IBM PS/2 Model 50 Capacitor Replacement Guide

This guide covers capacitor diagnosis and replacement on the IBM PS/2 Model 50 (machine type 8550) — the original 8550-021 and the Model 50 Z, 8550-031 and 8550-061. It covers the planar, the 94 W power supply, the diskette drive and the fixed-disk drives.

Safety Warning

The 94 W supply is a mains-rectified switching supply. Its primary side sits at mains potential and its reservoir capacitors hold a charge that can kill after the machine is unplugged. Recapping it is a reasonable thing to do, but treat it as live-mains work:

1. Power off and unplug the mains lead.
2. Discharge the primary reservoir capacitor through a 1 kΩ / 5 W resistor across its terminals, then confirm with a meter that it reads near zero before touching anything.
3. Work one-handed where practical, and do not probe a powered primary side unless you are using an isolation transformer.

There is no CRT in the system unit; the matching IBM 8503, 8512, 8513 and 8514 displays contain their own tube and supply and are serviced separately. See CRT Discharge Procedure if you are working on the display.

The supply is universal and voltage-sensing — there is no voltage selector switch to set wrongly.

What Actually Fails on a Model 50

This is worth stating plainly, because the Model 50 is often lumped in with the PS/2 models that suffer surface-mount electrolyte leakage.

• The planar does not carry surface-mount aluminium electrolytic capacitors. Photographs of both the Model 50 planar (P/N 90X9161 / 15F6969) and the Model 50 Z planar (FRU 90X9533) show surface-mount tantalum chip capacitors and ceramics for supply-rail decoupling. There is no equivalent on these boards of the leaking aluminium cans found on the P70/P75 video board and planar.
• The diskette drive does. The documented, repeatable electrolyte problem on PS/2 machines of this era is in the 3.5-inch drive's own logic board, where surface-mount aluminium electrolytics go open-circuit or high-ESR and leak onto the pads.
• The fixed-disk logic boards do. The integrated drive-plus-controller ("DBA") drives fitted to the 8550-031 and 8550-061 carry the same style of surface-mount electrolytic, and corrosion around their pads has been documented.
• The power supply is a normal 1980s switcher and ages like one, but IBM published no component-level data for it.

IBM never published capacitor values for any part of this machine. No parts list, technical reference, hardware maintenance reference or schematic set gives a capacitance or a working voltage for a planar, adapter or power-supply capacitor: IBM serviced these at FRU level and replaced whole assemblies. Any table of "Model 50 capacitor values" you find elsewhere is therefore somebody's reading of a particular board, not a manufacturer specification. Read the values off your own board before ordering parts.

Planar Capacitors

A Model 50 Z with the cover removed. The planar lies on the chassis floor, the drive structure sits over the front of it, and the drive riser card carries the diskette and fixed-disk connectors. The power supply slides on to an edge connector at the rear left.

The Model 50 and 50 Z planars use surface-mount tantalum chip capacitors (the amber or gold moulded blocks and beads visible across both boards) together with surface-mount ceramics. Tantalums fail short far more often than they fail open, and a shorted decoupling tantalum drags its rail down.

IBM did not mark these positions with values on the silkscreen, and no published designator-to-value map exists for either planar. Do not fit a replacement by reference to a value quoted for a different IBM machine — the widely-cited "10 µF / 16 V marked 106 16V" figure is documented for IBM 51xx (PC, XT, AT) motherboards, which use through-hole bead tantalums, and has never been shown to apply to a PS/2 planar.

Finding a shorted tantalum

1. Unplug the machine and discharge the supply.
2. Remove the planar. Take out the adapter cards, the diskette drive, the fixed disk and the drive riser card; on the Model 50 also remove the fixed-disk adapter and the exhaust fan assembly. The drive-structure removal tool is stowed under the battery/speaker assembly and is used to lift the white plastic snaps holding the drive structure down — including the one hidden under the speaker. Three screws hold the planar itself, and a centring pin at the rear locates it, so lift the planar straight up rather than at an angle.
3. Set a multimeter to its lowest resistance range or to diode test and probe from ground to the rail side of each tantalum. A healthy part settles to a high reading after a brief charging pulse; a shorted one sits near 0 Ω.
4. Everything on a given rail is in parallel, so a low reading points at a rail rather than at one part. Lift one end of the suspect capacitor and re-measure before condemning it.
5. Replace with an equivalent surface-mount tantalum or a low-ESR ceramic of the same value and equal or higher working voltage, read from the part you removed.

