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Macintosh Quadra 900 Capacitor Replacement Guide

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Macintosh Quadra 900

The Macintosh Quadra 900 logic board is built with tantalum capacitors, according to Recap-a-Mac, so it does not suffer the leaking surface-mount electrolytics of later Macintosh boards.[1] The aluminium electrolytics that age are in the 303 W power supply. Apple services the Quadra 900, the Quadra 950 and the Apple Workgroup Server 95 in one manual, which treats the power supply as a single module and gives no capacitor values for it or for the logic board.[2]

No populated capacitor list for the Quadra 900 or its power supply has been published by Apple, Console5 or Recap-a-Mac; Console5's Quadra 900 page carries only the service manual.[3] Read the values off your own boards before ordering parts.

Logic board

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Tantalum capacitors do not leak, but they fail short, which pulls the rail down and makes the supply shut down. A rail that measures as a short to ground with the supply disconnected is the sign to look for. If one has to be replaced, derate a solid tantalum by about half (a 10 V or higher part on a 5 V rail, 25 V on 12 V), or fit a polymer part.[4] On a tantalum the bar marks the positive terminal; on an aluminium electrolytic the stripe marks the negative. See Capacitor Failure Symptoms.

The PRAM battery on the logic board is a 3.6 V lithium cell.[5] Apple's check needs only the cover off: set the voltmeter to the 10 V DC scale and replace the cell below 3.2 V. To replace it, remove the cover, the drive shelf and the power supply, prise open the latch at the end of the holder and lift the cell out.[2] If it has leaked, see Battery Explosion, Capacitor or Corrosion Damage.

Power supply

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Apple's specification is 100-240 V AC RMS, self-configuring, 50-60 Hz single phase, 303 W maximum not including the monitor. The Service Source has no voltage-adjustment procedure and no rail tolerance table.[2]

The Quadra 950 supply is sold as Apple part 699-0394, a Delta SMP-291AB rated at 303 W.[6] Apple's shared manual gives the Quadra 900 the same 303 W rating but no part number, so check the label on your supply before assuming it is the same unit.[2]

The primary side runs at mains potential, and its reservoir capacitors hold a charge after the machine is unplugged. Discharge them and confirm with a meter before touching the board.

Known failure: C905A and C905B

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A 68kMLA member who opened his Quadra 950's Delta SMP-291AB found C905A and C905B on the input board, both 1000 µF 16 V Rubycon VXB parts, had leaked, while the other capacitors on that board still measured within specification. The two leaking parts measured close to their nominal capacitance with only slightly raised ESR and were found by eye.[7] Inspect the input board visually before powering a long-stored machine, and do not rely on a capacitance or ESR reading to clear these parts.

Removing the supply

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  1. Unplug the machine and lay it on its side. Press the two latches and lift the cover off; Apple warns that removing it with the machine standing may damage the cover.[2]
  2. Remove the drive shelf: disconnect the floppy and SCSI data cables from the logic board and the SCSI power cable from the supply, remove the two drive shelf screws, slide the shelf towards the rear and lift it out by the cable tie and metal tab.[2]
  3. Remove the speaker bezel: disconnect the speaker cable and release the four plastic latches inside the bezel.[2]
  4. Disconnect the power supply cable from the logic board, remove the three mounting screws, release the two latches of the blank bezel and remove it, then lift the supply straight up by its two handles, pulling evenly.[2]
  5. The fan is mounted on the supply: disconnect its cable and remove the four screws to take it and its grille off.[2]

Photograph the boards before removing any capacitor, fit replacements of the same value and at least the same voltage rating, and reflow every joint you disturb before the supply sees mains again.

Symptoms Apple sends to the supply

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  • Does not power on (black screen, fan not running, LED off) once the power cables, the monitor's own power and the power cord have been checked.
  • Clicking, chirping or thumping (power supply, then logic board, then floppy cable and floppy drive).
  • Intermittent shutdowns not explained by the side and rear vents (the thermal protection circuit recovers after 30 to 40 minutes), the power cord or the battery.
  • No internal hard drives operate, after checking SCSI IDs and the data cable.

[2]

The front-panel keyswitch in its secure position stops the floppy drive, mouse and keyboard working; check it before suspecting the supply or logic board.[2]

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References

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  1. ↑ Recap-a-Mac, "Recapping Service", FAQ "Why do you use tantalum capacitors as replacements?" (retrieved 2026-09-30): "Apple actually used tantalum capacitors instead of surface mount electrolytics for some of their vintage Macs (such as the Quadra 700 and Quadra 900)".
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Apple Computer, Macintosh Quadra 900/950/AWS 95 Service Source. Hosted here as Macintosh Quadra 900/950/ AWS 95 Service Source (File:Quadra_900.950.ws_95.pdf). Electrical (PDF p. 14); Symptom Charts (pp. 20-34); Take Apart: Cover (p. 36), Drive Shelf (pp. 37-40), Speaker Bezel (pp. 49-51), Power Supply (pp. 52-55), Fan (pp. 56-57); Additional Procedures: Battery Verification and Replacement (pp. 111-115).
  3. ↑ Console5 TechWiki, "Macintosh Quadra 900" (retrieved 2026-09-30).
  4. ↑ TinkerDifferent forum thread "Tantalum vs Electrolytic. Can I just always use tantalum?" (retrieved 2026-09-30). Secondary source for 50 per cent voltage derating of solid tantalums, their short-circuit failure mode and the benign failure of polymer types.
  5. ↑ Apple, "Macintosh Quadra 900: Technical Specifications" (retrieved 2026-09-30). Battery type 3.6 V lithium; 303 W.
  6. ↑ Recycled Goods, "Apple 699-0394 Delta SMP-291AB Power Supply for Quadra 950 303.25W" (retrieved 2026-09-30).
  7. ↑ 68kMLA forum thread "Quadra 950 (Delta SMP-291AB) Power Supply Recap", posts by dschnur. Secondary source: one member's teardown of his own Quadra 950 supply.


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