IBM PS/2 Model 70 Capacitor Replacement Guide

Reduced-size 20 MHz system board from an 8570-121. The orange and yellow beads are through-hole dipped tantalum capacitors; there are no aluminium electrolytics. This board has an Intel RapidCAD-2 in the 80387 socket (U42).

Capacitor work on the IBM PS/2 Model 70 (machine type 8570, all submodels) involves the system board, the drive riser, the 132 W power supply, the diskette drive, the fixed disk and, on 25 MHz machines, the processor card. IBM's documents for the Model 70 (the Technical Reference, the Hardware Maintenance Reference and the 1994 Hardware Maintenance Manual) stop at FRU level and give no capacitor values. The values below come from published repairs of particular boards and are credited to them. Read the value and voltage on each part before removing it.

Safety

The 132 W supply rectifies the mains, and its primary capacitors hold a charge after the machine is unplugged.

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

No schematic has been published for this supply, so identify the reservoir on the board yourself and treat every large capacitor on the mains side as charged until you have measured it. There is no CRT in the system unit; for an IBM display, see CRT Discharge Procedure.

Where the capacitors are

• System board: through-hole dipped tantalum capacitors and small surface-mount parts, and no aluminium electrolytics, on Ardent Tool's photographs of the full-size Type 1 board (16 MHz P/N 15F8379 and 20 MHz P/N 33F6083) and the 25 MHz board (P/N 33F8082), and on a photograph of a reduced-size Type 2 board from an 8570-121 (File:IBM PS2 Model 70 8570-121 motherboard.jpg).
• Drive riser: three resistor and capacitor pairs for supply buffering and decoupling.
• Power supply: the machine's electrolytics and mains filter capacitors.
• Diskette drive board: surface-mount electrolytics that go open or high-ESR. This is the best-documented capacitor fault on PS/2 machines of this age.
• Fixed disk logic board: surface-mount electrolytics that have been found corroding.

System board

There is nothing on the system board to recap on suspicion; replacing healthy tantalums on an old multilayer board risks lifting pads. A shorted tantalum pulls its rail down. The supply withholds Power Good, and the green power-good light stays off, unless the outputs are above +4.5 V, +10.8 V and −10.2 V, and a short on an output latches the supply off until the fault is removed and the switch has been off for ten seconds.

Finding a shorted tantalum

1. Unplug the machine and discharge the supply.
2. Set a multimeter to its lowest resistance range or diode test.
3. 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.
4. All parts on a rail are in parallel, so a low reading identifies a shorted rail. Lift one leg of the suspect capacitor and measure again to find the part.
5. Replace it with a tantalum of the value printed on the old part and an equal or higher voltage, the right way round.

No designator-to-value list has been published for these boards. The "10 µF 16 V, marked 106 16V" tantalum that minuszerodegrees.net describes applies to IBM's 51xx (PC, XT and AT) boards. Ardent Tool notes that every known 486 processor card carries yellow rework wires; they are not damage.

To take the system board out, Ardent Tool's procedure is to remove the adapters, the diskette drive, the fixed disk and the drive riser; lift the drive structure off its white snaps with the tool stored under the battery and speaker assembly; remove the supply, the three board screws and the three rear screws of the EMC shield; and lift the board straight up off its locating pin. On the 25 MHz machines IBM's tool 57F2859 also separates the processor card from the system board.

Drive riser

The drive riser (Ardent Tool records FRU 90X1111; IBM's 1994 FRU for the bus adapter is 90X9287) carries both diskette connectors and the fixed-disk connector. Peter Wendt notes that its three resistor and capacitor pairs buffer and decouple the supply to the drives, and that early risers without them lost data when two diskette drives, or a diskette drive and a tape drive, were fitted. No values have been published for them.

Power supply

Model 70 power supply

Item	Value
Maker and model	ASTEC AA15530
IBM numbers	P/N 90X9409, FRU 90X8626 (power supply and fan assembly)
Rating	132 W maximum continuous output
Outputs	+5 V, +12 V and −12 V to the system board; +5 V and +12 V to the drives; System Status and Power Good
Fuses	F1 and F2, one in each mains line: ASTEC part 084-00300030-0217, a Bel 3AG (6 × 32 mm) 5 A 250 V fast-blow
Case screws	Security Torx: T15 on the case, T10 at the edge connector

Sources: Ardent Tool, IBM's Technical Reference and parts list. No schematic or capacitor list has been published for this supply. The reverse-engineered schematic of the 225 W supply 72X6665 is for the Model 60 and 80 towers and does not apply. Photograph the board from both sides and record every value, voltage, size and polarity before desoldering anything.

A supply with no output at all may just have a blown fuse; Ardent Tool's advice for a dead supply is to suspect F1 or F2 first. Ardent Tool also found dust coating about half the components inside one supply.

