memory adapter slot.

=== 3xx — Keyboard ===

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Keyboard POST codes'''
! Code !! Meaning
|-
| 301 || Keyboard not responding || Wrong port? Try a known-good keyboard.
|-
| 302 || User-indicated keyboard error
|-
| 303 || Keyboard / system unit interface fault
|-
| 304 || Keyboard or system unit interface failed
|-
| 305 || +5 V fuse on planar (keyboard fuse)
|-
| 365 || Keyboard mouse problem (PS/2 ports)
|}

=== 6xx — Floppy Drive ===

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Floppy POST codes'''
! Code !! Meaning
|-
| 601 || Floppy / drive failure
|-
| 602 || Floppy drive boot record failure
|-
| 604 || Floppy diagnostic error
|-
| 611 || Floppy adapter / controller failure
|-
| 613 || Cable / drive A: failure
|-
| 621 || Drive seek failed
|-
| 622 || Drive CRC error
|-
| 624 || Floppy data bus addressing error
|-
| 626 || Floppy data compare error
|}

=== 8xx — Math Coprocessor ===

The Model 70 (E61 / 121 / A21 / A61) supports an optional '''80387DX''' coprocessor. The Power Platform B21 / B61 has the math unit built into the 80486DX.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Coprocessor POST codes'''
! Code !! Meaning
|-
| 801 || Coprocessor test failed
|}

=== 104xx — ESDI Fixed Disk Adapter/A ===

The Model 70 uses the '''IBM ESDI Fixed Disk Adapter/A''' for its 60 / 120 MB ESDI drive. The 104xx error family is the same as on the Model 50 / 60 ESDI submodels.

See [[IBM PS/2 Model 60 Troubleshooting Guide]] for the complete 104xx code table — the same codes apply to the Model 70.

In summary:

* '''10450 / 10451 / 10452''' — drive-side errors.
* '''10455 / 10499''' — controller failure. Recap controller card before condemning.
* '''10463''' — write/read sector error. Same.
* '''10464''' — primary boot record read failure (drive or filesystem).
* '''10480 / 10481''' — drive 0 / 1 fatal error.
* '''10491 / 10492 / 10493''' — user-indicated read/write/controller error during Advanced Diagnostics.

=== Display POST Codes ===

The Model 70 has '''onboard VGA''' on the planar. The 2401 / 2402 / 24xx code family covers the planar VGA. A 24xx error is a planar / VGA fault — and on the Model 70 is one of the more common SMD electrolyte leak symptoms (electrolyte corrodes traces near the VGA controller chip first because they are the most densely-packed on the planar).

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Display POST codes'''
! Code !! Meaning
|-
| 2401 || Onboard VGA POST failure
|-
| 2402 || VGA video memory failure
|-
| 2403 || VGA palette DAC failure
|-
| 2408 || User-indicated VGA failure (Advanced Diagnostics)
|-
| 2409 || VGA test failure
|-
| 2410 || Planar VGA card failure
|}

== SMD Electrolyte Leak Diagnostic Workflow ==

The Model 70 / 80 planar's surface-mount aluminium electrolytic capacitors are the leading cause of "the Model 70 won't POST" complaints. A leaky SMD cap can produce:

* A '''104''' (protected mode) error if the leak is near the CPU.
* A '''201''' (memory) error if the leak is near the SIMM controller.
* A '''2401''' (VGA) error if the leak is near the VGA chip.
* Random, intermittent reboots if the leak is on a bus signal.
* No POST at all if the leak has shorted a power rail.

If a Model 70 is exhibiting any persistent or intermittent POST fault, '''inspect the planar for SMD electrolyte leakage before any other diagnosis'''. Read [[IBM PS/2 Model 70 Capacitor Replacement Guide]] for the recap procedure.

== MCA-Specific Faults ==

=== 165 Card-ID Mismatch ===

The Model 70's three MCA slots make 165 less common than on a Model 80, but the fault mode is identical. Causes:

* Card was added or removed without running Auto Configuration.
* MCA edge contact oxidation.
* RTC battery died and CMOS lost the configuration record (cluster with 161/162/163).
* CMOS battery is fine but the saved configuration was corrupted.

Fix:

# Boot the Reference Diskette.
# Select '''Set Configuration → Run Auto Configuration'''.
# Insert option diskettes as prompted.
# Save and reboot.

If 165 persists after Auto Configuration:

* Reseat every MCA card. Clean edge fingers with eraser and contact cleaner.
* Try removing all cards, run Auto Configuration with bare planar, then reinstall cards one at a time.
* Suspect SMD electrolyte leakage on the planar near the MCA bus controller.

