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  • Earth leakage

    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.

  • #2
    the audio output of an amplifier is 70 volts. this is measured across the final amplifier transformer. current is negligible as it is mostly audio power - varying frequencies. an oscilloscope is the only instrument to measure output. its impedance is low enough so as not to offer stray capacitance or resistance to the circuit. an analog meter might offer some assistance - digital meters won't work.

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    • #3
      Thanks for your reply.

      Maybe I didn't make myself clear. The stray voltage is on the audio inputs to the amp, not the outputs, and also, when I measure it open circuit, on the inputs to the sat box and the TV. Surely if I took a, say, 1K resistor and connected it between the audio input and ground, then used a multimeter to measure the voltage across that resistor, I should be able to get a reading for the power flowing through that resistor (I can just about remember school-boy equations for voltage, current, resistance and power). But what I am unsure of is whether I should be using a 1K, 100K or 1 ohm resistor and, depending on what I do choose, what an acceptable voltage would be. I take your point without an oscilloscope, which I don't have, I won't have any idea about the frequency, but I'm not bothered about that. All I want to do is isolate which item, if any, is faulty so I can get it serviced.

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      • #4
        before you can measure the input the circuit must be loaded with a speaker or is ohmic equivelant - 8 ohms. formulas - power = P, voltage = E, current = I, resistance = R. P=EI, P=I [squared]R, E=IR, R=E divided by I, e = P divided by I.
        A capacitor will pass AC[frequwncy} and block DC. an inductor will block AC and pass DC. look for a shorted mylar or ceramic capacitor to ground.

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