Tuesday, October 13, 2015

Anatomy of a miswired power supply

This Thermaltake power supply was working fine with an IDE/PATA hard drive. Then when I added a SATA SSD, the computer would turn off immediately after turning on. I first thought the motherboard was doing something because of some weird incompatibility, but the same thing happened with the data cable disconnected.

After finding that the SSD doesn't work anymore in another PC, I checked the SATA power connectors. The colours seemed right, but the black wires were at +12V. The other wires were fine. That means the SSD got -7V power instead of +5V, and its ground was at -7V relative to SATA signal ground. Surprisingly, I managed to fix the SSD by bypassing a damaged component. This post is about fixing the power supply.

Here's the power supply label:

It has passed all the quality control checks, and the warranty sticker is intact:

The SATA power connectors look fine based on the wire colours:

Here's an external demonstration that they're not okay, by measuring resistance from a yellow +12V wire to their black wires. This is a cheap multimeter, and less than 3 ohms basically means zero ohms.

Now it's time to open up the PSU. It looks nice inside, without obviously bulging capacitors, and with very little dust considering how long it has been in use:

Note that the two big capacitors at the bottom of the picture could hold a dangerous high voltage charge, which could give you a big shock. The low voltage capacitors at the top should not be a risk.

The circuit board is held in by 4 screws, but also fan had to be unscrewed to free it more. I couldn't unplug the fan connector, because it was glued.

Here's a closeup of the problem. There are markings on top of the circuit board, indicating areas which connect to a specific voltage. One of the black ground wires connects to the wrong area, among the yellow +12V wires. Further up, the wire, you can heat shrink tubing covering where the wire splits into two wires. That way this one connection goes to both of the SATA ground wires.

There already was an unused hole in the ground area. A high wattage soldering iron made the repair easy.

Putting the power supply back together was a bit tricky. There are several places where things need to interlock properly. Take care around the grommet where the wires come out. The top of the case is supposed to fit into a groove in the grommet, so the wires don't chafe against the metal edge.

Here's a photo of the nicely voided warranty sticker. I still hope Thermaltake will reimburse me for this. Power supplies should not have defects like this, and most users have no way to protect themselves from this. It would be easy to check for miswiring with a tester at the factory.

Finally, here's a photo of the SSD repair. I'm not sure what was damaged, because it's hard to find information on some surface mount parts. I'm guessing it's a regulator that comes before the regulators which provide voltages that the SSD actually needs, providing greater voltage stability. I just bypassed it with a diode, which had to be filed down to fit inside the SSD case.