IBM PS/2 Model 25 Capacitor Replacement Guide

This guide documents capacitor diagnosis and replacement procedures for the IBM PS/2 Model 25 (machine type 8525) planar, the integrated 90 W / 115 W PSU, the integrated 12" CRT subsystem, the PS/2 single-cable floppy and hard-disk drive logic boards, and ISA expansion cards.

The Model 25 uses three distinct families of capacitors that age differently:

Tantalum capacitors (typically 10 µF / 16 V, marked "106 16V") on the planar +5 V and +12 V rails. Fail short-circuit; pull rails into PSU fold-back.
Aluminium electrolytic capacitors in the PSU primary and secondary, in the CRT yoke driver, in the drive logic boards, and (smaller values) on ISA cards. Dry out, increase ESR, leak electrolyte. Symptoms vary from PSU sag to drive failure to CRT focus drift.
X2-class line suppression capacitors on the PSU primary. Crack and leak fluid; produce mains smoke and a strong fishy odour.

Safety Warning

Repeat from IBM PS/2 Model 25 Maintenance Guide: the Model 25 contains an integrated CRT. Lethal voltages on the CRT anode (15–20 kV) and on the PSU bulk capacitor (200 V or higher). Wait 30 minutes after power-off, then discharge the CRT anode through a 1 MΩ / 10 W bleed resistor, and discharge the PSU bulk capacitor through a 1 kΩ / 5 W resistor, before beginning any work inside the chassis. Verify with a multimeter that the discharge is complete.

Tools and Materials

Temperature-controlled soldering iron, fine chisel or flat tip, 350–380 °C.
Solder wick (3 mm) and a desoldering pump or hot-air vacuum.
60/40 leaded solder or lead-free with the appropriate flux for old joints.
10 µF / 16 V tantalum replacement caps (or low-ESR 25 V ceramics).
Aluminium electrolytic replacements: low-ESR, 105 °C, equal or higher voltage rating. Common Model 25 values:
- 1000 µF / 16 V (PSU secondary +5 V)
- 470 µF / 35 V (PSU secondary +12 V)
- 220 µF / 200 V or 220 µF / 250 V (PSU bulk capacitor)
- 47 µF / 16 V (drive logic boards)
- 10 µF / 16 V (planar bypass, replaces tantalums in some restoration approaches)
X2 line suppression capacitor replacements (typically 0.1 µF / 275 VAC, sometimes 0.22 µF).
Anti-static strap connected to chassis ground.
Digital multimeter with diode-test or 200 Ω resistance range.
1 MΩ / 10 W bleed resistor for the CRT anode.
1 kΩ / 5 W resistor for the PSU bulk capacitor.
A paint pen and a phone camera for marking polarity before removal.

Planar Capacitors

The Model 25 8086 planar carries fewer tantalums than a 5170 motherboard, because the PSU rails are routed through the planar to the drives via the single-cable interface, and a number of the bulk-decoupling positions are populated as ceramic chip capacitors rather than as tantalums. However, the tantalum positions are present in the +5 V and +12 V planar input regions and on the riser card.

This guide does not enumerate each tantalum's reference designator on the Model 25 planar because IBM did not publish a unified silkscreen reference and the 8086, 286 and SX planars are all electrically different. Treat every orange or yellow bead tantalum on the planar and on the riser card as a candidate for inspection if the system will not start.

Failure Mode

The tantalums on the planar are commonly marked "106 16V" (10 µF, 16 V working voltage). They are polarised. Failure modes:

Short circuit (most common): pulls +5 V or +12 V down. The PSU detects the over-current and folds back. Symptom: chassis fan may spin briefly, audible click from the PSU, then nothing. Or continuous click cycle as the PSU retries.
Open circuit: no immediate symptom because decoupling is partly redundant, but the planar may become noise-sensitive (random 201 errors, random hangs).

