USB short caused by polarity of AC Current?

I've got a HP plotter that connects via USB. The last time I plugged oit into my server it shorted the usb hub and fried the motherboard. The only connection was the USB plug. I replaced the MOBO thinking it was the USB hub that shorted but when I went to plug in the cable, it shorted to the metal bracket on IO plate. Luckily the computer wasn't on and didn't short the mobo this time.

I called a local 'computer' repairman ( I use the term loosely as you'll see) who took the unit home to repair. He brought it back a couple of days later and reported that he felt it was a polarity issue on the house current. It is an old house and does not run a fixed ground. I had been using the plotter in the back room off the usb port with my Laptop that wasn't plugged in, i.e. grounded. When it shorted above, it was plugged into a different outlet than the computer and hub it fried.

1. Is this a plausible scenario? I didn't really trust the repairman as he never got the printer up and was using WIN98 for diagnostics and never even got the printer loaded. (I don't think there's even drivers for Win98 for this beast.) He only informed me of this when he returned the unit with a bill for $20. Needless to say, I'm very skeptical of his analysis. HP service wants $300 a hour starting when they leave Atlanta an hour and 1/2 from here so you can see the impracticality of that. So, does this sound like a plausible explanation?

2. Secondly, How can I check the voltage on the USB cable coming off the printer? I've got a volt meter and other tools and extra cables I could strip one down and build a tester but need the directions to cobble it together. Any suggestions on how to build a USB voltage tester?

I really can't afford to blow up my server or laptop for that matter but either need to get this equipment up or get rid of it. Of course, HP has discontinued the model but I can still find ink and printheads online. Any help someone can give me would be appreciated.

Polarity on AC? Huh?

AC stands for alternating current, there is no + and - like in DC.

1) I don't see how being in a different socket would fry it. The power from the laptop isn't used if the hub has its own power source.

2) Here is the pinout for it.

Actually, yes. Kind of. Basically you have some parts of your house that are wired properly, ie earth grounded, and some parts that have a floating ground. So since the cases themselves should be connected to a true ground (not a return line) the server has this (is actually at 0 volts) and the plotter does not (some XX volts) so when you connect them together there is current flow and something fries.

But I would have assumed that was an extremely unlikely case.

What you should do is plug the plotter into whatever outlet the server is plugged into, which ideally would put both units onto a circuit that has a true earth ground, and you should be okay.

To check the voltages, find said earth ground (outside of the case or something) and then put the other end of the voltmeter to whatever you want to test. In this case, the outside metal shield on the USB port. If this is to blame it should have some voltage on it.

What you should do is plug the plotter into whatever outlet the server is plugged into, which ideally would put both units onto a circuit that has a true earth ground, and you should be okay.

Click to expand...

My concern is if it wasn't the polarity, do I really want to plug in the usb port without checking it first, regardless of AC source. The short didn't appear to a be a 5v charge, it melted the end of the usb cable where it touched. I had to wiggle it loose from the bracket. I lucked out the first time and was able to find a replicable board and swap the entire contents and have it fire and run the first time off the original windows install. However, I might not be as fortunate next time. I'll build a tester and run a voltage check before I plug it into anything.

Thanks for the response.

So I checked the leads coming off the printer and it is zero voltage on all points and possible combinations of wires. I ran a continuity test on each wire prior to inserting the cable. Inserted the cable and then checked each wire for a charge, which is registering zero voltage. I'm assuming that since the port isn't in use this would be correct. I'm fixing to load up drivers and connect the laptop. Any last minute suggestions besides to back up the laptop?

Printers up and running. Thanks for the input.