Removal and replacement

Leaking or failed surface-mount capacitors contaminate their own solder joints. Jim Shorney's account of PS/2 capacitor work is worth heeding: the joints often refuse to reflow, and the legs pull out of the contaminated solder before it melts. Work at a controlled temperature, add fresh leaded solder and flux to each joint first, and try to desolder rather than twist — but expect some parts to come away without the solder ever melting. Clean each pad and check it is still bonded to the board before fitting the new part.

Match polarity to the board. A tantalum fitted backwards will usually short on first power-up.

Power Supply (94 W)

The Model 50 and 50 Z share one supply: an ASTEC AA13621, IBM P/N 90X9527, IBM FRU P/N 90X9366, rated 94 W maximum continuous output. It is the same physical family as the Model 70 supply but with no built-in blower — on the Model 50 the fan is a separate assembly that plugs into the planar and exhausts through the rear panel.

Opening it: the case is held by security Torx screws, five on the side and two on the top, and opens with a T10 security bit. There is no ultrasonic welding and no glue.

What is published, and what is not

Model 50 / 50 Z power supply — published data

Item	Value	Source
Rating	94 W maximum continuous output	IBM announcement letter; IBM HMR
Input	100–125 V or 200–240 V, automatically selected, 50/60 Hz	IBM HMR
Outputs	+5 V, +12 V, −12 V (there is no −5 V rail)	IBM HMM pinout
Internal fuses	Two (F1, F2) — a dead supply with no outputs usually means one has blown	Ardent Tool
Capacitor values	Not published by IBM, and no reverse-engineered schematic of this unit is known to exist

Note the last row. The 94 W ASTEC unit has not been reverse-engineered and documented the way the tower supply has, so a recap means reading each can before you remove it. Photograph the board, write down every value and working voltage, and order to match.

Recapping the 94 W supply

1. Discharge the primary reservoir capacitor and confirm with a meter.
2. Remove the supply: two screws at the back of the case, one screw at the front inner corner, then pull it off the planar edge connector.
3. Open the case (T10 security Torx).
4. Photograph the board from both sides before touching anything. Record each electrolytic's value, working voltage, diameter, lead spacing and polarity.
5. Replace like for like: same capacitance, equal or higher working voltage, 105 °C rating, low-ESR parts on the secondary side where the originals were low-ESR types.
6. Inspect the mains-side X- and Y-class suppression capacitors. These are safety-rated parts and must be replaced with equivalently rated X or Y class parts, never with ordinary film capacitors. Any RIFA-branded part showing crazing of the epoxy should be replaced on sight; they crack, absorb moisture and eventually vent.
7. Check both fuses for continuity before reassembly.
8. Reassemble and bench-test the outputs under a modest resistive load before refitting it to the machine. The connector is a 50-pin edge connector: pin 1 is −12 V, odd pins 3–13 are +12 V, odd pins 15–47 are +5 V, even pins 2–48 are ground, pin 49 is System Status and pin 50 is Power Good.

IBM's own service limits, measured at the planar connector, are given on the maintenance page; a rebuilt supply that sits outside them has not been fixed.

Diskette Drive Logic Board

This is the repair most Model 50s actually need.

PS/2 3.5-inch drives take power and signals through a single interface with no separate power connector, and were supplied by several manufacturers — Mitsubishi, ALPS, Sony and YE Data — under the same IBM part numbers (P/N 90X6766, FRU 72X8523 for the card-edge type used on the Model 50). The drives are physically different inside and most parts do not interchange, though they behave identically at the interface.

Al Savage's teardown of failed PS/2 Mitsubishi drives found the cause to be surface-mount electrolytic capacitors on the drive's logic board that had gone open-circuit or very high-ESR. Replacing them recovered the drives. Reported values differ between drive revisions — one recap account names 22 µF, 1 µF and 0.22 µF parts, another replaced two capacitors with 10 µF — so read the values off your own board.

Symptoms that point at the drive's capacitors rather than its mechanics:

• Will not read any diskette, including known-good ones written elsewhere.
• Intermittent read and write errors that worsen as the drive warms.
• A 6xx diskette error at POST. Mitsubishi drives in particular are noted for capacitors that produce 6xx electronic errors.
• Brown crust or discoloured, grey, rough solder joints around a can.

