IBM PS/2 Model 50 Capacitor Replacement Guide

A Model 50 Z with the cover off, showing the planar, the drive structure and the drive riser card. (Image: Ruben de Rijcke, CC BY-SA 4.0, via Wikimedia Commons)

Capacitor work on the IBM PS/2 Model 50 (machine type 8550), both the original 8550-021 and the Model 50 Z (8550-031 and -061), involves the planar, the 94 W power supply, the diskette drive and the fixed-disk drive boards. IBM's service documents for the Model 50 list parts at FRU level and give no capacitor values for any of them. Read the value and voltage on each part before removing it.

Safety

The 94 W supply rectifies the mains. Its primary reservoir capacitor holds a lethal charge after the machine is unplugged.

1. Switch off and unplug the mains lead.
2. Discharge the reservoir capacitor through a resistor (for example 1 kΩ, 5 W) and check with a meter that it is near zero before touching the board.
3. Do not probe a powered primary side without an isolation transformer.

The supply selects its input range automatically; there is no voltage switch. There is no CRT in the system unit. For an IBM 8503, 8512, 8513 or 8514 display, see CRT Discharge Procedure.

Where the capacitors are

• On the planar, Ardent Tool's photograph of the Model 50 planar (P/N 90X9161) shows yellow dipped tantalum capacitors, such as C72, C79 and C116, and no aluminium electrolytics.
• The power supply, an ASTEC AA13621 switching supply, has its own electrolytics and mains suppression capacitors.
• The diskette drive board has surface-mount electrolytics that go open or high-ESR and leak. This is the commonest capacitor fault on PS/2 machines of this age.
• The 30 MB and 60 MB fixed disks of the 031 and 061 have their controller built in. The capacitors on PS/2 drives of this kind have been found corroding.

Planar

A shorted tantalum pulls its rail down. The supply only raises Power Good, and lights the green lamp on the front, once every rail is above its minimum under-voltage sense level (+4.5 V, +10.8 V, −10.2 V). A short on any output latches the supply off until the fault is cleared and the switch has been off for ten seconds.

Taking the planar out

Ardent Tool's procedure for the Model 50 and 70:

1. Remove all 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.
2. Find the drive-structure removal tool under the battery and speaker assembly. Use it to lift all the white plastic snaps, including the one under the speaker assembly, and pull the drive structure straight up.
3. Unscrew and remove the power supply.
4. Remove the three screws holding the planar, and the three screws at the rear holding the EMC shield over the ports.
5. Lift the planar straight up. A 1/8-inch pin in the base locates it through a hole beside inductor LN2, and tilting the board levers against the pin.

Finding a shorted tantalum

1. Set a multimeter to its lowest resistance range or diode test.
2. Measure from ground to the positive side of each tantalum. A good part reads high after a brief charging pulse; a shorted one reads near 0 Ω.
3. All parts on a rail are in parallel, so a low reading identifies a shorted rail. To find the part, lift one leg of the suspect capacitor and measure again.
4. Replace it with a tantalum of the value printed on the old part and an equal or higher voltage, the right way round.

The "10 µF 16 V, marked 106 16V" figure that minuszerodegrees.net gives for IBM tantalums refers to 51xx (PC, XT and AT) boards, not the Model 50.

Power supply

The Model 50 and 50 Z use the same supply, IBM FRU 90X9366, an ASTEC AA13621 rated 94 W maximum continuous output. It has no fan of its own: the fan is a separate assembly on the planar. IBM published no internal parts list or schematic for it, so record every value before removing parts.

1. Discharge the reservoir capacitor and check it with a meter.
2. Remove the supply: two screws at the back of the case and one at the front inner corner, then pull it off the planar edge connector.
3. Open the case with a T10 security Torx bit (five screws on the side, two on the top).
4. Photograph both sides of the board. Record each electrolytic's value, voltage, size and polarity.
5. Fit 105 °C parts of the same capacitance and an equal or higher voltage, low-ESR where the original was.
6. Replace a cracked or bulging suppression capacitor with one of the same capacitance, the same or higher AC voltage and the same safety class (X2 for X2, Y2 for Y2). An aged RIFA part ruptures with smoke and a strong odour.
7. If the supply gave no output at all, check its fuses F1 and F2.
8. Test it out of the machine before refitting.

Testing the supply

Ardent Tool's test, with the supply out of the machine and switched on, measures at the 50-pin connector with the negative lead on pin 2:

Model 50 supply test (supply removed)

Output	+ lead	Minimum	Maximum
−12 V	Pin 1	−9.0 V	−15.0 V
+12 V	Pin 3	+9.0 V	+15.0 V
+5 V	Pin 15	+3.7 V	+6.2 V

IBM's 1994 Hardware Maintenance Manual prints the same table for the Models 50 and 70 with the +12 V minimum given as +15.0 V and the +5 V minimum as +6.2 V, which are misprints. These are pass/fail limits for a supply out of the machine. In the machine, each output must stay above its under-voltage sense level or the supply withholds Power Good. Pin 1 is −12 V, odd pins 3 to 13 are +12 V, odd pins 15 to 47 are +5 V, even pins 2 to 48 are ground, pin 49 is System Status and pin 50 is Power Good.

