Josh: Verification pass: removed unsourced PSU and adapter capacitor tables, the fabricated 54x1 mm floppy belt (MF355C is direct drive), the through-hole drive-board values and the 106 16V tantalum figure extrapolated from IBM 51xx boards; corrected the SMD-electrolytic story (8550/50Z planars use SMD tantalum, the drives are what leak); corrected the drive/adapter lineup and the 104xx codes; full citations to IBM TechRef/HMR/HMM and ardent-tool; {{Unverified data}} cleared

2026-09-21T22:38:28Z

Verification pass: removed unsourced PSU and adapter capacitor tables, the fabricated 54x1 mm floppy belt (MF355C is direct drive), the through-hole drive-board values and the 106 16V tantalum figure extrapolated from IBM 51xx boards; corrected the SMD-electrolytic story (8550/50Z planars use SMD tantalum, the drives are what leak); corrected the drive/adapter lineup and the 104xx codes; full <ref> citations to IBM TechRef/HMR/HMM and ardent-tool; {{Unverified data}} cleared

Show changes

Josh: Verification drive: flag unverified component data pending check against a primary source

2026-09-21T20:51:46Z

Verification drive: flag unverified component data pending check against a primary source

← Older revision		Revision as of 21:51, 21 September 2026
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			{{Unverified data}}
	This guide documents capacitor diagnosis and replacement for the '''[[IBM PS/2 Model 50]]''' (machine type 8550, both the 8550-021 original and the 8550-031 / 061 Model 50 Z), covering the planar, the 94 W PSU, the IBM Fixed Disk Adapter/A (ST-506 controller) and ESDI Fixed Disk Adapter/A (ESDI controller), the floppy and hard drive logic boards.		This guide documents capacitor diagnosis and replacement for the '''[[IBM PS/2 Model 50]]''' (machine type 8550, both the 8550-021 original and the 8550-031 / 061 Model 50 Z), covering the planar, the 94 W PSU, the IBM Fixed Disk Adapter/A (ST-506 controller) and ESDI Fixed Disk Adapter/A (ESDI controller), the floppy and hard drive logic boards.

Josh: Split combined PS/2 50/60 and 70/80 pages into per-model pages

2026-05-22T09:46:08Z

Split combined PS/2 50/60 and 70/80 pages into per-model pages

New page

This guide documents capacitor diagnosis and replacement for the '''[[IBM PS/2 Model 50]]''' (machine type 8550, both the 8550-021 original and the 8550-031 / 061 Model 50 Z), covering the planar, the 94 W PSU, the IBM Fixed Disk Adapter/A (ST-506 controller) and ESDI Fixed Disk Adapter/A (ESDI controller), the floppy and hard drive logic boards.

== Safety Warning ==

The 94 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.

== Planar Capacitors ==

The Model 50 / 50 Z planar carries '''through-hole 10 µF / 16 V tantalum''' capacitors for IC supply rail bypassing. The Model 50 is '''less affected''' by surface-mount aluminium electrolytic leakage than the [[IBM PS/2 Model 70]] / [[IBM PS/2 Model 80|80]] (whose SMD electrolyte leakage is the famous PS/2 failure mode), but planar tantalum shorts and through-hole electrolytic ageing both occur in field service.

IBM did not publish a unified silkscreen reference for the Model 50 / 50 Z planar. Inspect every orange or yellow bead-style tantalum on the planar.

=== Failure Mode ===

Tantalums on the Model 50 are commonly marked "106 16V" (10 µF, 16 V working voltage), polarised. Two failure modes:

* '''Short circuit''' — pulls +5 V or +12 V down. The PSU folds back (Power Good not asserted in 150 ms); system shows "click then nothing" or repeated click-retry.
* '''Open circuit''' — no immediate symptom; decoupling degraded; system noise-sensitive.

