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  #21  
Old March 31st 17, 01:32 PM posted to uk.tech.digital-tv
Max Demian
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On 31/03/2017 09:11, Brian Gaff wrote:
ERm thanks for that unsettling news. Time was that all hi fi kit was
earthed, but not any more, but in the main with these devices all you feel
is a 50 hz tingle as you run your finger over brushed ally parts etc like
front panels and phono socket earths. You could be right about the aerial.
Being blind I tend to feel for the socket with one hand not being able to
see it.


I've got one of those touch table lamps with three brightnesses, which
seems to feed AC mains to the metalwork, presumably via a big resistor.
I can feel a tactile (but not electrical) tingling when I touch it, and
a neon screwdriver lights up dimly.

--
Max Demian
  #22  
Old March 31st 17, 02:01 PM posted to uk.tech.digital-tv
Bill Wright[_3_]
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On 31/03/2017 09:51, NY wrote:


Were old live chassis TVs full-current half-mains or did they have hefty
current-limiting resistors? Was there a risk of the aerial being at
half-mains when it was plugged in? I realise that all other exposed
parts of the chassis (eg volume and channel changing knobs on front)
were big bakelite things which would have provided plenty of insulation.


The isolating caps were usually built into the socket. If the socket got
broke some repair men would fit a stock one that didn't isolate, or they
might solder a bit of coax to the tuner input and run it to the outside
of the set.

My dad taught me to test every aerial with a dry finger end before
grasping it.

The old valve radios were terrible for this. I don't know what the
voltage or current was but it certainly hurt. It would send you flying.

Bill
  #23  
Old March 31st 17, 02:29 PM posted to uk.tech.digital-tv
Johnny B Good[_2_]
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On Fri, 31 Mar 2017 09:49:07 +0000, Huge wrote:

On 2017-03-31, David Wade wrote:


The original Sky Digital box, the Amstrad DRX100 had its chassis
earthed via the three core mains lead.
I think there was a slim Sony box that was also earthed.

I wonder why Sky specified this for some early boxes, then embraced
the Class II paradigm thereafter?

PAT testing failures?


I expect it was cheaper.


'Twas ever thus. I first met this design paradigm almost half a century
ago[1] when I signed up as a GPO apprentice technician.

Most components in a typical telephone handset (instrument) in both the
Tele 300 and Tele 600 series, along with various switches and selectors
used in Strowger exchanges had two or more functions. For example the
bell capacitor not only provided a path for the 16.67Hz ringing current
to operate the magneto bell, it also formed part of the anti-sidetone
circuit as well as part of the spark quench (snubber) circuit for the
rotary dial impulse interrupter contacts.

Back then, "Mass Production" meant product counts in the mere tens of to
hundreds of thousands so any component costs savings tended to be much
less frivolous than what we see today's bean counters enforcing on,
admittedly low value "Throw-away" consumer goods being produced in their
millions to satisfy 'consumer demand.

Two classic examples of such bean counter abuse are offered by those
packs of 3 to 5 disposable cigarette lighters available from pound shops
and the less trivial 3KW fast boil electric jug kettles available from
the likes of Argos and other major retailers in the 10 to 30 quid price
range.

In the case of the disposable lighters, the bean counters have insisted
(obviously against the recommendations of their design engineers) on
shaving off about a tenth of a penny's worth of plastic from the
operating levers of the whole pack of 3 to 5 lighters.

This guarantees premature failure before either the flint or the one
time gas fill is consumed by even as much as 50%. It also guarantees a
disgruntled consumer (or it fekin' well ought to except it seems the bean
counters know exactly how far they can "Push The Envelope" (their luck,
if you will) as far as consumer expectations here in the UK are
concerned).

What surprises me more is the lack of "Howls of Rage" from the 'Greenies'
over this abuse of resources - it just shows you how 'selective' your
typical 'Greeny' can be.