Diskette drive board

IBM's 1994 FRU number for the Model 50's 1.44 MB drive is 64F0207. PS/2 drives came from several makers, including Mitsubishi, ALPS and Sony, and their boards differ, so read the values on the board in front of you. Published repairs:

Published PS/2 diskette drive recaps

Drive	Capacitors found	Source
Mitsubishi MF355W-99M3, P/N 90X6766, FRU 72X8523 (card-edge)	Two 22 µF 16 V, one 1 µF 50 V and two 0.22 µF 50 V, open or high-ESR	Al Savage
Mitsubishi PS/2 drive	Two surface-mount electrolytics, replaced with 10 µF through-hole parts	ibmfiles.com
ALPS PS/2 drive	One 4.7 µF 25 V, one 10 µF 16 V and three non-polarised 4.7 µF 16 V	Mac84

Mitsubishi MF355W-99M3 drive board (FRU 72X8523) with the five failed surface-mount electrolytics marked. (Photo: Al Savage, via Ardent Tool of Capitalism)

Alan Douglas, quoted by Ardent Tool, used surface-mount electrolytics for the 22 µF and 1 µF positions and ceramics for the 0.22 µF ones, which are hard to find as electrolytics. Leaking surface-mount capacitors contaminate their joints. Jim Shorney found that the solder often will not melt and the legs pull out of it instead, with a risk of lifting the pad; Alan Douglas cut the old capacitors in two with flush cutters to reach the leads. Add fresh solder and flux first, and check each pad is still bonded before fitting the new part.

Signs that point to the drive's capacitors:

• the drive reads nothing, even known-good diskettes;
• read and write errors that get worse as the drive warms;
• 6xx errors at POST; Peter Wendt notes that Mitsubishi drives in particular have capacitors that cause them;
• brown residue or grey, discoloured solder around a capacitor.

A 162 caused by the drive is usually mechanical. During POST the heads are stepped out from track 0 and back within a set time; a sticking head carriage, a dirty track-0 sensor or a loose head cushion makes the drive miss it, and it is dropped from the configuration. The drives are direct drive and have no belt.

Fixed disks and adapters

Model 50 fixed-disk arrangements

Submodel	Drive (IBM FRU)	Adapter
8550-021	20 MB ST-506, 72X8522	ST-506 adapter in slot J10: FRU 72X8505 in IBM's 1994 list; Ardent Tool records 90X8070 (early) and 90X8642 (later)
8550-031	30 MB with integrated controller, 6128287	Passive riser
8550-061	60 MB with integrated controller, 6128294	Passive riser, FRU 90X9441
8550-021 with IBM's 60 MB upgrade	60 MB with integrated controller	Riser with its own BIOS, FRU 90X9571

Sources: IBM's parts list and Ardent Tool.

• On the 031 and 061 the capacitors that matter are on the drive's own board. The ibmfiles.com repair fitted through-hole capacitors to the surface-mount pads, taking care not to shear the taller parts off.
• The bare 90X9441 riser has no components to replace. One version of its board (15F8434) has outlines for two capacitors that IBM never fitted.
• No capacitor list is published for the 021's ST-506 adapter or the 90X9571 riser. Replace only parts that are visibly damaged or measure bad.

Before blaming capacitors on an integrated drive, read the error code. In Ardent Tool's copy of IBM's list, 10455 and 10456 are controller errors, 10462 a controller seek error and 10464 a read error, and for 10483 the FRUs are the adapter and the system board, with a pointer to stiction on older drives. For the 021's ST-506 drive, 1702 points to the adapter; most other 17xx codes point to the drive, cable, adapter, system board and power supply in that order. See IBM PS/2 Model 50 Troubleshooting Guide.

After the work

1. Test the supply on its own first, against the limits above.
2. Refit it and start from the Reference Diskette. A successful POST ends with one short beep.
3. Run Automatic Configuration if anything changed, then Advanced Diagnostics (Ctrl-A at the Reference Diskette main menu).
4. Run the memory tests repeatedly to find marginal decoupling.

If a fault appears after a recap, check the polarity of every part you fitted.

When not to recap

A Model 50 that starts reliably, passes POST, keeps its configuration and reads diskettes does not need its planar touched. Act when there is:

• fluid, crust or grey, discoloured solder at a capacitor;
• a supply output outside the limits above, or no Power Good (the green lamp stays off);
• diskette errors on a drive that is mechanically sound;
• a supply that whines, smells or will not start when warm;
• a shorted 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
• Capacitor Failure Symptoms
• Battery Explosion, Capacitor or Corrosion Damage
• Floppy Drive Repair
• Recommended Tools