IBM PS/2 Model 70 Capacitor Replacement Guide

This guide documents capacitor diagnosis and replacement for the IBM PS/2 Model 70 (machine type 8570, all submodels), covering the planar, the 132 W PSU, the IBM ESDI Fixed Disk Adapter/A, the floppy and ESDI drive logic boards, and the Power Platform daughtercard (8570-B21 / B61).

The Model 70 planar is one of the two most famously-leaky PS/2 planars (the other being the Model 80). The surface-mount aluminium electrolytic capacitors on the planar are the defining failure mode of the system and the primary subject of this guide.

Safety Warning

The 132 W PSU contains mains-rectified bulk capacitors that hold a lethal charge after power-off. Before any PSU work:

Power off and unplug the mains lead.
Wait at least 30 seconds.
Discharge the bulk capacitor through a 1 kΩ / 5 W resistor.
Verify with a multimeter.

No CRT in the system unit.

The SMD Electrolyte Plague

The Model 70 planar (and the Model 80 planar) carries surface-mount aluminium electrolytic capacitors — typically 10 µF / 16 V and 47 µF / 16 V — scattered across both sides of the planar for IC supply rail bypassing. After 30+ years the rubber bung seal at the base of these SMD caps degrades and the corrosive aluminium electrolyte leaks onto the planar.

How the Leak Damages the Planar

The leaked electrolyte is mildly conductive (causing intermittent shorts on adjacent traces) and is corrosive (eating through copper traces and IC pins). The damage progresses on the following timeline once leakage starts:

Mild discolouration of the solder mask near the cap base (months).
Crusty deposits forming on adjacent vias and traces (1–2 years).
First intermittent POST faults — system passes cold, fails warm (2–3 years from start of leak).
Permanent loss of specific functional blocks (VGA, RTC, MCA bus controller) as traces corrode through (3–5 years).
Dead planar with corroded traces and lifted pads requiring extensive board-level repair (5+ years).

The leakage continues at room temperature with no power applied. Storage in a damp environment accelerates the damage.

Detection

Power off, unplug, discharge PSU.
Remove the planar from the chassis floor.
Inspect both sides under a USB microscope or magnifier with bright lighting. Look for:
- Brown or green residue near a SMD cap base.
- Discolouration of solder mask.
- Crusty deposits on adjacent vias.
- Lifted SMD cap (one end of the cap raised from the pad).
- Cracked solder mask near a cap.
- Any "fishy" smell from the planar.
Photograph every cap location for reference before any work.

If any SMD cap shows leakage, all of them should be replaced — the rest are at the same age and will leak within a few years.

Replacement Procedure

Photograph the planar from both sides at high resolution. Record every cap's location, value (top marking) and orientation.
Remove every SMD electrolytic with a hot-air rework station (set to 320 °C, 30 seconds per cap). Lift each cap straight up; do not pry sideways or you may lift pads.
Clean each pad with solder wick.
Wash the planar with isopropyl alcohol (90%+) and a soft brush. For severely contaminated planars, an ultrasonic cleaner (with planar-safe detergent) is the best option. Dry thoroughly (compressed air or a low-temp oven; ensure all moisture is gone before any rework).
Inspect every pad for damage. Where pads are lifted or traces corroded through, repair with 30 AWG enamelled wire or with a UV-cure trace repair kit.
Fit replacements: use through-hole 10 µF / 16 V tantalum (preferred) or SMD low-ESR aluminium 10 µF / 16 V or through-hole 47 µF / 16 V equivalents. Many restorers prefer to substitute through-hole tantalum for SMD aluminium because tantalum does not leak (though tantalum has its own failure mode — short circuit if exceeded on voltage).
Verify polarity on every cap before soldering.

Capacitor Locations on the Planar

The Model 70 planar carries approximately 36–40 SMD electrolytics in total (count varies by planar revision). Common locations:

Near the 80386DX socket — 10 µF / 16 V bypass.
Near the VGA chip and VRAM — 10 µF / 16 V bypass × 4–6.
Near the SIMM sockets — 47 µF / 16 V bulk × 2.
Near the MCA bus controller — 10 µF / 16 V bypass × 2–3.
Near the floppy controller — 10 µF / 16 V bypass × 1–2.
Near the RTC (DS1287) — 10 µF / 16 V bypass × 1.
Near the power-input filter — 47 µF / 16 V bulk × 2.

