Editing IBM PS/2 Model 25 Maintenance Guide

This guide documents preventive maintenance procedures for the IBM PS/2 Model 25 (machine type 8525), covering all three CPU generations (Model 25 8086, Model 25 286, Model 25 SX) and the integrated 12" CRT, the all-in-one chassis, the PS/2 keyboard and pointing-device ports and the proprietary PS/2 single-cable drive interface.

== Safety Warning ==

The Model 25 contains an '''integrated CRT'''. The CRT anode can hold a '''lethal high-voltage charge''' (15&ndash;20 kV) for hours after the system is powered off. The integrated PSU primary side can also hold a lethal charge on its bulk capacitor.

Before any work that opens the chassis:
# Power off the system and unplug the mains lead from the wall.
# Wait at least 30 minutes before opening the case.
# Even after 30 minutes, treat the CRT anode and the PSU primary side as live until measured otherwise.
# Discharge the CRT anode through a 1 MΩ / 10 W bleed resistor (one end clipped to the chassis ground, the other slid under the rubber anode boot to touch the metal anode cap). Do not use a screwdriver to short the anode directly &mdash; the inrush can damage the CRT or your tools.
# Discharge the PSU bulk capacitor (typically 220 µF / 200 V or higher) through a 1 kΩ / 5 W resistor between its two terminals.

These steps apply to every Model 25 service procedure that involves opening the system unit.

== Identifying Your Model ==

Before any work, identify which Model 25 generation you have:

* '''Model 25 8086''' &mdash; front badge reads "Personal System/2" with no "286" or "SX" suffix. CPU socket near the upper-left of the planar is empty or contains an 8086-2/V20. MCGA video (15-pin analog VGA connector but limited modes). No hardware RTC; date and time must be set every boot.
* '''Model 25 286''' &mdash; front badge reads "286" prominently. VLSI chipset visible on the planar (ZM2&ndash;ZM6). VGA video. Battery-backed CMOS in the RTC.
* '''Model 25 SX''' &mdash; front badge reads "SX" or "386SX". 80386SX CPU at U2. Crystal at Y1 = 32.768 kHz indicates a hardware RTC. IDE 40-pin connector at J10 (not a single-cable PS/2 interface).

Submodel and IBM machine type are printed on a label on the rear of the chassis (e.g. "Type 8525-004").

== Opening the System Unit ==

The Model 25 case is one solid moulding around the CRT. The system unit cover is held by screws on the bottom of the case (Model 25 8086 and 286) or with a single thumbscrew at the rear (Model 25 SX). The CRT and the planar share the chassis; once the cover is off, the CRT is exposed on its left and the planar/PSU are on the right.

# Disconnect the keyboard, mouse, monitor passthrough (Model 25 286 / SX), serial and parallel cables.
# Place the system upside-down on a soft surface; on the 8086 / 286 remove four screws on the bottom; on the SX remove the rear thumbscrew.
# Lift the cover. Avoid sliding the cover over the CRT yoke.

Do '''not''' lift the system by the CRT bezel. The bezel is plastic and only clipped to the chassis; lifting by the bezel will detach it and may strain the yoke neck.

== Regular Cleaning ==

* Use a soft brush and low-pressure compressed air to clear dust from the planar, the riser card, the floppy drive(s), the optional hard drive cradle and the PSU vents. Hold any fan blades by hand if using compressed air.
* '''Do not''' use compressed air to clean inside the CRT bezel. Dust on the inside of the bezel and the front of the CRT screen is best removed with a microfibre cloth lightly damp with isopropyl alcohol, with the system off and the CRT discharged.
* Clean the CRT face from the outside with a microfibre cloth and (optionally) IPA. Do not spray cleaner directly onto the bezel &mdash; mist into the cloth.
* Clean the keyboard and mouse Mini-DIN connectors with a deoxidising contact cleaner on a foam swab. Pins on the Mini-DIN connector bend easily; verify pin alignment before reseating.

