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IBM PS/2 Model 25 Maintenance Guide

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IBM PS/2 Model 25 (8525). The system unit, display and power supply share one case.

The IBM PS/2 Model 25 (machine type 8525) puts the system board, drives, power supply and a 12-inch display in one case. The power supply is inside the display assembly,[1] so routine work on the system board and drives means working next to the CRT. Fault diagnosis is in the IBM PS/2 Model 25 Troubleshooting Guide.

Safety

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  • The display assembly contains the CRT and the mains power supply. Unplug the machine before opening it, and follow the CRT Discharge Procedure before working near the tube or its anode lead.
  • On a colour machine the supply sends 450 mA at 115 V DC to the display assembly; on a monochrome machine, 800 mA at +39 V DC.[1]
  • The input voltage is set by a switch on the rear. Set it to the 100–125 V or 200–240 V range before the power cord goes into an outlet.[1][2]

Identifying the machine

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The type, model and serial number are on a label under the lower left-hand corner of the display bezel.[3]

Model 25 variants
Machine Models System board FRU (IBM, 1994) Clock and battery
Model 25 (8086) 001, G01, 004, G04, L01, L04 96F7390 None
Model 25 286 006, G06, 036, G36 96F7435 DS1287 clock module at ZM35, FRU 8509237
Model 25 SX K00, K01, L02 42G2802 (1 MB), 42G2803 (4 MB) CR2032 at B1, FRU 33F8354

Sources: IBM's maintenance manual for the FRUs and ardent-tool for the board positions.[4][5][6] System board designators for all three boards are on the IBM PS/2 Model 25 page.

Opening the system unit

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The Model 25 opens from underneath:[7]

  1. Switch off the computer and everything attached to it.
  2. Remove any diskettes.
  3. Unlock the keylock if there is one.
  4. Unplug all power cords and note which cable goes where at the rear.
  5. Lay a thin towel or cloth over the screen, then place the system unit face down on a flat surface.
  6. Remove the two cover screws.
  7. Lift the bottom cover about 25 mm (1 inch).
  8. Take the power and display cables out of the cable hook as you lower the cover, so they are not pulled out of their connectors.
  9. Swing the bottom cover out and down.

To reassemble, reverse the steps. Keep the power and display cables clear of everything in the bottom half of the case, and put them back in the cable hook. The hook often falls off because its adhesive pad has dried out; glue it back to the pad before refitting the cables.[7]

Power supply checks

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The supply feeds the system board through the 12-pin connector J7: pin 1 Power Good, pins 2 and 5 to 8 ground, pin 3 +12 V, pin 4 −12 V, pin 9 −5 V, pins 10 to 12 +5 V.[1][4] Measure with the machine switched on; pin 1 is towards the rear.[2]

Models 25, 25 SX and 25-286, service limits at J7
Rail − lead + lead Minimum Maximum
+5 V J7-5 J7-10 +4.8 V +5.2 V
−5 V (leads reversed) J7-9 J7-5 +4.5 V +5.4 V
+12 V J7-5 J7-3 +11.5 V +12.6 V
−12 V (leads reversed) J7-4 J7-8 +10.8 V +12.9 V

Power Good should read +3.5 to +5 V on a working machine. If it does not, remove adapters, cables and extra memory and try again, then refit them one at a time.[2]

For the 8086 machine, IBM's regulation limits are +5 % to −3 % on +5 V and +12 V and +10 % to −8 % on −5 V and −12 V, with ripple limits of 100 mV peak to peak on +5 V, 50 mV on −5 V and 120 mV on ±12 V. The supply's under-voltage sense levels are much lower (+3.2 V, −2.4 V, +10.5 V and −8.6 V), so a rail can be well out of limits without the supply shutting down; measure against the service limits above.[1] The system board and drives take about 4 A of the 7 A available on +5 V, leaving about 3 A for the two slots.[1]

A Model 30 supply does not fit: the power connectors differ.[8][9] IBM's replacement part for the supply is the whole display assembly.[4]