Moving on to what might be considered a 'less trivial' example of "Bean
Counteritis", the humble fast boil electric jug kettle, the trick used
here is to deliberately overlook the basic design rule of moulding (and
forging) component parts that are expected to withstand regularly
repeated cycles of dynamic stress such as a con-rod in an ICE or, in this
case, the wishbone shaped plastic lever in the base of the jug kettle by
which the user switches the kettle on or off.

Here, we have a vaguely wishbone shaped plastic part that, of necessity,
incorporates a kink or two in each arm to avoid fouling other components
in the base of the kettle over its normal range of movement. "Good Design
Practice"(tm) dictates that any such kinks not involve sudden angular
changes of shape (not even with the most obtuse of angles), instead,
requiring that such 'kinks' take the form of smooth bends which not only
eliminates a stress concentration point but is also a desired shape to
facilitate the efficacy of the moulding process itself.

Quite clearly, not just *one* rule but *two* rules of good manufacturing
process have been blatantly disregarded by the bean counters, not so much
to save manufacturing costs (in this case, it most definitely *doesn't*!
), so much as to create a "Replacement Kettle Market" (aka, the Gillette
Strategy) to keep the factory production lines running in perpetuity at
an artificially inflated rate.

Here, the manufacturer is playing a potentially disastrous game with his
target market demographic. On the one hand, he doesn't want to gain a
notoriety for being less reliable than his competitors who are also
playing the same game, whilst on the other hand, he doesn't want to
reduce his turnover in the 'Replacement Kettle' market.

Depending on the region these kettles are being marketed into, the
desired 'Lifetime' can range from as little as 15 months to maybe as high
as 36 months in regions where expectations are considerably higher than
what seems to me to be the case, here in the UK.

Assuming a UK marketing strategy, the manufacturer will have a target
lifetime endurance for that wishbone lever component in the region of 15
to 18 months so as to safely avoid having to deal with 'within warranty
returns' beyond the 5 to 10 percent 'over production' allowance to cover
such 'outliers', yet also avoid needlessly providing too generous a
service life at the expense of 'repeat trade'.

The manufacturer will also want to make any such 'failures' as silent
and undramatic as possible, especially where there may be the slightest
suggestion of a potentially hazardous failure involving even the
slightest risk of electrocution. Ergo, you can rest assured that the most
reliable component will be the most expensive to design and manufacture,
the heating element base assembly, the bit that is the essential heart of
an electric jug kettle's primary function. This is very likely to be able
to survive a decade or more's worth of rigorous service before finally
failing in a typically ostentatious fashion (flash or bang or both!).

Such dramatic failure modes are the very last thing the manufacturer
wants to see their product exhibiting at "End of Economic Life" (They say
there's no such thing as bad publicity but, as rules go, this is the
exception that proves otherwise as far as electric jug kettle
manufacturers are concerned).

Since it costs no more to design and manufacture a heating element
assembly that can last a decade or longer than it does to design one to
fail at around the 18 to 24 months mark, the manufacturer has to look at
some other, less safety critical part to "Do The Dirty Deed".

Quite obviously, the cheaper the 'Life Shortening' part can be made, the
better it is for the manufacturer, even better if said proprietary part
can be readily replaced should the worst come to the worst and he's faced
with having to ship a bag or three's worth of these cheap proprietary
plastic parts to the local repair agent's depot to deal with a flood of
within warranty returns that overwhelms the 'over-provisioning allowance'
that had been supplied to cover the (now mis) calculated requirement.

Whilst the only sensible solution to the question of needless pollution
by disposable cigarette lighters is simply to avoid buying the damn
things in the first place, the issue of 'sabotaged by design' in the case
of your typical fast boil electric jug kettle is probably worth a bit of
remedial effort on the part of a D-I-Yer, especially if they have access
to a 3D printer or someone who owns such a piece of kit all too eager to
demonstrate the point of owning such kit.

