On Sun, 13 Jul 2008 18:47:12 +0100, "David Looser"
wrote:

"Dave Plowman (News)" wrote in message
...


Dunno. Faults happen. What you have to remember is when TV sets became
available much of the country still used two pin mains sockets - ie no
earth and no polarisation. 13 amp final ring circuits were a post WW2
device to save on materials - and although used on new builds, plenty
still had pre-war wiring - some even converted from DC days. So you simply
couldn't rely on the plug being correctly inserted to make the chassis
neutral.

DC lasted in some areas right up to the 60s. If you were unlucky enough to
have a DC supply with the +ve pole earthed you had no choice but to make the
chassis live.

Also if you looked at the design of such devices, any metalwork
connected to the chassis was protected from being touched accidently -
things like the grub screws holding on knobs would have plastic inserts
over them.

Until they got taken out and not replaced, or the knob got broken and the
metal shaft of the control was exposed. In practice many sets had touchable
metalwork. One fatality I heard about was to a child who touched the heads
of screws that had been used to attach a stand to the underside of the TV
cabinet and were long enough to make contact with the chassis.

The reason TVs had live chassis was to reduce costs

And to allow operation in the remaining DC areas.

- the 500vA or so
transformer needed to isolate the chassis was an expensive component.


The standard all-valve "large screen" TV of the 50s/early 60s consumed 150W,
of which half went into keeping the heaters alight. So a 250VA transformer
would have been plenty big enough.

David.


Not audio or video related, but we still use DC - up to 10K volts at
1.6 amps - we try not to touch it :-)
--
Cheers

Peter