This is a problem that comes up on a regular bases here. Faulty or suspected faulty power supply. I am sure most here new about this little device, but I did not. At least not this cheap. By the way I bought one locally for $25. I work on enough computers to justify the purchase of one. A user who may need it only once may be better off paying another $15 for a $40 power supply.
I just thought some members may want one.http://www.geeks.com/details.asp?invtid=PS-124&cat=GDT
I have tried it on bad, good, and a weak power supply that I compared voltage to with a voltmeter. On the 12v lead I had 9.6 volts and the tester showe it as bad. It was weak but should be replaced.
I will let you know when I have more data on it.