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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
In for switch replacement but owner says he has , sometime back , got a
shock off the mains lead by touching the earth pin when just unplugged from the mains. I told him probably static discharge rather than anything to do with mains , but explored , just in case. The large conventional type mains transformer is mounted on a metal plate that is held to the wood of the casing by 4 bolts. There is no earth strap , or ever has been , to this plate or transformer frame, so not earthed. The 4 bolts are exposed through casing on base of the amp. Hum issue if earth strap put in place ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
#2
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
"N_Cook" wrote in message ... In for switch replacement but owner says he has , sometime back , got a shock off the mains lead by touching the earth pin when just unplugged from the mains. I told him probably static discharge rather than anything to do with mains , but explored , just in case. The large conventional type mains transformer is mounted on a metal plate that is held to the wood of the casing by 4 bolts. There is no earth strap , or ever has been , to this plate or transformer frame, so not earthed. The 4 bolts are exposed through casing on base of the amp. Hum issue if earth strap put in place ? Most consumer-grade amps have two-prong plugs, so what are you going to earth it to? The ground in such an amp is always relative, often the CT on the power transformer itself or back end of the bridge rectifier, quite possibly floating a few volts away from earth ground. It must be rare enough to satisfy regulators that the mains transformer fails open circuit but shorted to the case. If the unit has a three prong plug, I'd say give a ground strap a try if you're concerned... most power trannies are bolted onto a metal case, not wood, and the case is grounded in the case of a three prong plug. Ground loop noise problems would depend quite a lot on the grounding scheme used in the amp, i.e. whether or not power and signal grounds are appropriately isolated to minimize interference. Dave |
#3
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
Dave wrote in message
news:Bbf%j.235$i74.16@edtnps91... "N_Cook" wrote in message ... In for switch replacement but owner says he has , sometime back , got a shock off the mains lead by touching the earth pin when just unplugged from the mains. I told him probably static discharge rather than anything to do with mains , but explored , just in case. The large conventional type mains transformer is mounted on a metal plate that is held to the wood of the casing by 4 bolts. There is no earth strap , or ever has been , to this plate or transformer frame, so not earthed. The 4 bolts are exposed through casing on base of the amp. Hum issue if earth strap put in place ? Most consumer-grade amps have two-prong plugs, so what are you going to earth it to? The ground in such an amp is always relative, often the CT on the power transformer itself or back end of the bridge rectifier, quite possibly floating a few volts away from earth ground. It must be rare enough to satisfy regulators that the mains transformer fails open circuit but shorted to the case. If the unit has a three prong plug, I'd say give a ground strap a try if you're concerned... most power trannies are bolted onto a metal case, not wood, and the case is grounded in the case of a three prong plug. Ground loop noise problems would depend quite a lot on the grounding scheme used in the amp, i.e. whether or not power and signal grounds are appropriately isolated to minimize interference. Dave 240V , UK use. There is just the input ground wire to the plate housing the mains fuse holder. There is no specific bond wire to the main PA just relying on metal brackets/spacers and self-tap screws "bonding" back to the inlet plate. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
#4
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
Dave wrote in message
news:Bbf%j.235$i74.16@edtnps91... "N_Cook" wrote in message ... In for switch replacement but owner says he has , sometime back , got a shock off the mains lead by touching the earth pin when just unplugged from the mains. I told him probably static discharge rather than anything to do with mains , but explored , just in case. The large conventional type mains transformer is mounted on a metal plate that is held to the wood of the casing by 4 bolts. There is no earth strap , or ever has been , to this plate or transformer frame, so not earthed. The 4 bolts are exposed through casing on base of the amp. Hum issue if earth strap put in place ? Most consumer-grade amps have two-prong plugs, so what are you going to earth it to? The ground in such an amp is always relative, often the CT on the power transformer itself or back end of the bridge rectifier, quite possibly floating a few volts away from earth ground. It must be rare enough to satisfy regulators that the mains transformer fails open circuit but shorted to the case. If the unit has a three prong plug, I'd say give a ground strap a try if you're concerned... most power trannies are bolted onto a metal case, not wood, and the case is grounded in the case of a three prong plug. Ground loop noise problems would depend quite a lot on the grounding scheme used in the amp, i.e. whether or not power and signal grounds are appropriately isolated to minimize interference. Dave 240V , UK use. There is just the input ground wire to the plate housing the mains fuse holder. There is no specific bond wire to the main PA just relying on metal brackets/spacers and self-tap screws "bonding" back to the inlet plate. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
#5
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
N_Cook wrote:
Dave wrote in message news:Bbf%j.235$i74.16@edtnps91... "N_Cook" wrote in message ... In for switch replacement but owner says he has , sometime back , got a shock off the mains lead by touching the earth pin when just unplugged from the mains. I told him probably static discharge rather than anything to do with mains , but explored , just in case. The large conventional type mains transformer is mounted on a metal plate that is held to the wood of the casing by 4 bolts. There is no earth strap , or ever has been , to this plate or transformer frame, so not earthed. The 4 bolts are exposed through casing on base of the amp. Hum issue if earth strap put in place ? Most consumer-grade amps have two-prong plugs, so what are you going to earth it to? The ground in such an amp is always relative, often the CT on the power transformer itself or back end of the bridge rectifier, quite possibly floating a few volts away from earth ground. It must be rare enough to satisfy regulators that the mains transformer fails open circuit but shorted to the case. If the unit has a three prong plug, I'd say give a ground strap a try if you're concerned... most power trannies are bolted onto a metal case, not wood, and the case is grounded in the case of a three prong plug. Ground loop noise problems would depend quite a lot on the grounding scheme used in the amp, i.e. whether or not power and signal grounds are appropriately isolated to minimize interference. Dave 240V , UK use. There is just the input ground wire to the plate housing the mains fuse holder. There is no specific bond wire to the main PA just relying on metal brackets/spacers and self-tap screws "bonding" back to the inlet plate. Is there a capacitor strapped across the mains switch, or the input socket? That would account for the clout he got when touching the mains plug pins Ron(UK) |
