I have recently discovered some issues with some ac adapters, USB hub 5VDC adapters, external hard drive enclosure 12VDC adapters from no-name or small-name manufacturers.
The ground pin carries a large and persistent 60Hz AC voltage against earth ground. My scope maxed out at about +-38V when measuring the ground of these adapters. It looks like sine wave with some signs of rectification. Where the voltage crosses ground is not a single point, but a small range, similar to that of a diode's turn-on curve. The voltage is AC. I grabbed the negative of my scope with my right hand and positive (connected to the adapter ground) with left hand to provide a poor ground path but the signal didn't disappear.
Out of curiosity, I tested a few other adapters, from netgear etc. The ground carries a small random voltage of a few volts as I expected and grabbing connections the same way I described the voltage vanishes into a volt or less, as I expected.
I didn't notice these voltages until yesterday. I've been having issues with my HDMI output. Occasionally the output will black out for a second or so. Sometimes it's more frequent and annoying. I thought maybe it was the old monitor. I bought a new monitor and the issue still showed up. I thought my laptop has issues, until yesterday. I was unplugging my powered USB hub to take my laptop from my desk (very rarely I do that). As I was unplugging the USB cable that connects to my USB hub, my monitor blacked out again with some popping sound on the monitor's integrated speaker. Out of curiosity, I tried to plug and unplug the USB cable, and same thing happened again! I then tried not to fully insert the USB plug, but only touching the shield to the shield of the port, and it happened again!
So I tested its output. Relative voltage between power and ground is about 5V, that is if I ground its ground with my scope ground. If I test the adapter's ground or power, I get overload: 40V+ AC at 60Hz. Several cheap adapters 5V and 12V have the same issues.
Then I tested a few brand-name adapters for routers and found they had good results.
So what the heck? Are these manufacturers too cheap to produce adapters out of the right designs? My computer has been plagued by this random occasional black out for over a year. I think that matches with how long I have owned that powered USB hub. This sucks. I wonder if any USB devices have been damaged by this voltage.