Symptoms that are not capacitor faults, and should be chased mechanically first:

• A 162 at POST with the drive otherwise dead. During POST the heads are driven off track 0 and back within a fixed time; a sticking head carriage, a dirty track-0 photo-sensor or a detached head cushion will miss that window and the drive is thrown out of the configuration. The ALPS drives are known for losing the damper cushion glued to the head assembly; the Mitsubishi 355 collects dirt around the read/write head actuator spindle.
• Speed or timing complaints blamed on a belt. There is no belt. The Mitsubishi MF355C specification states that the drive uses a DC brushless direct-drive motor coupled straight to the spindle at 300 rpm. Nothing in a PS/2 3.5-inch drive is belt-driven, and any guide that tells you to fit one is wrong.

Because the drive takes its supply through the interface, a drive fault can also be a supply fault — IBM lists the power supply among the FRUs for unclassified 6xx errors.

Fixed Disk Drives and Adapters

The Model 50 family uses three different arrangements, and they are not interchangeable:

Model 50 fixed-disk arrangements

Machine	Drive	Adapter
8550-021	20 MB MFM / ST-506, FRU 72X8522	ST-506 adapter in the dedicated adapter slot, FRU 90X8070 (early) or 90X8642 (later)
8550-031	30 MB integrated drive-plus-controller, FRU 6128287	Bare passive riser, FRU 90X9441
8550-061	60 MB integrated drive-plus-controller, FRU 6128294	Bare passive riser, FRU 90X9441
8550-021 with the 60 MB upgrade	60 MB integrated drive-plus-controller	Riser carrying its own BIOS ROMs, FRU 90X9571

The units IBM's error codes call "ESDI" on the desktop machines are not true ESDI drives on a separate controller card. They are a hard disk and a Micro Channel controller built as one physical assembly, plugged into a riser that is mostly or entirely passive; the bare 90X9441 riser is an empty board that only routes signals. True ESDI controllers and separate ESDI drives belong to the towers — the Model 60 and Model 80.

The practical consequences for capacitor work:

• On the 8550-031 and 8550-061, the capacitors that matter are on the drive's own logic board, not on the riser. These are surface-mount electrolytics of the same family as the diskette-drive parts, and corrosion around their pads has been photographed on PS/2 DBA drives. Read the values from your board.
• On the bare 90X9441 riser there is nothing to recap: the board is a passive interconnect. One variant, circuit board 15F8434, carries unpopulated silkscreen outlines for two capacitors that IBM never fitted — do not read those as missing parts.
• On the 8550-021 ST-506 adapter and the 90X9571 riser, no published capacitor list exists. Inspect, and replace only what is visibly failed or measures bad.

If an integrated drive reports a 104xx error, note which one before reaching for a soldering iron: 010455 and 010456 are controller errors, 010462 a controller seek error, 010464 a drive read error, and 010483 points at the adapter and the system board — and, for older drives, at spindle stiction rather than electronics. For the ST-506 machine, 1702 indicts the adapter specifically while the rest of the 17xx range points first at the drive, then the cable, then the adapter, the planar and the power supply.

After a Recap

1. Bench-test the supply on its own first, under load, against IBM's service limits.
2. Refit and start from the Reference Diskette rather than the hard disk.
3. Run Automatic Configuration if any adapter changed, then run Advanced Diagnostics (from System Programs, press Ctrl-A at the main menu).
4. A successful POST ends in one short beep.
5. Leave the machine running a long memory test to shake out marginal decoupling.

If something fails immediately after a recap, check the polarity of every part you fitted before suspecting anything else.

When Not to Recap

A Model 50 that starts reliably, POSTs clean, holds its configuration and reads and writes diskettes does not need its planar touched. Blanket-replacing surface-mount tantalums on a working board risks lifting pads for no gain.

Do act if you see:

• Any fluid, crust or crystalline residue near a capacitor, or discoloured, grey or rough solder at its pads.
• A rail outside IBM's service limits, or Power Good never asserting (the green front-panel light stays off).
• Diskette read/write failures or 6xx errors on a mechanically sound drive.
• A supply that whines, smells, or will not start when warm.
• A shorted decoupling capacitor confirmed out of circuit.

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 — the tower, which uses a different and better-documented power supply
• Capacitor Failure Symptoms
• Battery Explosion, Capacitor or Corrosion Damage
• Floppy Drive Repair
• Recommended Tools