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/