In this case, it's just a matter of making a date in the calender for a
year and a day after purchase to disassemble the kettle to retrieve said
bit of plastic with a view to either smoothing out the abrupt kinks or
else having said part duplicated in a modified form by a 3D printer since
the chances are extremely high that the rest of the kettle will last
another 9 years of hard service before failing in a satisfyingly dramatic
pyrotechnic fashion as electric kettles once did in more innocent times
past. :-)

[1] Minimising "Component Count" by minimising 'redundancy' in the
circuitry of an assemblage such as a telephone instrument or a rotary
selector switching stage in a Strowger exchange was a well established
practice that was old even back then (50 years or older!).

--
Johnny B Good
  #24  
Old March 31st 17, 02:39 PM posted to uk.tech.digital-tv
Bill Wright[_3_]
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On 31/03/2017 14:29, Johnny B Good wrote:

Most components in a typical telephone handset (instrument) in both the
Tele 300 and Tele 600 series, along with various switches and selectors
used in Strowger exchanges had two or more functions. For example the
bell capacitor not only provided a path for the 16.67Hz ringing current
to operate the magneto bell, it also formed part of the anti-sidetone
circuit as well as part of the spark quench (snubber) circuit for the
rotary dial impulse interrupter contacts.


blah blah blah erudition knowledge blah blah blah

[1] Minimising "Component Count" by minimising 'redundancy' in the
circuitry of an assemblage such as a telephone instrument or a rotary
selector switching stage in a Strowger exchange was a well established
practice that was old even back then (50 years or older!).


I calculate that you were typing and thinking at 45wpm when you wrote
that. 1,271 words in 28 minutes. Remarkable.

Bill
  #25  
Old March 31st 17, 03:09 PM posted to uk.tech.digital-tv
Johnny B Good[_2_]
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On Fri, 31 Mar 2017 09:11:49 +0100, Brian Gaff wrote:

ERm thanks for that unsettling news. Time was that all hi fi kit was
earthed, but not any more, but in the main with these devices all you
feel is a 50 hz tingle as you run your finger over brushed ally parts
etc like front panels and phono socket earths. You could be right about
the aerial.
Being blind I tend to feel for the socket with one hand not being able
to see it.


With just one or two such "Half Live" bits of kit linked together, it's
not too big an issue but, once you have three or more of these double
insulated boxes all commoned together, it all starts getting rather out
of hand.

I'd overlooked the issue of you literally having to blindly grope around
with an aerial lead and half live kit so my suggested use of Marigolds
may prove to be your best option to avoid any such further 'nasty
surprises' the next time you're obliged to delve around the back of the
TV set.

Regarding that phenomena with feeling a 50 (or is it 100?) Hz tingle as
you lightly brush a finger or the back of a hand against brushed
aluminium surfaces of double insulated Hi-Fi kit, it was this experience
with an Akai GX630DB reel to reel tape deck that led me to discover that
all bar one or two of the mains sockets in my very first house did not
have working safety earths. The tape deck wasn't of double insulated
design having its exposed metalwork connected via its 3 core mains lead
to the plugtop's earth pin.

This lead to my poking a test wire through the front room window
connected to a meat skewer pushed a few inches into the soil of my front
garden to provide a reference earth by which to check the ring main
socket earths with a multimeter. This let me track down the breaks in the
CPC which had allowed the earth pins to float at an induced halfish mains
voltage level. It was just a case of poor wiring connections which were
readily remedied once tracked down.

Once I'd sorted out this rather dangerous situation, I no longer felt
that mains tingle effect that I had vaguely noticed during the first 12
months or so that I'd had my Hi-Fi set up. I was rather glad I'd
discovered and remedied the fault in time to avoid a potentially fatal
accident since just about anything plugged into that ring main which
relied on a good earth connection to its exposed metalwork could have
resulted in electrocution in the event of a framing fault to the live
rather than the desired blowing of a safety fuse.