#6
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
Ron(UK) wrote in message
... N_Cook wrote: Dave wrote in message news:Bbf%j.235$i74.16@edtnps91... "N_Cook" wrote in message ... In for switch replacement but owner says he has , sometime back , got a shock off the mains lead by touching the earth pin when just unplugged from the mains. I told him probably static discharge rather than anything to do with mains , but explored , just in case. The large conventional type mains transformer is mounted on a metal plate that is held to the wood of the casing by 4 bolts. There is no earth strap , or ever has been , to this plate or transformer frame, so not earthed. The 4 bolts are exposed through casing on base of the amp. Hum issue if earth strap put in place ? Most consumer-grade amps have two-prong plugs, so what are you going to earth it to? The ground in such an amp is always relative, often the CT on the power transformer itself or back end of the bridge rectifier, quite possibly floating a few volts away from earth ground. It must be rare enough to satisfy regulators that the mains transformer fails open circuit but shorted to the case. If the unit has a three prong plug, I'd say give a ground strap a try if you're concerned... most power trannies are bolted onto a metal case, not wood, and the case is grounded in the case of a three prong plug. Ground loop noise problems would depend quite a lot on the grounding scheme used in the amp, i.e. whether or not power and signal grounds are appropriately isolated to minimize interference. Dave 240V , UK use. There is just the input ground wire to the plate housing the mains fuse holder. There is no specific bond wire to the main PA just relying on metal brackets/spacers and self-tap screws "bonding" back to the inlet plate. Is there a capacitor strapped across the mains switch, or the input socket? That would account for the clout he got when touching the mains plug pins Ron(UK) There is a .47uF , 330 V polycarbonate between neutral and live on the amp side of the mains switch which would self discharge through the transformer in mS but nothing else except a fuse. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
#7
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
Is there a capacitor strapped across the mains switch, or the input socket? That would account for the clout he got when touching the mains plug pins Ron(UK) There is a .47uF , 330 V polycarbonate between neutral and live on the amp side of the mains switch which would self discharge through the transformer in mS but nothing else except a fuse. Are you sure it`s on the transformer side of the switch? Ron |
#8
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
Ron(UK) wrote in message
... Is there a capacitor strapped across the mains switch, or the input socket? That would account for the clout he got when touching the mains plug pins Ron(UK) There is a .47uF , 330 V polycarbonate between neutral and live on the amp side of the mains switch which would self discharge through the transformer in mS but nothing else except a fuse. Are you sure it`s on the transformer side of the switch? Ron Mystery solved, yes that cap is on the mains cord side of the switch. So what class of electric shock do you call that, high impedance , DC , of little medical consequence in itself, but surprise effect could have indirect consequences. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
#9
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
second attempt
Ron(UK) wrote in message ... Is there a capacitor strapped across the mains switch, or the input socket? That would account for the clout he got when touching the mains plug pins Ron(UK) There is a .47uF , 330 V polycarbonate between neutral and live on the amp side of the mains switch which would self discharge through the transformer in mS but nothing else except a fuse. Are you sure it`s on the transformer side of the switch? Ron Mystery solved, yes that cap is on the mains cord side of the switch. So what class of electric shock do you call that, high impedance , DC , of little medical consequence in itself, but surprise effect could have indirect consequences. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
#10
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
"Ron(UK)" wrote: Is there a capacitor strapped across the mains switch, or the input socket? That would account for the clout he got when touching the mains plug pins That's what I was thinking too but it shouldn't happen by touching the earth pin alone. Of course, likely he didn't just touch the one pin. Graham |
#11
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
N_Cook wrote: There is a .47uF , 330 V polycarbonate between neutral and live on the amp side of the mains switch Do you seriously mean POLYCARBONATE ? They are UNSAFE for across the mains use because they have very poor pulse tolerance. It should be a safety X-type cap. Graham |
#12
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
N_Cook wrote: Mystery solved, yes that cap is on the mains cord side of the switch. As suspected. So what class of electric shock do you call that, high impedance , DC , of little medical consequence in itself, but surprise effect could have indirect consequences. The IEC safety regs (60065) specify the maximum energy allowable for such filtering caps. I forget the clause and I don't fancy looking it up right now. Graham |
#13
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Safety issue ? -Phonic Power Pod 1062,mixer amp, from 2002
Eeyore wrote in message
... N_Cook wrote: There is a .47uF , 330 V polycarbonate between neutral and live on the amp side of the mains switch Do you seriously mean POLYCARBONATE ? They are UNSAFE for across the mains use because they have very poor pulse tolerance. It should be a safety X-type cap. Graham X1 type, large yellow cased , I thought were polycarbonate but I stand corrected. I thought the critical factor was they had to be self-healing which could be from pulse or surge. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
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