Diagnostic Procedure

Discharge the CRT and PSU bulk capacitor.
Remove the planar from the chassis (Model 25 286: planar is held by 3–4 screws to the bottom of the chassis; Model 25 SX: 4–5 screws).
Set a multimeter to diode test or 200 Ω resistance.
Probe each visible tantalum in-circuit: black probe to ground (any chassis screw), red probe to the rail side of the cap. A good tantalum reads as an open or high resistance after a brief charge pulse. A failed (shorted) tantalum reads close to 0 Ω.
If a candidate short is found, remove the cap to confirm. Other planar components on the same rail can give false shorts.
Replace the failed cap with a fresh 10 µF / 16 V tantalum of equal or higher voltage rating. Match the polarity orientation.

Removal and Replacement

Mark the polarity of the failed cap with a paint pen or photograph the cap.
Apply fresh solder + flux to both leads from the underside of the planar.
Heat one pad and lever the cap up on that side with tweezers. Heat the other pad and lift the cap clear.
Clean both holes with solder wick.
Insert the new tantalum: + marking on the cap to the + marking on the silkscreen. The bar on the cap body indicates the negative leg.
Solder both leads from the underside. Inspect for clean fillets.
Trim leads flush.

Power Supply Recap

The Model 25 PSU is integrated into the chassis. It is rated 90 W (8525-001/C02) or 115 W (8525-004/C05) and uses 110/220 VAC switchable input. Internally it contains:

Primary side: mains rectifier (typically a 1N400x bridge), X2 line suppression capacitor on the mains input, bulk capacitor (220 µF / 200 V or 220 µF / 250 V), switching transistor, and switching transformer.
Secondary side: rectifier diodes, smoothing electrolytics (1000 µF / 16 V on +5 V; 470 µF / 35 V on +12 V; smaller values on −5 V and −12 V), output filter chokes, and a feedback optocoupler.

After 30+ years, the secondary electrolytics typically have aged enough to need replacement. Symptoms:

Reduced or sagging rails under load.
Audible whine from the PSU.
System resets under heavy disk activity.
PSU refuses to start when hot.

Recap procedure:

Discharge the CRT and PSU bulk capacitor. Verify discharge with a multimeter.
Remove the PSU from the chassis (typically 4–5 screws).
Open the PSU housing. Many IBM PSUs are screwed; some are ultrasonically welded and require careful seam cutting.
Photograph the PSU board before any work. Mark polarity of each electrolytic.
Identify the bulk capacitor (largest, primary side) and verify it is discharged.
Desolder each electrolytic, beginning with the secondary side. Use solder wick on each lead.
Fit replacement electrolytics of equal capacitance, equal or higher voltage rating, and equal or higher temperature rating (105 °C preferred over 85 °C).
If the X2 mains suppression cap shows any cracking, bulging, or fluid leakage, replace it. RIFA-branded X2 caps in IBM PSUs from this era have a well-documented tendency to crack, smoke, and emit a strong odour.
Reassemble the PSU. If the housing was welded, use small screws to refit it.
Verify each rail with a multimeter under no load before refitting to the chassis. Then verify under a resistive dummy load (e.g. 1 A on +5 V, 0.5 A on +12 V) before refitting the planar.

If the PSU has been showing intermittent symptoms before recap, also inspect the optocoupler in the feedback path. A weak optocoupler can cause regulation drift even after the electrolytics are replaced.

Integrated CRT Subsystem

The integrated 12" CRT has its own driver board (mounted near the CRT neck) and electrolytic capacitors that age. Symptoms:

Loss of focus or focus drift after warm-up.
Geometric distortion (pincushion, trapezoidal).
Brightness drop.
Flyback transformer whine.

CRT driver boards typically use:

1 µF, 2.2 µF, 4.7 µF or 10 µF at 50–100 V working voltage on the yoke driver outputs.
47 µF at 100 V or 200 V on the EHT driver.

