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I have a fairly complex AV system, the main components of which are old but still good: a Toshiba 36" CRT TV, a Yamaha AX2 amp, a Sky box and a small HTPC. A few days ago the HTPC wouldn't switch on, so I thought I'd try the usual - unplug it, plug it in and reboot. But when I pulled the 12v supply out of the case, I received a very definite shock. When I unplugged the HTPC from the amp, and plugged it back in to its power supply, it switched on fine. Unfortunately, I then plugged it back in and tried to switch it on and this seems to have fried the internal 12v - 5v power card. I can get that replaced, but clearly I need to trace the source of the electric shock.
The HTPC is in a metal case and is supplied by a 12v power brick. The negative of the 12v supply is connected to earth and, when it is plugged in, so is the case of the HTPC. I presume when I unplugged the supply, I was touching the 12v negative and the case, which must have been receiving power through an audio lead from the amp.
The amp isn't earthed (2-core power cord). Nor is the sky box or, I think, the TV. I don't know if that is relevant or not. What I want to determine is which component, if any, is faulty or whether it is some sort of compatibility problem.
In attempting to isolate the problem, I've been measuring the voltage from the audio negative on the amp, sky box and TV to earth, and I am somewhat surprised to see voltages from 30V AC up to 120V AC, but I'm guessing these are maybe just stray capacitance because I can't feel them (I would prefer not to have to keep using my fingers as a test tool!).
So my first question is, how should I take a sensible measurement? I presume the very high voltages I'm seeing are accompanied with negligeble current. Should I be measuring the voltage across some resistance to get a more sensible reading? If so, what sort of resistance?
Hopefully, if somebody can guide me to taking sensible readings, then I can start plugging the various bits of equipment back together again and see when the problem reappears. I guess some guidance as to what are acceptable readings would be useful too.
The HTPC is in a metal case and is supplied by a 12v power brick. The negative of the 12v supply is connected to earth and, when it is plugged in, so is the case of the HTPC. I presume when I unplugged the supply, I was touching the 12v negative and the case, which must have been receiving power through an audio lead from the amp.
The amp isn't earthed (2-core power cord). Nor is the sky box or, I think, the TV. I don't know if that is relevant or not. What I want to determine is which component, if any, is faulty or whether it is some sort of compatibility problem.
In attempting to isolate the problem, I've been measuring the voltage from the audio negative on the amp, sky box and TV to earth, and I am somewhat surprised to see voltages from 30V AC up to 120V AC, but I'm guessing these are maybe just stray capacitance because I can't feel them (I would prefer not to have to keep using my fingers as a test tool!).
So my first question is, how should I take a sensible measurement? I presume the very high voltages I'm seeing are accompanied with negligeble current. Should I be measuring the voltage across some resistance to get a more sensible reading? If so, what sort of resistance?
Hopefully, if somebody can guide me to taking sensible readings, then I can start plugging the various bits of equipment back together again and see when the problem reappears. I guess some guidance as to what are acceptable readings would be useful too.
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