Editing IBM PS/2 Model 50 Capacitor Replacement Guide

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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.<ref name="hmm">IBM, ''IBM Personal System/2 Hardware Maintenance Manual'', S52G-9971-02, October 1994. Hosted on this wiki as [[IBM Personal System 2 Hardware Maintenance Manual (S52G-9971-02, October 1994)]] ([[:File:IBM PS2 HMM S52G-9971 Oct 1994.pdf|PDF]]). Models 50 and 70 power supply connector, p. 141; Model 50 parts list (power supply 90X9366, fan 72X8515, system boards 72X8516 and 10G4005, 1.44 MB drive 64F0207, drives 72X8522, 6128287 and 6128294, adapters 72X8505, 90X9571 and 90X9441), p. 240.</ref><ref name="m50hmr">IBM, ''IBM PS/2 Model 50 Hardware Maintenance Reference'', S64F-3989-00, March 1990. Hosted on this wiki as [[IBM PS2 Model 50 Hardware Maintenance Reference]] ([[:File:IBM PS2 Model 50 HMR.pdf|PDF]]). Sections 3.1 (automatic input switching, under-voltage sense levels, the green power-good light) and 3.2 (one short beep after a successful POST).</ref> 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.

# Switch off and unplug the mains lead.
# 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.
# Do not probe a powered primary side without an isolation transformer.

The supply selects its input range automatically; there is no voltage switch.<ref name="m50hmr" /> 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.<ref name="at8550">Louis Ohland, Tomáš Slavotínek ''et al.'', [https://www.ardent-tool.com/8550/Planar.html "8550 Planar"], Ardent Tool of Capitalism. Designator map of planars 90X9161 and 15F6969 and a photograph of the component side of 90X9161.</ref>
* The power supply, an ASTEC AA13621 switching supply, has its own electrolytics and mains suppression capacitors.<ref name="atpwr50">Louis Ohland, Tomáš Slavotínek ''et al.'', [https://www.ardent-tool.com/50_70/Power.html "8550 / 8570 / 756x – Power Supply"], Ardent Tool of Capitalism. The 50-pin connector pinout; the test-voltage table (−12 V −9.0 to −15.0 V at pins 2 and 1; +12 V +9.0 to +15.0 V at pins 2 and 3; +5 V +3.7 to +6.2 V at pins 2 and 15), measured with the supply out of the machine; the ASTEC AA13621 (IBM P/N 90X9527, FRU 90X9366), 94 W maximum continuous output; the separate case fan; T10 security Torx case screws (five on the side, two on the top); the removal procedure; the lint screen on the underside; and the note that F1 or F2 is the suspect in a supply with no output.</ref>
* 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.<ref name="atfdd">[https://www.ardent-tool.com/floppy/8580.html "8580 Diskette Drives"], Ardent Tool of Capitalism, content by Fred Spencer and Al Savage. Al Savage's recap of Mitsubishi MF355W-99M3 card-edge drives (P/N 90X6766, FRU 72X8523), with the capacitor values and positions marked on his photograph; the diskette controller dirt problem; Peter Wendt's causes of a 162 from a drive that fails its POST seek test.</ref><ref name="ibmfiles">[http://www.ibmfiles.com/pages/ps2capacitors.htm "PS/2 Surface Mount Capacitors"], ibmfiles.com (2016). Two surface-mount electrolytics on the Mitsubishi PS/2 drive board, replaced with 10 µF through-hole parts on the pads; corrosion on the capacitors of PS/2 DBA hard disks and the same repair.</ref>
* The 30 MB and 60 MB fixed disks of the 031 and 061 have their controller built in.<ref name="ann50z">IBM Announcement Letter 188-080, ''IBM Personal System/2 Model 50 Z Desktop Systems with Improved Hardfile and Memory'', 2 June 1988, text at [https://www.ardent-tool.com/8550/188-080.txt ardent-tool.com]. The 30 MB and 60 MB drives, with "The fixed disk controller is integrated with the drive".</ref> The capacitors on PS/2 drives of this kind have been found corroding.<ref name="ibmfiles" />

== 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).<ref name="m50hmr" /> A short on any output latches the supply off until the fault is cleared and the switch has been off for ten seconds.<ref name="m50tr90">IBM, ''IBM Personal System/2 Model 50 Technical Reference'', October 1990. Hosted on this wiki as [[IBM PS2 Model 50 Technical Reference (October 1990)]] ([[:File:IBM PS2 Model 50 Technical Reference Oct90.pdf|PDF]]). "Output Protection", p. 1-12.</ref>

=== Taking the planar out ===

Ardent Tool's procedure for the Model 50 and 70:<ref name="atcommon50">[https://www.ardent-tool.com/50_70/Common.html "8550 and 8570 – Common Devices"], Ardent Tool of Capitalism. Opening the case, opening a locked unit, closing the case, and removing and refitting the planar, including the drive-structure removal tool under the battery/speaker assembly, the three planar screws, the EMC shield screws and the 1/8-inch centring pin beside inductor LN2.</ref>

# 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.
# 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.
# Unscrew and remove the power supply.
# Remove the three screws holding the planar, and the three screws at the rear holding the EMC shield over the ports.
# 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 ===

# Set a multimeter to its lowest resistance range or diode test.
# 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.
# 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.
# 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.<ref name="mzd">minuszerodegrees.net, [https://www.minuszerodegrees.net/failure/failure.htm "Commonly Failing Electronic Components (in vintage computers)"]. Tantalum failure modes and polarity; "On the IBM 51xx motherboards and cards, the tantalums used by IBM are commonly 10µF/16V"; line-suppression capacitor replacement rules; ruptured RIFA capacitors.</ref>

== Power supply ==

The Model 50 and 50 Z use the same supply, IBM FRU 90X9366, an ASTEC AA13621 rated 94 W maximum continuous output.<ref name="atpwr50" /><ref name="hmm" /> It has no fan of its own: the fan is a separate assembly on the planar.<ref name="atpwr50" /> IBM published no internal parts list or schematic for it, so record every value before removing parts.

