USB troubleshooting - Don’t forget to check any USB hubs!

Today I was involved in troubleshooting problems with a couple of USB devices that had just stopped working.  To be accurate, it was one that appeared to have stopped working (a USB fingerprint reader). 

Rather then start messing about with drivers I began the investigation by tracing the wire from the device to the port it was connected to.  The fingerprint reader was connected to a 4-port USB hub, so I unplugged it and temporarily hooked it up to one of the front-panel USB ports - Success!

It was then that I noticed that there were no LEDs lit up on the hub.  Strange.  Next I traced the power cable to the adaptor in the wall outlet and checked that it was plugged in.  All seemed OK.

I unplugged the USB hub and the adaptor and took it to my laptop for testing.  Nothing.  Dead.  Seemed that the USB hub was dead. 

I don't like to chuck things away without knowing for sure if they are dead and trying to figure out what killed it.

"Felix qui potuit rerum cognoscere causus"
Happy is he who knows the cause of things.

I whipped out my trusty multimeter and checked out the voltage being given out by the adaptor and found it to be over 12V.  Checking the information on the adaptor body and the hub itself told me that the hub required 7.5V/2.1A.  This was definitely the right adaptor so more that likely the adaptor malfunctioned, fed the hub too much juice (over a long period probably) and burned out it out.  Just to check, I use a variable power supply to give the hub the right amount of power.  Still dead.

I went back and found where the adaptor plugged into the wall.  It was on the back of a movable PC trolley that was pushed against a wall.  By pushing trolley against a wall the adaptor was pushed right up to the wall too and probably caused it to overheat.

The moral of this story:

Work methodically to find out what's wrong - don't jump to conclusions (for example, I could have assumed it was the fingerprint reader and started uninstalling drivers and such)
Once you've found the problem, learn all you can about it (here, I tried to to work out if it was the hub, adaptor, or both that was dead)
Try to figure out why the problem happened and take steps to prevent the problem happening again.