Opening the supply

1. Remove the standard screw at the front of the supply (there is an access hole through the drive shelf), then the two standard screws at the back. The supply slides out off the system-board edge connector.
2. Remove the security Torx screws. One at the upper rear inner side has a toothed lock washer.
3. Ease the two halves apart at the retaining rib at top centre with a small screwdriver under the sheet metal. The inner cover with the fan pivots on tabs at the lower inner side; unplug the fan lead.
4. To reach F1 and F2, pull the edge connector up at the rear and then back. The fuses are at the lower left of the board, towards the mains socket.

Source: Ardent Tool. Fit 105 °C electrolytics of the same capacitance and an equal or higher voltage, low-ESR where the original was. Replace a mains filter capacitor only with a part 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 smell. Test the supply before refitting it, against IBM's limits in the maintenance guide.

Diskette drive board

IBM's 1994 FRU for the Model 70's 1.44 MB drive is 64F0207. Al Savage's Mitsubishi below is FRU 72X8523, the half-height drive IBM lists for the Models 30 286, 60, 65 and 80, so its board may not match a Model 70 drive. 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 PS/2 diskette drive recaps

Drive	Capacitors found	Source
Mitsubishi MF355W-99M3, P/N 90X6766, FRU 72X8523	Two 22 µF 16 V, one 1 µF 50 V and two 0.22 µF 50 V surface-mount electrolytics, 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, and cut the old capacitors in two with flush cutters to avoid tearing the pads. Jim Shorney found that the solder on leaking surface-mount capacitors often will not melt and the legs pull out instead. Add fresh solder and flux first, and check each pad is still bonded before fitting the new part. Through-hole parts on the surface-mount pads work, but stand taller and can be knocked off.

Faults that look like drive capacitors and are not:

• A 162 at POST caused by the drive. During POST the heads are stepped away 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. Older ALPS drives lose the damper cushion glued to the head assembly.
• 6xx errors that stay with the machine: dirt on the diskette controller leaking current between its pins, which Ardent Tool describes on the Model 80. Clean the system board before condemning the drive.

Mitsubishi's MF355C specification describes a brushless DC motor turning the spindle directly at 300 rpm, so a drive of that design has no belt.

Fixed disk

The Model 70's fixed disk has its controller built in and plugs straight into the drive riser. IBM's 1994 FRUs are 6128294 (60 MB), 56F8894 (80 MB), 6128291 (120 MB) and 56F8895 (160 MB). The ibmfiles.com author found the electrolytics on later PS/2 drives of this kind corroding, with slight pad corrosion, and replaced them with through-hole parts on the surface-mount pads. No value list has been published. Ardent Tool's map of the 60 MB WD387 board names its ICs only, and the 160 MB board (P/N 79F6370) has no designators printed on it.

Before suspecting capacitors:

• A drive that will not spin up after storage has stuck heads. Peter Wendt's remedies are a few power cycles, or unplugging the system unit, shaking it up and down and turning it sharply clockwise; never lift the front and drop it. He names the Seagate ST-177I 60 MB drive in some Model 70-061 machines as prone to it. Copy the data off as soon as it starts.
• 104xx codes: IBM's list gives 10454 as a sector-buffer test error, 10455 and 10456 as controller errors, 10462 as a controller seek error and 10464 as a drive read error, all "run Advanced Diagnostics". For the read/write group (10450, 10451, 10490 to 10492, 10499) the FRUs after diagnostics are the riser card, the drive and the system board. The full table is in the Model 50 troubleshooting guide.

Processor cards

The 25 MHz machines carry the processor on a separate card: the 386 card (FRU 15F7659) with its 82385 cache controller and eight SRAMs, or the 486 card (FRU 64F0123) with the 486DX-25 and a 50 MHz oscillator. No capacitor data has been published for either card.

Memory

No capacitor data has been published for IBM's memory adapters. Memory errors on this machine usually come from the modules: the 25 MHz boards need 80 ns parity modules, and IBM says the 85 ns kits must not be used on them. One Ardent Tool contributor reports 215 errors when the 25 MHz board's modules were fitted starting at J17 or J18 instead of J21 and J20.

After the work

1. Test the supply on its own against IBM's limits.
2. Refit it and start from the Reference Diskette, with the diskette in the drive at power-on. A successful POST ends with one short beep.
3. Run Automatic Configuration if an adapter changed, then Advanced Diagnostics (Ctrl-A at the main menu).
4. Check that the green power-good light is on.
5. Run the memory tests repeatedly to find marginal decoupling, and format and verify a diskette if you worked on the drive.

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

When not to recap

A Model 70 that starts reliably, passes POST, keeps its configuration and reads diskettes does not need anything replaced, and the system board should be left alone. Act when there is:

• fluid, crust or grey, discoloured solder at a surface-mount capacitor on a drive board;
• a supply output outside IBM's limits, or no Power Good (the green light 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 70
• IBM PS/2 Model 70 Maintenance Guide
• IBM PS/2 Model 70 Troubleshooting Guide
• IBM PS/2 Model 50 Capacitor Replacement Guide
• IBM PS/2 Model 80 Capacitor Replacement Guide
• Capacitor Failure Symptoms
• Battery Explosion, Capacitor or Corrosion Damage
• Floppy Drive Repair
• Recommended Tools