Clock and battery

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  • Model 25 (8086): no real-time clock and no battery. The BIOS reports "No Real-time clock", and the date and time must be set at each start.[10][11]
  • Model 25 286: a Dallas DS1287 clock module at ZM35, with its cell sealed inside. Ardent-tool lists the DS12887, DS12887+ and 'A'-suffix parts as replacements, and warns against the DS12C887.[5][11]
  • Model 25 SX: a CR2032 at B1. It should read 2.5 to 3.7 V out of the holder; replace it below 2.5 V, because low voltage can corrupt CMOS bits.[6][11] On this board U10 is the 8042 keyboard controller.[6]

After replacing the battery or module, run the configuration program from the Reference Diskette and reset the date and time. A 162 straight after adding or removing a device means the configuration has changed; IBM's first action is to run setup or Automatic Configuration.[12] See Battery Explosion, Capacitor or Corrosion Damage for cleaning up after a leaking cell.

Connectors

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  • On the 8086 machine the keyboard and pointing-device interfaces are the same, so either device can plug into either 6-pin connector.[13] The 286 and SX boards use an 8042 controller.[5][6]
  • The ports are fused: F1 and F2 on the Type 1 8086 board, one F1 on the Type 2, 286 and SX boards. For a 305 (keyboard voltage error) IBM's first FRU is the keyboard port fuse.[8][14]
  • The earphone connector takes a 1/4-inch plug; inserting one disables the internal beeper. IBM recommends a mono earphone of 15 to 35 Ω, and some up to 100 Ω work.[15] It is not a line output; ardent-tool suggests a 33 kΩ series resistor to bring it down to line level.[3] On the 286 the jack is on a separate board (P/N 33F4433, 57F2165 or 64F0184).[5]

Drives

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Drive power and data share one cable from the system board. On the 40-pin diskette connector J12, pin 38 is +5 V and pin 40 is +12 V. On the 44-pin fixed-disk connector J11, pins 30 and 32 are +5 V and pins 40, 42 and 44 are +12 V.[15] Because supply pins share these connectors, check that pin 1 lines up at both ends before switching on.

  • The 8086 machines shipped with 720 KB drives and the 286 and SX with 1.44 MB drives.[16][17] The 8086 machine also works with some 1.44 MB drives, mainly Sony.[8]
  • The drives have no dust shutter, and their electrolytic capacitors leak and corrode the drive's logic board.[9][18] See IBM PS/2 Model 25 Capacitor Replacement Guide and Floppy Drive Repair.
  • Mitsubishi was one of the PS/2 drive makers.[19] Its MF355C specification describes a direct-drive brushless spindle motor, so a drive of that design has no belt.[20]
  • IBM's error codes 662 and 604 mean the wrong drive type, and 663 the wrong media type.[12]
  • On the 8086 and 286 machines the fixed disk connects to J11, a dedicated I/O channel with the controller on the drive side. The Model 25 SX has a 40-pin IDE connector at J10 and a 4-pin drive power connector at J14.[15][6] See Hard Drive Maintenance and Repair.

Bolt-down feature

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The system unit has three holes for bolting it to a desk. Using only the centre hole lets it rotate and tilt; using the two outer holes lets it tilt but not rotate.[3]

Things that are not faults

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  • The 8086 Model 25 forgetting the date and time at every start.[10]
  • False errors, or a false report that a power-on password is set, after soft-booting the Reference Diskette. Power on with the diskette in the drive instead.[21]
  • A Model 25 SX with a 20 MHz 386SX running at 16 MHz. Most of them left the factory that way.[6]

Tools

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  • Screwdriver, anti-static strap and a digital multimeter.
  • A cloth to lay the display on.
  • A CRT discharge tool; see CRT Discharge Procedure.
  • The Reference or Starter Diskette for the model, or the Advanced Diagnostics diskette for the SX.[6]

See Recommended Tools.