# Discharge the reservoir capacitor and check it with a meter.
# 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.<ref name="atpwr50" />
# Open the case with a T10 security Torx bit (five screws on the side, two on the top).<ref name="atpwr50" />
# Photograph both sides of the board. Record each electrolytic's value, voltage, size and polarity.
# Fit 105 °C parts of the same capacitance and an equal or higher voltage, low-ESR where the original was.
# 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.<ref name="mzd" />
# If the supply gave no output at all, check its fuses F1 and F2.<ref name="atpwr50" />
# 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:<ref name="atpwr50" />

{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''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.<ref name="hmm" /> 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.<ref name="m50hmr" /> 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.<ref name="atpwr50" />

== Diskette drive board ==

IBM's 1994 FRU number for the Model 50's 1.44 MB drive is 64F0207.<ref name="hmm" /> 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.<ref name="atfdd" /> Published repairs:

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''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<ref name="atfdd" />
|-
| Mitsubishi PS/2 drive || Two surface-mount electrolytics, replaced with 10 µF through-hole parts || ibmfiles.com<ref name="ibmfiles" />
|-
| 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<ref name="mac84">[https://mac84.net/web/repair-guides-documents/ibm-ps-2-floppy-disk-drive-recapping-guide-alps-drive/ "IBM PS/2 Floppy Disk Drive Recapping Guide (ALPS drive)"], Mac84. Values and positions on the ALPS drive board; the three 4.7 µF 16 V parts are non-polarised; the separate controller board is not covered.</ref>
|}

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.<ref name="atcommon60">[https://www.ardent-tool.com/60_65_80/Common.html "60, 65 SX, and 80 – Common Devices"], Ardent Tool of Capitalism. Alan Douglas on replacing the 22 µF, 1 µF and 0.22 µF drive capacitors and on cutting the old parts in two to avoid tearing pads; Peter Wendt on ALPS and Mitsubishi drives (dust, head actuator dirt, the ALPS head-damper pad, Mitsubishi capacitors causing 6xx errors).</ref> 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.<ref name="atp75">[https://www.ardent-tool.com/8573/P75_Caps.html "8573 P75 Failing Capacitors"], Ardent Tool of Capitalism. Jim Shorney on removing leaking PS/2 surface-mount electrolytics; discoloured, grey solder pads as a sign of leakage.</ref><ref name="atcommon60" /> 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;<ref name="atcommon60" />
* brown residue or grey, discoloured solder around a capacitor.<ref name="atp75" />

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.<ref name="atfdd" /> The drives are direct drive and have no belt.<ref name="mf355c">Mitsubishi Electric, ''3.5 inch Flexible Disk Drive Standard Specifications, MF355C'', UGD-0527A, February 1987. Hosted on this wiki as [[:File:Mitsubishi MF355C 3.5 inch Diskette Drive Specifications (UGD-0527A, Feb 1987).pdf]]. Section 3: brushless DC direct-drive spindle motor at 300 rpm.</ref>

== Fixed disks and adapters ==

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''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.<ref name="hmm" /><ref name="athdd50">Louis Ohland, Tomáš Slavotínek ''et al.'', [https://www.ardent-tool.com/8550/HDD_Adapters.html "8550 HDD Adapters"], Ardent Tool of Capitalism. The ST-506 adapters FRU 90X8070 and 90X8642 for the 021; the bare riser FRU 90X9441, "completely empty otherwise", with a board variant 15F8434 that has silkscreen outlines for two capacitors that are not fitted; the riser with BIOS FRU 90X9571 for an 021 with the 60 MB upgrade; and Peter Wendt's explanation that the integrated drives combine a Micro Channel controller and disk in one unit.</ref>

* 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.<ref name="ibmfiles" />
* The bare 90X9441 riser has no components to replace. One version of its board (15F8434) has outlines for two capacitors that IBM never fitted.<ref name="athdd50" />
* 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.<ref name="aterr">[https://www.ardent-tool.com/trouble/ps2error.html "PS/2 Error Codes"], Ardent Tool of Capitalism, with the tables for [https://www.ardent-tool.com/trouble/error_06.html 1500–2999] (17xx, ST-506) and [https://www.ardent-tool.com/trouble/error_08.html 10000–11999] (104xx, integrated drives); the Advanced Diagnostics (Ctrl-A) instruction.</ref> See [[IBM PS/2 Model 50 Troubleshooting Guide]].

== After the work ==

# Test the supply on its own first, against the limits above.
# Refit it and start from the Reference Diskette. A successful POST ends with one short beep.<ref name="m50hmr" />
# Run Automatic Configuration if anything changed, then Advanced Diagnostics (Ctrl-A at the Reference Diskette main menu).<ref name="aterr" />
# 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;<ref name="atp75" />
* a supply output outside the limits above, or no Power Good (the green lamp stays off);<ref name="m50hmr" />
* 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]]

== References ==

<references />

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