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This machine runs fine with 122-123volts AC in. With normal 120volts AC, internal and external USB-powered HDD’s fail. I assume the power supply is failing. What is the experience with this? A particular capacitor or caps (as in G5’s) or other component? Or should I just replace the PSU? Update (11/29/2016) Hi Mayer, to clarify: At 120vac the internal HDD fails, and the external, USB3 HDD backup fails as well. Additional info: About a year ago, after 2 years of service, at 120vac, the external HDD failed. I suspected voltage drop across the connectors joining a 2M USB extension cable to the short USB2/USB3 cable shipped with the HDD. I replaced the 2-part cable with a single 2m USB2/USB3 cable and all was well for a year. I did not measure voltages at that time. So evidently power drop has progressed since then. BTW, I also got my Apple II in ‘79. Quickly got another one for my office. Interfaced it with my Telex machine. Then got my secretary into word processing on it.

To start with I would strongly recommend getting a good UPS for your system as its sounds like your AC power is not very stable. The power supply offers power at different voltages across your system. Its possible the 5 Volt rail your USB ports offer to your external drive may have a problem (as well as causing your internal HD issues). A useful tool here is a USB power meter like this one: AboveTek which you can see whats happening without opening you system as well as check your USB devices. Most likely your power supply has issues as to fixing it Vs replacing. I would just replace it as it’s just easer and safer. Here’s the IFIXIT guide: iMac Intel 20" EMC 2133 and 2210 Power Supply Replacement. Update (12/01/2016) Yes, your power supply offers other voltages as well as multiple feeds of some. Your internal HD has access to 3.3 and 12 volts feeds as well as 5 volts I spoke about before. Here’s a good diagram which lists the power lines. The power pin’s (3 per) are carrying the same voltage feed each.

As you stated your external drive was having a problem the USB ports offer only 5 volts which is why I suspected the 5 volt rail was having problems.

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