== PSU Voltage Checks ==

The Model 25 PSU is integrated into the chassis and supplies the planar through a non-standard connector. There are '''no Molex 4-pin drive connectors''' &mdash; drive power is routed through the planar via the same single-cable interface that carries the data lines.

To check PSU rails:
# Identify the PSU connector on the planar. Refer to the planar silkscreen at J7 (8086) or J7/J14 (286).
# With the system powered on (cover off, no CRT discharge needed because the rails are accessible from the data side), measure +5 V and +12 V at the planar power connector with a digital multimeter.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Model 25 PSU rail tolerances (CBIOS specification)'''
! 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 Model 25 PSU is rated 90 W (8525-001/C02 monochrome variants) or 115 W (8525-004/C05 colour variants). The colour variant draws more current because the colour CRT requires the higher PSU output to drive its three guns and a larger flyback. '''Do not''' fit a Model 30 PSU as a drop-in &mdash; the planar connectors are different.

== CMOS / RTC Battery ==

The Model 25 8086 has '''no hardware RTC''' &mdash; there is no battery to replace, and the system always boots with the date and time at the BIOS default (the user must set them from DOS each boot).

The '''Model 25 286 and Model 25 SX''' have a hardware RTC backed by a battery. The original battery is typically a '''Dallas DS1287''' or '''DS12887''' integrated RTC+battery module on the planar (look for a square IC with a "DALLAS" or "STMicroelectronics M48T..." marking). When the integrated battery fails, the recommended fix is to fit a '''Dallas DS12887+''' replacement (the "+" indicates a fresh manufacture date).

Symptoms of a dead RTC battery:
* '''161''' error at POST &mdash; CMOS configuration empty.
* '''162''' error at POST &mdash; CMOS checksum mismatch.
* '''163''' error at POST &mdash; time and date not set.
* On the Model 25 SX, the system also defaults to '''720 KB''' floppy operation regardless of the diskette type (it will not read 1.44 MB diskettes) until the CMOS is restored.

Replacement procedure:
# Power off and unplug the system.
# Discharge the CRT and PSU bulk capacitor.
# Identify the RTC module on the planar (Model 25 286: search near the front-left of the planar; Model 25 SX: at U10 or nearby, labelled DALLAS DS1287/DS12887).
# Carefully desolder the original module (it is a 24-pin DIP). Use solder wick and a fine-tipped iron; the planar pads are not very tolerant.
# Fit a socket if you wish to allow future replacements, or solder the new module directly.
# Boot the Reference Diskette and run '''SETUP''' to re-enter memory size, drive types, date and time.

Some restorers cut the DS1287 plastic case to expose the internal cell and connect an external lithium coin cell. This works but is not as clean as a full replacement module.

== Connector Care ==

The Model 25 uses several connector types that need specific care.

* '''PS/2 keyboard / mouse (Mini-DIN-6)''': Plastic key in the connector; aligning the connector against the slot mark is essential. Plugging a mouse into the keyboard port (or vice versa) produces a 301 error at POST. The two ports are functionally identical at the 8042 controller, but the BIOS expects each device on its designated port.
* '''F1 keyboard fuse on the planar''' (3 A): Protects the combined +5 V to the keyboard and pointing device. A blown F1 produces a 305 error and prevents the system from booting past POST (the system may also continuously beep). Replace with the same value and type.
* '''Serial port (DB-25 male)''' on the 8086; DB-9 on the 286 / SX.
* '''Parallel port (DB-25 female)''': not at risk of mis-insertion with the serial.
* '''Earphone connector (1/4" mono)''' at the rear: routes the PC speaker through a passive divider to an external amplifier or headphones. On the Model 25 286 a special break-out adapter routes the PC speaker to an external source via a TRS audio jack.