CRT board recap is best left to enthusiasts experienced with CRT service. Mishandling the EHT lead, the focus pot, or the screen pot can result in catastrophic CRT failure or personal injury. If you are not familiar with CRT service, replace the entire system unit or seek out a CRT specialist.

Drive Logic Boards

PS/2 floppy drives (ALPS, Mitsubishi, Sony, YE-Data) carry a small number of aluminium electrolytics on the drive logic board, typically:

47 µF / 16 V near the motor driver (most common to leak).
10 µF / 25 V near the read-write amplifier.

Symptoms of drive board capacitor failure:

Drive does not seek.
Drive seeks but cannot read or write reliably.
Slow seek (timing capacitor leaked).
Drive belt slip (rubber belt has aged separately; not a capacitor issue).

Procedure: remove the drive from the chassis, remove the drive logic board, inspect for leaked electrolyte (brown residue on the PCB or surrounding traces), clean with isopropyl alcohol, replace each electrolytic with a low-ESR equivalent.

The Mitsubishi MF355C / F floppy is particularly prone to drive belt slippage; replace the belt at the same time as the recap.

ISA Card Capacitors

ISA cards in the Model 25 (mostly memory expansion cards, the optional Token Ring or Ethernet adapter, or aftermarket VGA cards on the 8086) carry small numbers of tantalums and electrolytics. The standard 10 µF / 16 V tantalum failure mode applies. If the planar tests clean off-system but the system will not start with the cards installed, pull each card and re-test.

Polarity Reference

The polarity-marking convention on IBM 5xxx and PS/2 motherboards is consistent: the + marking on the silkscreen is the positive (rail) side; the bar / minus marking on the cap body is the negative side. Aluminium electrolytics show the negative side with a stripe; tantalums show the positive side with a small "+" near one lead.

Polarity reference for IBM motherboard tantalum capacitors. The same convention applies to the Model 25 planar. (Image: minuszerodegrees.net)

A failed tantalum often shows no visible damage. Do not rely on visual inspection alone.

A failed tantalum capacitor on an IBM 5xxx motherboard. The small black hole on the body is the only visual indication of failure — in most cases there is no visual sign at all. (Image: minuszerodegrees.net)

The recommended through-hole soldering technique (SNCTOL — "solder needs cleaning to other leg") is shown below.

SNCTOL soldering technique for through-hole component removal on IBM boards. (Diagram: minuszerodegrees.net)

Post-Recap Verification

After any capacitor work, do the following before fitting the planar (or PSU) back into the chassis:

Bench-test the planar with a known-good PSU and no peripherals. Verify each rail at the planar power connector.
Refit the planar in the chassis.
Boot the Reference Diskette and run Advanced Diagnostics (Ctrl-A). Run the full battery of tests.
If any test fails, return to the relevant section above and re-inspect.

The expected outcome of a successful planar recap is: PSU comes up immediately, system POSTs to "1 short beep," Reference Diskette boots, Advanced Diagnostics runs all tests without errors.

When Not to Recap

A blanket recap is not always necessary. If the Model 25 boots, POSTs cleanly, and Advanced Diagnostics passes:

Caps are working within tolerance.
Replacement risks introducing solder bridges, lifted pads (a known Model 25 planar risk with the older masks), and other restoration faults.
Leave the planar alone until a specific fault appears.

Always recap if:

Fluid leak visible from any electrolytic.
PSU smoke or fishy odour.
Visible cap bulge or top vent.
Diagnostic procedure identifies a shorted tantalum.

Related Pages

References

Commonly Failing Electronic Components, minuszerodegrees.net. Reference for tantalum failure modes.
IBM PS/2 Model 25 — DOS Days. Reference for the floppy drive logic-board capacitor failure pattern and PSU rating.
PS/2 Error Codes, Ardent Tool of Capitalism. Reference for the "Power Good in 150 ms" PSU spec and shorted-component dead-system isolation.
IBM, IBM Personal System/2 Hardware Maintenance Reference.