Re: Aerial Amp - feeling a tingle

The message <4t2qacF11rto5U1@xxxxxxxxxxxxxxxxxx>
from Dickie mint <richard_taylorspam01@xxxxxxxxxxxxxxx> contains these words:

Johnny B Good wrote:

It's not unusual for older TV sets (and possibly newer sets too) to
induce 'half mains' voltage into an unearthed co-ax courtesy of the
coupling caps intended to 'isolate' a potentially live chassis in the
case of old designs or the filter capacitors on the mains input side of
modern SMPSU designs intended to 'RF bond' the chassis to the 'screening
ground reference' of the antenna input circuit when 'double insulated'
rules allow the manufacturer to avoid use of a safety earth connection.

<snip>


I had this problem back in 1998, when DTT was starting up and I had to
organise a monitor stack of STBs connected to TVs.

Our AV technician was going on about having to use isolating aerial
sockets. He quoted some article somewhere I never have got my hands on.

There are high value resistors involved in this chassis mid pointing too?

Anyone know of it?

Ah, yes, forgot about the 1Meg static discharge resistor wired across
the chassis 'earth' coupling capacitor to provide a 'leakage path' to
prevent static charge build up.

The capacitors were of the ceramic disc type which, ISTR, could
tolerate high voltage breakdown without going permanently short circuit.
These were usually rated at a kilovolt or higher to reduce the risk of
'flash-over' due to (the possibility of) having a 350volt peak amplitude
mains voltage superimposed on a few hundred volts dc drop across the
static discharge resistor during periods of low humidity high
atmospheric potential gradient conditions.

--
Regards, John.

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.

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