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IBM PS/2 Model 25 Capacitor Replacement Guide

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The IBM PS/2 Model 25 (machine type 8525) has electrolytic capacitors in its power supply and display electronics, which share the display assembly, and on the logic boards of its diskette and fixed-disk drives. IBM's documents give the supply only as ratings and limits,[1] and IBM's service manual treats the whole display assembly, supply included, as one replaceable part.[2] Neither document gives a capacitor value for the system board, the supply or the display, and the Technical Reference has no logic diagrams, so the values have to be read from the parts fitted. The drive boards are the best-documented recap on this machine, from community teardowns.

Safety

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  • The power supply and the CRT circuits are in the display assembly. Unplug the machine and follow the CRT Discharge Procedure before working inside the display assembly.
  • The supply sends 115 V DC at 450 mA to the display on colour machines, and +39 V DC at 800 mA on monochrome machines.[1]
  • Set the rear voltage switch to the correct range before plugging the machine in.[1][3]

Diskette drive boards

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The PS/2 diskette drives have no dust shutter, and their electrolytic capacitors leak and corrode the drive's logic board.[4][5] William Walsh's advice on ardent-tool is to plan on replacing every surface-mount electrolytic on the drive's board and cleaning up the corrosion.[6] PS/2 drives were made mostly by Alps and Mitsubishi, with Sony and YE-Data less common, and the capacitors differ by maker.[5]

Published PS/2 diskette drive recaps (community teardowns)
Drive Capacitors Source
Alps 4.7 µF 25 V (×1) and 10 µF 16 V (×1), the two that usually leak; three non-polarised 4.7 µF 16 V Mac84[7]
Mitsubishi Two surface-mount electrolytics on the drive board, replaced with 10 µF through-hole parts soldered to the pads ibmfiles.com[8]

The three 4.7 µF parts on the Alps board are non-polarised and should not be replaced with polarised capacitors.[7] These are repair notes from individual drives, not manufacturer data; check the parts on your own board.

Mitsubishi's specification for its MF355C 3.5-inch drive describes a brushless DC motor that turns the spindle directly at 300 rpm, so a drive of that design has no belt to replace.[9] See Floppy Drive Repair.

Power supply and display assembly

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IBM's ratings for the 8086 machines are 113 W (colour) and 90 W (monochrome), and 124.5 W for the Model 25 286 and SX.[1][10][11] IBM's parts list gives the display assembly, which includes the supply, as separate FRUs for monochrome and colour, for the 8086, 286 and SX machines, and for different mains regions.[2] Photograph the board and record each capacitor's value, voltage and temperature rating before removing anything, and fit parts of the same capacitance and at least the same voltage rating.

IBM lists the display assembly as a possible FRU for no-beep faults and for diskette (6xx) and fixed-disk (17xx) errors, alongside the power supply.[2] Check the supply outputs at J7 before recapping and again afterwards.

The line-suppression capacitors on the mains input of supplies of this era can rupture; RIFA parts do so with smoke and a strong smell. Replace them with parts of the same capacitance, at least the same AC voltage rating, and the same safety class (X2 for X2, Y2 for Y2).[12]

System board

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The Model 25 Technical Reference has no logic diagrams or parts list for the system board.[13] On IBM's 51xx boards, minuszerodegrees.net reports that tantalum capacitors fail short or open, that a short overloads the supply so it will not run, and that failed tantalums often show no damage. It also warns that not every orange capacitor on an IBM board is a tantalum.[12] On the Model 25 a shorted part holds Power Good down: ardent-tool's 8525/8530 power page says Power Good should read +3.5 to +5 V, and if it does not, remove adapters, cables and extra memory and refit them one at a time.[3]

To find a short on the system board, switch off and measure the resistance from each rail to ground at J7 (+5 V on pins 10 to 12, +12 V on pin 3, −5 V on pin 9, −12 V on pin 4, ground on pins 2 and 5 to 8).[1] A reading near zero points to a shorted part on that rail; lift the capacitors on the rail one at a time to find it.

A failed tantalum on an IBM 51xx system board. The small black hole is the only sign of failure. (Image: minuszerodegrees.net)

After recapping

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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
  • Measure at J7 with the machine running; pin 1 is towards the rear.[3]
  • Power Good (J7-1) should read +3.5 to +5 V.[3]
  • For the 8086 machine, IBM's ripple limits are 100 mV peak to peak on +5 V, 50 mV on −5 V and 120 mV on ±12 V.[1]
  • Run Advanced Diagnostics from the Reference or Starter Diskette (Ctrl-A at the main menu), powering on with the diskette in the drive.[14]

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 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). Symptom-to-FRU Index (AT Bus), pp. 12 to 17; Model 25 parts, pp. 223 to 225.
  3. ↑ 3.0 3.1 3.2 3.3 Ardent Tool of Capitalism, "8525 / 8530 Power". J7 measurement limits for the Models 25, 25 SX and 25-286, and Power Good reading.
  4. ↑ 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.
  5. ↑ 5.0 5.1 DOS Days, "IBM PS/2 Model 25". Community reference: drive makers, dust shutter, capacitor leakage and corrosion.
  6. ↑ Ardent Tool of Capitalism, "8525 Planar", based on content by William R. Walsh. Diskette drive notes.
  7. ↑ 7.0 7.1 Mac84, "IBM PS/2 Floppy Disk Drive Recapping Guide (ALPS drive)". Community repair guide with a location diagram and Mouser part numbers.
  8. ↑ ibmfiles.com, "PS/2 Surface Mount Capacitors". Community repair notes for Mitsubishi PS/2 diskette drives.
  9. ↑ 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.
  10. ↑ IBM, Announcement Letter 190-082, "IBM Personal System/2 Model 25 286", 10 May 1990, transcribed at ardent-tool.com.
  11. ↑ IBM, Announcement Letter 192-019, "IBM Personal System/2 Model 25 SX", 21 January 1992, transcribed at ardent-tool.com.
  12. ↑ 12.0 12.1 minuszerodegrees.net, "Commonly Failing Electronic Components (in vintage computers)". Community reference based on IBM 51xx machines: tantalum failure modes and line-suppression capacitor replacement rules.
  13. ↑ IBM, IBM Personal System/2 Model 25 Technical Reference, June 1987, contents, pp. v to viii: sections on the system board, coprocessor, power supply, keyboard, BIOS, instruction set and characters, with block diagrams but no logic diagrams or parts list.
  14. ↑ Ardent Tool of Capitalism, "PS/2 Error Codes".
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