== Single-Cable Drive Interface ==

The PS/2 single-cable drive interface combines power and data in one ribbon cable from the planar to the floppy or hard drive. Both ends use a card-edge or BERG header. Care points:

* Pin 1 is marked with a red or blue stripe on the cable. Always align stripe-to-stripe.
* The drive end of the cable carries +5 V and +12 V; reversing the cable will not damage the drive in most cases (the drive logic includes protection) but the drive will not respond.
* Drives manufactured by '''ALPS''', '''Mitsubishi''', '''Sony''' and '''YE-Data''' all appear in genuine IBM PS/2 systems. Sony drives are the most likely to support 1.44 MB media in machines where the BIOS strictly limits the floppy to 720 KB (the Model 25 8086).
* Floppy drives have '''no dust shutter''' on the front bezel &mdash; dust can enter and contaminate the heads. Clean with a head-cleaning diskette periodically.

== Floppy Drive Maintenance ==

* The Model 25 8086 ships with '''720 KB''' 3.5" diskettes. The Reference Diskette and DOS 3.30 use only DSDD media. '''Do not''' use 1.44 MB media in the 720 KB drive &mdash; the drive write current is set for low-coercivity (DSDD) media and 1.44 MB media will accept a poor magnetisation.
* The Model 25 286 and Model 25 SX use '''1.44 MB''' 3.5" diskettes. Both DSDD (720 KB) and DSHD (1.44 MB) media are supported.
* Replace the rubber drive belt if the drive is the '''Mitsubishi MF355C''' family; the belt is a known wear item.
* Aluminium electrolytic capacitors on the drive logic board age and can leak; if the drive becomes slow or fails to read, inspect the drive logic board and consider a drive-board recap. See [[IBM PS/2 Model 25 Capacitor Replacement Guide]].

== Hard Drive Maintenance ==

* The Model 25 8086 originally shipped without a hard drive. Owner-fitted ST-506 hard drives use the planar's single-cable interface.
* The Model 25 286 with ST-506: the Seagate ST-506 / ST-225 30 MB drive is mounted in the second floppy bay with a mounting cradle.
* The Model 25 SX uses standard 40-pin IDE on J10. Drive power is supplied through the planar.

== Cable Routing ==

When reassembling the chassis, route cables to avoid:
* Contact with the CRT yoke (mechanical wear and electromagnetic interference).
* Contact with the PSU bulk capacitor or transformer (heat).
* Sharp folds at connectors (cable fatigue).

== Recommended Tools ==

* Philips #2 screwdriver, T15 Torx (some 286 / SX submodels).
* Anti-static strap connected to chassis ground.
* Digital multimeter with high-voltage probe for CRT discharge verification.
* 1 MΩ / 10 W bleed resistor for CRT anode discharge.
* 1 kΩ / 5 W resistor for PSU bulk capacitor discharge.
* Microfibre cloth, isopropyl alcohol, foam swabs.
* Soldering iron (fine tip), solder wick, lead-free or 60/40 solder.
* Spare DS12887+ for Model 25 286 / 25 SX RTC.
* Period-correct or modern equivalent Model 25 Reference Diskette image (use a Greaseweazle or KryoFlux + DSDD diskette to write a new copy).

== Related Pages ==

* [[IBM PS/2 Model 25]]
* [[IBM PS/2 Model 25 Troubleshooting Guide]]
* [[IBM PS/2 Model 25 Capacitor Replacement Guide]]
* [[IBM PC AT Maintenance Guide]]
* [[Recommended Tools]]

== References ==

* [https://www.dosdays.co.uk/computers/IBM%20PS2%20Model%2025/ibm_ps2_model_25.php IBM PS/2 Model 25 &mdash; DOS Days].
* [https://www.ardent-tool.com/qtechinfo/8525.html PS/2 Model 25 Quick Reference], Ardent Tool of Capitalism.
* [https://www.ardent-tool.com/qtechinfo/8525-286.html PS/2 Model 25 286 Quick Reference], Ardent Tool of Capitalism.
* IBM, ''IBM Personal System/2 Model 25 Hardware Maintenance Reference'' (SY31-9088 / SY31-9094).
* IBM, ''IBM Personal System/2 Reference Diskette'' (model-specific; for Model 25, the diskette is in the System Disk / Reference Diskette section of the kit).

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[[Category:IBM]]
[[Category:Maintenance Guides]]