Planar Capacitors Summary

**Model 70 planar SMD electrolytic replacement summary**
Value	Voltage	Type	Quantity	Notes
10 µF	16 V	SMD aluminium (original) → tantalum / SMD low-ESR replacement	~30	IC bypass throughout
47 µF	16 V	SMD aluminium (original) → tantalum / SMD low-ESR replacement	~6–10	Bulk filtering near SIMM sockets and power input
22 µF	16 V	SMD aluminium (original) → SMD low-ESR replacement	~2–4	On some planar revisions

The exact count depends on planar revision. Inspect every SMD electrolytic on the planar before ordering replacements.

132 W PSU Recap

The Model 70 132 W PSU is a single-board switching supply. Common cap values:

**Model 70 132 W PSU capacitor replacement summary**
Value	Voltage	Type	Typical position	Notes
2200 µF	16 V	Low-ESR aluminium electrolytic, 105 °C	+5 V smoothing × 2	Critical for +5 V stability
1000 µF	16 V	Low-ESR aluminium electrolytic, 105 °C	+5 V auxiliary × 1
470 µF	35 V	Low-ESR aluminium electrolytic, 105 °C	+12 V smoothing × 1	Drive motor rail
220 µF	35 V	Aluminium electrolytic, 105 °C	−12 V smoothing × 1
100 µF	50 V	Aluminium electrolytic, 105 °C	Primary auxiliary × 2
100 µF	16 V	Aluminium electrolytic, 105 °C	Bypass / feedback × 2–3
47 µF	50 V	Aluminium electrolytic, 105 °C	Primary startup × 1
220 µF	200 V	Aluminium electrolytic, 105 °C	Primary bulk × 1	Lethal charge — discharge before work
0.1 µF	275 VAC	X2 class	Mains suppression × 1	Replace if RIFA-branded

Recap Procedure

Discharge the bulk capacitor; verify with a multimeter.
Remove the PSU from the chassis (4 screws).
Open the PSU housing. The Model 70 PSU shell is normally screwed.
Photograph the board. Mark each electrolytic's polarity.
Desolder each electrolytic with solder wick on each lead.
Fit low-ESR, 105 °C replacements, equal capacitance, equal or higher voltage rating.
Inspect the X2 mains suppression cap. If RIFA-branded or cracked / bulging, replace.
Reassemble. Verify rails on the bench with a multimeter under a 1 A resistive load before refitting to the chassis.

ESDI Fixed Disk Adapter/A Capacitors

The IBM ESDI Fixed Disk Adapter/A drives the 60 / 120 MB ESDI hard drive. Cap values are the same as on the Model 50 / 60 ESDI controller:

10 µF / 16 V (multiple positions) — IC bypassing.
22 µF / 16 V (one or two positions) — sector buffer power.
47 µF / 16 V (one position) — on-card 5 V regulation.

If the ESDI controller reports a 10455 or 10463 on a known-good drive, recap the controller card capacitors before condemning the card.

ESDI Drive Logic Board

The 60 / 120 MB ESDI drives carry their own drive logic board with aluminium electrolytics. Typical values:

47 µF / 16 V — near the spindle motor driver.
10 µF / 25 V — near the head-amp section.
22 µF / 16 V — sector buffer.

Symptoms of aged drive caps include the drive not spinning up reliably, CRC errors on reads, and the drive becoming unreliable when warm.

Floppy Drive Logic Board

Same as Model 50 / 60. The PS/2 1.44 MB floppy drives carry 47 µF / 16 V (motor driver) and 10 µF / 25 V (head amplifier) electrolytics. Recap with 105 °C low-ESR equivalents. The Mitsubishi MF355C family also needs a 54 × 1 mm flat rubber drive belt at the same time.

