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Amiga Technical References
A4000 makes rattling noises, fan stalls on powerup, or fan does not turn. Solution: replace fan. Replacement fans: Radio Shack #273-243 or Panasonic FBK-08A12M, available from Digi-Key, Hosfelt, and others (see Sources).
Be aware that there are at least two styles of power supplies in the 4000; the fans may differ slightly. Some older 4000s had a power supply with a large hole for the fan, and a bolt-on grill protecting it. This power supply used an actual Panasonic FBM-08A12M. A newer power supply design has a built-in grill formed by holes in the side of the power supply; in this case, the fan is held in place by four odd-looking screws that are tapped directly into the holes in the fan's plastic frame, and the fan itself is a "MAX FLOW" generic.
Christopher Laprise has suggested that the A4000 power supply hot air vent is "horribly wrong," and I have to agree. Removing one or more of the horizontal strips of metal can make for a dramatic drop in temperature inside the A4000. I'd suggest using a nibbling tool for this, as it won't leave metal filings inside the power supply. As Chris notes, if you remove more than one section, make them alternate ones to leave something there to keep things from falling into the power supply.