== Power Platform-Specific Faults (8570-B21 / B61) ==

The Power Platform daughtercard adds its own failure modes:

* '''Power Platform regulator failure''' — the on-card +5 V regulator can fail and the daughtercard receives no power. Diagnosis: probe the +5 V test point on the Power Platform.
* '''Power Platform clock generator failure''' — the 80486DX receives no clock.
* '''Power Platform Reference Diskette mismatch''' — if a standard E61 / 121 / A21 Reference Diskette is booted on a Power Platform-equipped 8570, the 80486-specific BIOS extensions are not loaded and some diagnostics will fail.
* '''Cache test failure''' — the Power Platform exposes the 80486 on-die 8 KB cache for testing in Advanced Diagnostics. A cache test failure is a daughtercard fault, not a planar fault.

== ECA Recalls and Service Bulletins ==

The Model 70 was affected by several Engineering Change Authorisation (ECA) bulletins:

* '''ECA 087''' — Reference Diskette compatibility update for newer MCA cards.
* '''ECA 092''' — Planar fix for one revision of the Model 70 with an erratic memory timing.
* '''ECA 117''' — ESDI controller firmware update.

These ECAs apply only to specific FRU part numbers; check the planar / card FRU against the bulletin before applying.

== PSU Faults ==

Symptoms and diagnosis:

* '''Dead — no fans, no power''': Bulk capacitor or mains rectifier; verify mains; PSU recap required. See [[IBM PS/2 Model 70 Capacitor Replacement Guide]].
* '''Fans spin briefly, then click-retry''': Power Good not asserted in 150 ms. Could be PSU fold-back or shorted planar tantalum / leaked SMD electrolyte.
* '''Boots cold, fails when warm''': Secondary electrolytics aged. Recap.
* '''Audible whine, smell of fish''': RIFA X2 cap is venting.
* '''Rails low/high''': PSU feedback path issue. Recap and retest.

== Drive Stiction ==

ESDI drives in the 8570 are prone to spindle stiction after long storage. Same procedure as the Model 60:

* Power off.
* Open the chassis. Locate the drive.
* Gently rotate the drive case 45–90° around its spindle axis in both directions to free the heads.
* Re-install. Power on.

After the drive boots, '''immediately''' image its contents to a modern disk image file. The drive cannot be trusted to spin up reliably again.

== Floppy Faults ==

Same as Model 60 (see [[IBM PS/2 Model 60 Troubleshooting Guide]]).

== Keyboard / Mouse Faults ==

* '''301''': Keyboard or mouse in the wrong port.
* '''305''': +5 V fuse on planar blown.
* '''365''': PS/2 mouse fault.

== Memory Faults ==

* '''201''' with address in the on-planar range: SIMM (planar memory) failure. Note: the Model 70 uses IBM proprietary 72-pin SIMMs.
* '''201''' with address in an MCA memory adapter range: Memory adapter card failure.
* '''215 / 216''': MCA Memory Adapter configuration error.

== When to Suspect the Planar ==

* 1xx errors that persist after CMOS battery replacement and Auto Configuration.
* 10x errors with no other peripheral fault visible.
* No video and no beeps after PSU verified known-good.
* Repeated 165 errors after every card removed and reseated and a full Auto Config done.
* '''Any''' visible SMD electrolyte leak.

The single most common Model 70 planar failure is '''SMD aluminium electrolyte leakage'''. See [[IBM PS/2 Model 70 Capacitor Replacement Guide]].

== Diagnostic Workflow ==

# Visually inspect planar for SMD electrolyte leak first. If found, do not power on.
# Power on. Listen for beep.
# Note POST screen — any leading numeric code.
# If 161/162/163 cluster — replace DS12887 module.
# If 165 — run Auto Configuration.
# If 17xx (n/a — no ST-506 on Model 70) / 104xx — verify drive type in CMOS; reseat drive cable; consider stiction; consider controller cap failure.
# If 24xx — planar VGA fault, often SMD electrolyte leak.
# If 104 — 80386 protected mode fault, often SMD electrolyte leak near CPU.
# If no beep and no video — PSU first (rail check), then planar SMD inspection.
# Run Advanced Diagnostics (Ctrl-A) once basic POST passes.

== ⚠️ 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 70]]
* [[IBM PS/2 Model 70 Maintenance Guide]]
* [[IBM PS/2 Model 70 Capacitor Replacement Guide]]
* [[IBM PS/2 Model 80 Troubleshooting Guide]] — tower sibling, same planar family
* [[IBM PS/2 Model 60 Troubleshooting Guide]] — predecessor

== References ==

* [https://www.ardent-tool.com/trouble/ps2error.html PS/2 Error Codes — Ardent Tool of Capitalism]. Source for the 1xx–104xx error code tables.
* [https://www.ardent-tool.com/qtechinfo/GJAN-43VS3S.html IBM PS/2 Model 70 — Ardent Tool Quick Reference]. FRU data and ECA history.
* IBM, ''IBM Personal System/2 Hardware Maintenance Manual'' (S52G-9971-02, October 1994). POST error code reference.

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