Power Platform Daughtercard Capacitors (8570-B21 / B61)

The Power Platform 80486 daughtercard carries its own +5 V regulator and a small number of decoupling caps. Typical values:

10 µF / 16 V tantalum × 4–6 — IC bypass around the 80486.
47 µF / 16 V aluminium electrolytic × 1 — +5 V smoothing on the regulator output.
22 µF / 16 V tantalum × 2 — clock generator bypass.

If a Power Platform-equipped Model 70 fails to POST or shows 80486-specific faults, inspect the daughtercard for cap leakage and recap if needed.

Replacement Parts Summary

**Model 70 system-wide cap replacement summary**
Value	Voltage	Type	Where
10 µF	16 V	Tantalum (preferred) or SMD low-ESR	Planar SMD positions, ESDI / Power Platform card bypass
22 µF	16 V	Tantalum or SMD low-ESR	Planar SMD on some revisions, ESDI sector buffer, Power Platform clock generator
47 µF	16 V	Tantalum or SMD low-ESR	Planar SMD bulk, drive logic board, Power Platform regulator output
100 µF	16 V	Aluminium electrolytic, low-ESR, 105 °C	PSU bypass / feedback
220 µF	35 V	Aluminium electrolytic, 105 °C	PSU −12 V smoothing
220 µF	200 V	Aluminium electrolytic, 105 °C	PSU primary bulk (lethal-charge component)
470 µF	35 V	Low-ESR aluminium electrolytic, 105 °C	PSU +12 V smoothing
1000 µF	16 V	Low-ESR aluminium electrolytic, 105 °C	PSU +5 V auxiliary
2200 µF	16 V	Low-ESR aluminium electrolytic, 105 °C	PSU +5 V smoothing × 2
0.1 µF	275 VAC	X2 class	PSU mains suppression
100 µF	50 V	Aluminium electrolytic, 105 °C	PSU primary auxiliary
47 µF	50 V	Aluminium electrolytic, 105 °C	PSU primary startup

Polarity Reference

A failed tantalum often shows no visible damage.

Failed tantalum on an IBM motherboard. (Image: minuszerodegrees.net)

Post-Recap Verification

Bench-test the planar with a known-good PSU and no peripherals.
Verify rails at the planar power connector.
Refit; boot the Reference Diskette.
Run Auto Configuration if any MCA cards changed.
Run Advanced Diagnostics (Ctrl-A) and walk through each test, paying special attention to:
- Protected-mode test (104-class fault → SMD cap near CPU).
- Memory test (full pass).
- Onboard VGA test.
- ESDI controller test.
Run a memory test pass for several hours under DOS to confirm planar memory reliability.

If any test fails after a recap, re-inspect the polarity of every replaced cap before suspecting another fault — reversed polarity is the most common error.

When Not to Recap

If the Model 70 boots, POSTs cleanly and runs reliably with no diagnostic errors and visual inspection shows no SMD leakage, the caps are within tolerance and continued use is acceptable. However, on a 30+ year old Model 70, this combination is increasingly rare; most surviving 8570s either have visible SMD leakage or are showing the first intermittent faults of impending leakage.

Always recap if:

Any SMD electrolyte leak visible on the planar.
Fluid leak visible from any electrolytic in the PSU or drive logic boards.
PSU smoke, fishy odour or audible whine.
ESDI controller reports 10455 / 10463 on a known-good drive.
Drive becomes unreliable when warm.
System passes POST cold but fails when warm.

Related Pages

IBM PS/2 Model 70
IBM PS/2 Model 70 Maintenance Guide
IBM PS/2 Model 70 Troubleshooting Guide
IBM PS/2 Model 80 Capacitor Replacement Guide — tower sibling, same planar family of SMD electrolytics
Capacitor Failure Symptoms

References

Commonly Failing Electronic Components, minuszerodegrees.net. Reference for SMD aluminium electrolyte leakage on the Model 70 / 80 planars.
IBM PS/2 Model 70 — DOS Days. Reference for the 132 W PSU rating and ESDI controller architecture.
IBM PS/2 Model 70 — Ardent Tool Quick Reference. FRU breakdown.
IBM, IBM Personal System/2 Hardware Maintenance Manual (S52G-9971-02, October 1994).