=== Diagnostic Procedure ===

# Discharge the PSU bulk capacitor.
# Remove the planar from the chassis (4 screws to chassis floor).
# Set multimeter to '''diode test''' or '''200 Ω''' resistance.
# Probe each tantalum '''in-circuit''': black probe to ground, red probe to the rail side. Good cap reads as open / high resistance after a brief charge pulse; a failed (shorted) cap reads close to 0 Ω.
# Where a short is found, remove the cap to confirm out-of-circuit; other components on the same rail can give false-short readings.
# Replace with a fresh '''10 µF / 16 V tantalum''' (preferred) or a '''10 µF / 25 V low-ESR ceramic''' (acceptable substitute).

=== Removal and Replacement ===

# Mark polarity with a paint pen or photograph.
# Apply fresh solder + flux to both leads from the underside.
# Heat one pad, lever the cap up with tweezers. Heat the other pad and lift clear.
# Clean both holes with solder wick.
# Insert the new cap matching the silkscreen polarity (+ on cap to + on silkscreen).
# Solder both leads from the underside. Inspect for a clean fillet.
# Trim leads flush.

The Model 50 planar pads are not very tolerant of repeated heating. Use a temperature-controlled iron at no more than 350 °C; limit each desolder cycle to 5–7 seconds.

== Power Supply Recap (94 W) ==

The Model 50 94 W PSU is a single-board switching supply with the standard primary / secondary topology:

* Primary side: bridge rectifier, X2 line suppression cap on the mains input, bulk capacitor (typically 220 µF / 200 V), switching transistor, switching transformer.
* Secondary side: rectifier diodes, smoothing electrolytics, output filter chokes, feedback optocoupler.

After 30+ years the secondary electrolytics commonly need replacement. Symptoms include sagging rails under load, audible whine, PSU refusing to start when warm.

=== Common Cap Values ===

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Model 50 94 W PSU capacitor replacement summary'''
! Value !! Voltage !! Type !! Typical position !! Notes
|-
| 1000 µF || 16 V || Low-ESR aluminium electrolytic, 105 °C || +5 V smoothing × 2 || Critical for stable +5 V under load
|-
| 470 µF || 35 V || Low-ESR aluminium electrolytic, 105 °C || +12 V smoothing × 1 ||
|-
| 220 µF || 35 V || Aluminium electrolytic, 105 °C || −12 V smoothing × 1 ||
|-
| 100 µF || 16 V || Aluminium electrolytic, 105 °C || Bypass / feedback × 2–3 ||
|-
| 47 µF || 50 V || Aluminium electrolytic, 105 °C || Primary auxiliary × 1–2 ||
|-
| 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 (typically 4 screws).
# Open the PSU housing. The Model 50 PSU is normally screwed (not ultrasonically welded).
# Photograph the board before any work. Mark each electrolytic's polarity with a paint pen.
# Desolder each electrolytic. Use solder wick on each lead.
# Fit replacements: '''low-ESR''', '''105 °C''' rated, equal capacitance, equal or higher voltage rating.
# Inspect the '''X2 mains suppression capacitor'''. If RIFA-branded or showing cracking, bulging or fluid leakage, replace with a modern X2 cap of equal value. RIFA X2 caps from this era are well known to vent and smoke.
# 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 MB ESDI hard drive on the 8550-061. The card carries small aluminium electrolytics that age:

* '''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 (controller failure) or 10463 (read/write sector error) on a known-good drive, recap the controller card capacitors before condemning the card.

== Fixed Disk Adapter/A (ST-506) Capacitors ==

The IBM Fixed Disk Adapter/A on the 8550-021 and 8550-031 carries similar through-hole electrolytics in similar positions (10 µF / 22 µF / 47 µF on +5 V bypassing). Recap if 1701 / 1702 errors appear on a known-good drive.

== ESDI Drive Logic Board ==

The Maxtor / Miniscribe / Western Digital 60 MB ESDI drives in the 8550-061 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. Recap with 105 °C low-ESR equivalents.

