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A friend has offered to give me an iMac (either 2009 0r 2012 model but I haven't seen it close up yet). It works, other than a broken power supply.

This ifixit page details the power supply replacement process:

https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+EMC+2308+Power+Supply+Replacement/1771#s9148

However, this guide warns you not to touch the exposed face of the power supply as the capacitors can shock you if they have retained a charge.

Now I'm no stranger to the inside of a computer, but I haven't replaced a power supply with exposed capacitors before. So to avoid the risk altogether, how long would it take for the capacitors to discharge if it's simply left unplugged. How can I safely test for discharge with a multimeter?

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Usually, if the iMac lost power suddenly (e.g. pulling the AC plug from power while it's still switched on) you would need to allow some time for the capacitors to discharge before working on it. However, if the iMac was shut down properly (e.g. Apple > Shut Down) then it should discharge the power supply capacitor fairly quickly (not immediately, but close).

Typically, the input capacitors on a computer power supply have between a 180 to 360 volt charge on them. How long they hold this charge depends on the circuit, but a bleeder resistor sits across them to dissipate this charge.

Regardless, in your case it sounds as if the iMac hasn't been powered up for a while and the power supply isn't working. Of course, since it's broken (or faulty) there's always the potential for a problem. And, since you can't be 100% sure what the problem is, make sure there is no AC cable plugged in while working on it.

TIP: DO NOT wear a watch, bracelet, necklace, etc (or any metallic objects) which could accidentally make contact with the power supply circuitry.

Regarding the use of a multimeter, you could test across the capacitors with one. If you actually get a voltage reading then you could even use the multimeter in this way to discharge the capacitors slowly until the voltage reads zero. If you're wanting to use a multimeter to discharge, I'd recommend an analogue one as a high impedance digital meter would take a while to discharge.

However, as I mentioned above, it sounds as if the iMac hasn't been powered up for a while, so you should be right to go.

  • You've probably repaired more machines than me over the years, but I'd always leave the power cable connected, with the socket off, for proper earthing [grounding] & as a static discharge precaution, so you're at the same potential as the chassis before even touching anything else. You do have to be certain that off is off, of course, but UK sockets are pretty good in that respect. – Tetsujin Mar 9 '17 at 13:13
  • @Tetsujin, Regardless of whether the machine is plugged into a grounded circuit, disconnected, or running off of a hamster wheel, you’re charges are immediately equalized the moment you touch the chassis. A grounding strap will ensure you’re maintaining the same potential when you’re breaking contact to utilize tools etc... I would never advise anyone to leave a machine plugged into mains, even if you have the outlet shut off. Having the machine grounded to earth will actually do nothing to mitigate the potential difference in static charges between you and the machine... – Mikeatine Jan 21 at 19:24
  • ...You may be carrying a difference of 10’s of thousands of volts by walking across the carpet floor, hence the risk of damaging sensitive microelectronics that are designed to handle max loads in the 10’s of volts but the actual wattage being generated by static electrify is minuscule with only milliamps of current. The lack of current differenctial means that the computer being grounded to earth will do nothing to balance your static charges. It’s good safety practice to always makes habit of visually noting that the machine is physically unplugged whenever possible. – Mikeatine Jan 21 at 19:24

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