References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 IBM, IBM Personal System/2 Model 25 Technical Reference, June 1987, section 3, "Power Supply", pp. 3-3 to 3-6, hosted as IBM PS2 Model 25 Technical Reference (June 1987).
  2. ↑ 2.0 2.1 2.2 Ardent Tool of Capitalism, "8525 / 8530 Power". J7 measurement limits for the Models 25, 25 SX and 25-286, Power Good reading, and the voltage switch warning.
  3. ↑ 3.0 3.1 3.2 Ardent Tool of Capitalism, "8525 Common Devices", based on content by William R. Walsh. Label location, earphone connector and bolt-down feature.
  4. ↑ 4.0 4.1 4.2 IBM, Personal System/2 Hardware Maintenance Manual, S52G-9971-02, October 1994, hosted as IBM Personal System 2 Hardware Maintenance Manual (S52G-9971-02, October 1994). Power supply connector, p. 138; system board locations, pp. 185 to 188; Model 25 parts, pp. 223 to 225.
  5. ↑ 5.0 5.1 5.2 5.3 Ardent Tool of Capitalism, "8525-286 Planar".
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 Ardent Tool of Capitalism, "8525 SX Planar", based on content by William R. Walsh.
  7. ↑ 7.0 7.1 Ardent Tool of Capitalism, "Opening The Model 25", based on content by William R. Walsh.
  8. ↑ 8.0 8.1 8.2 Ardent Tool of Capitalism, "8525 Planar", based on content by William R. Walsh. 8086 board designators, power connector note, diskette drive notes.
  9. ↑ 9.0 9.1 DOS Days, "IBM PS/2 Model 25". Community reference.
  10. ↑ 10.0 10.1 IBM, IBM Personal System/2 Model 25 Technical Reference, June 1987, p. 5-45, hosted as IBM PS2 Model 25 Technical Reference (June 1987).
  11. ↑ 11.0 11.1 11.2 Ardent Tool of Capitalism, "PS/2 RTC/CMOS Batteries", based on content by Bob Eager and Peter H. Wendt.
  12. ↑ 12.0 12.1 IBM, Personal System/2 Hardware Maintenance Manual, S52G-9971-02, October 1994, Symptom-to-FRU Index (AT Bus), pp. 12 to 17, hosted as IBM Personal System 2 Hardware Maintenance Manual (S52G-9971-02, October 1994).
  13. ↑ IBM, IBM Personal System/2 Model 25 Technical Reference, June 1987, p. 1-8, hosted as IBM PS2 Model 25 Technical Reference (June 1987).
  14. ↑ Ardent Tool of Capitalism, "Error Codes 300 - 999".
  15. ↑ 15.0 15.1 15.2 IBM, IBM Personal System/2 Model 25 Technical Reference, June 1987, "Earphone Connector", p. 1-125, and "Diskette Drive Connector" and "Fixed Disk Connector", pp. 1-104 to 1-104.2, hosted as IBM PS2 Model 25 Technical Reference (June 1987).
  16. ↑ IBM, Announcement Letter 187-148, "IBM 8525, Personal System/2 Model 25", 4 August 1987, hosted as File:IBM Announcement Letter 187-148 PS2 Model 25 (August 1987).pdf.
  17. ↑ IBM, Announcement Letter 190-082, 10 May 1990, transcribed at ardent-tool.com.
  18. ↑ Ardent Tool of Capitalism, "60, 65 SX, and 80 – Common Devices". Peter Wendt on ALPS and Mitsubishi PS/2 diskette drives: both collect dust because no shield covers the slot, and the Mitsubishi drives have capacitors that fail and cause 6xx errors.
  19. ↑ ibmfiles.com, "PS/2 Surface Mount Capacitors". Community repair notes for Mitsubishi PS/2 diskette drives.
  20. ↑ Mitsubishi Electric, 3.5 inch Flexible Disk Drive Standard Specifications, MF355C, UGD-0527A, February 1987, section 1.1 (p. 3) and section 2.3, "Rotation Mechanism" (p. 9): brushless DC direct-drive spindle motor at 300 rpm. Hosted as File:Mitsubishi MF355C 3.5 inch Diskette Drive Specifications (UGD-0527A, Feb 1987).pdf.
  21. ↑ Ardent Tool of Capitalism, "PS/2 Error Codes".
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