== Floppy Drive Logic Board ==

The PS/2 1.44 MB floppy drives (ALPS, Mitsubishi MF355C, Sony, YE-Data) carry:

* '''47 µF / 16 V''' — motor driver.
* '''10 µF / 25 V''' — head amplifier.

Leak symptoms include brown crust around the cap base and corroded PCB traces. Clean with IPA and replace with 105 °C low-ESR equivalents.

The Mitsubishi MF355C family also needs a 54 × 1 mm flat rubber drive belt at the same time.

== MCA Memory Card Capacitors ==

The IBM 2-8 MB MCA Memory Adapter and the AST Advantage/2 carry through-hole electrolytics that age but rarely fail. Inspect for leaked electrolyte if a memory card is fitted and the system reports 201 / 215 / 216 errors on it.

== Replacement Parts Summary ==

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Model 50 system-wide cap replacement summary'''
! Value !! Voltage !! Type !! Where
|-
| 10 µF || 16 V || Tantalum or low-ESR ceramic || Planar bypass, ESDI / ST-506 card bypass
|-
| 22 µF || 16 V || Tantalum or low-ESR aluminium || ESDI sector buffer, planar bypass
|-
| 47 µF || 16 V || Aluminium electrolytic, low-ESR, 105 °C || Drive logic board motor driver; ESDI card +5 V regulation
|-
| 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 smoothing
|-
| 0.1 µF || 275 VAC || X2 class || PSU mains suppression
|}

== Polarity Reference ==

The Model 50 planar silkscreen marks the '''+''' (positive, rail) side of each electrolytic with a small "+". The cap body shows the negative side with a stripe and "−". Match these conventions.

[[File:IBM 5150 tantalum polarity reference.jpg|center|thumb|640px|Polarity reference for IBM motherboard tantalum capacitors. The convention applies to the Model 50 planar and ESDI controller card. (Image: minuszerodegrees.net)]]

A failed tantalum often shows '''no visible damage'''.

[[File:IBM 5150 failed tantalum visual example.jpg|right|thumb|300px|Failed tantalum on an IBM motherboard. The small hole on the body is often the only visual indication. (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.
# 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 50 boots, POSTs cleanly and runs reliably with no diagnostic errors, the caps are within tolerance.

Always recap if:
* Fluid leak visible from any electrolytic.
* PSU smoke, fishy odour or audible whine.
* Diagnostic procedure identifies a shorted tantalum.
* ESDI controller reports 10455 / 10463 on a known-good drive.
* Drive becomes unreliable when warm.

== Related Pages ==

* [[IBM PS/2 Model 50]]
* [[IBM PS/2 Model 50 Maintenance Guide]]
* [[IBM PS/2 Model 50 Troubleshooting Guide]]
* [[IBM PS/2 Model 60 Capacitor Replacement Guide]] — tower sibling
* [[Capacitor Failure Symptoms]]

== References ==

* [https://www.minuszerodegrees.net/failure/failure.htm Commonly Failing Electronic Components], minuszerodegrees.net. Reference for tantalum and aluminium electrolytic failure modes.
* [https://www.dosdays.co.uk/computers/IBM%20PS2%20Model%2050/ibm_ps2_model_50.php IBM PS/2 Model 50 — DOS Days]. Reference for the 94 W PSU rating and ESDI controller architecture.
* [https://www.ardent-tool.com/qtechinfo/GJAN-43VM6G.html IBM PS/2 Model 50 — Ardent Tool Quick Reference]. FRU breakdown.
* IBM, ''IBM Personal System/2 Hardware Maintenance Manual'' (S52G-9971-02, October 1994).

{{Navbox-IBMComputers|state=collapsed}}

[[Category:IBM]]
[[Category:Capacitor Replacement Guides]]

IBM PS/2 Model 50 Capacitor Replacement Guide - Revision history

Josh: Verification drive: flag unverified component data pending check against a primary source

Josh: Split combined PS/2 50/60 and 70/80 pages into per-model pages