--
Johnny B Good
  #26  
Old March 31st 17, 04:27 PM posted to uk.tech.digital-tv
Indy Jess John
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On 31/03/2017 14:01, Bill Wright wrote:

The old valve radios were terrible for this. I don't know what the
voltage or current was but it certainly hurt. It would send you flying.


If it was one with a rectifier valve, the voltage difference between the
two rectifier anodes was typically 700V, feeding 350V into the HT for
the rest of the set.
The HT voltage coming out of the alternative rectifier bridge
arrangement was typically 325V.

There would be current limiters in the circuitry, but even so, you would
get quite a shock from the HT.

Jim
  #27  
Old March 31st 17, 05:37 PM posted to uk.tech.digital-tv
NY
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"Indy Jess John" wrote in message
...
On 31/03/2017 14:01, Bill Wright wrote:

The old valve radios were terrible for this. I don't know what the
voltage or current was but it certainly hurt. It would send you flying.


If it was one with a rectifier valve, the voltage difference between the
two rectifier anodes was typically 700V, feeding 350V into the HT for the
rest of the set.
The HT voltage coming out of the alternative rectifier bridge arrangement
was typically 325V.

There would be current limiters in the circuitry, but even so, you would
get quite a shock from the HT.


I had a greater-than-mains shock (according to a needle voltmeter - ie not
high resistance) it was about 350 V. That was from a tape recorder with a
valve amplifier. That HURT!

  #28  
Old March 31st 17, 07:46 PM posted to uk.tech.digital-tv
Benderthe.evilrobot
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Posts: 148
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"Brian Gaff" wrote in message
news
Just to put the record straight. I had removed all other plugs from the
back during the tests, so it would have had nowhere to go but me, I guess.
I doubt there was much current behind it. I suppose it was the surprise
factor, considering that unlike most old tellies which often did have the
chassis at half mains volts,


In the valve era; most sets had one mains wire soldered to the metal
chassis - with a 2 pin plug you had a 50/50 chance of mains live to the
metalwork. Most early SMPSU sets had the negative end of the bridge
rectifier to chassis - so that was always live, but there was less metalwork
to touch. All of those sets had a high voltage isolation unit behind the
aerial socket.


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  #29  
Old March 31st 17, 07:59 PM posted to uk.tech.digital-tv
Indy Jess John
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On 31/03/2017 19:46, Benderthe.evilrobot wrote:

In the valve era; most sets had one mains wire soldered to the metal
chassis - with a 2 pin plug you had a 50/50 chance of mains live to the
metalwork. Most early SMPSU sets had the negative end of the bridge
rectifier to chassis - so that was always live, but there was less metalwork
to touch. All of those sets had a high voltage isolation unit behind the
aerial socket.


I have seen battery/mains "portable" radios wired like that (with a 120V
battery for the HT) but nearly all the mains only valve radios I have
messed around with had a three core input lead and the chassis was earthed.

Some even had connectors on the back called Aerial and Earth and the
earth one was wired to the chassis.

Either I was lucky or you were unlucky in getting rather different memories.

Jim
  #30  
Old March 31st 17, 08:52 PM posted to uk.tech.digital-tv
Benderthe.evilrobot
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Posts: 148
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"Indy Jess John" wrote in message
...
On 31/03/2017 19:46, Benderthe.evilrobot wrote:

In the valve era; most sets had one mains wire soldered to the metal
chassis - with a 2 pin plug you had a 50/50 chance of mains live to the
metalwork. Most early SMPSU sets had the negative end of the bridge
rectifier to chassis - so that was always live, but there was less
metalwork
to touch. All of those sets had a high voltage isolation unit behind the
aerial socket.


I have seen battery/mains "portable" radios wired like that (with a 120V
battery for the HT) but nearly all the mains only valve radios I have
messed around with had a three core input lead and the chassis was
earthed.


Battery mains portables almost always had a mains transformer that also
provided isolation.

Most mains only sets were live chassis - I assume the ones with a mains
transformer must've been deluxe models, there were much fewer about.

Only very early TVs had mains transformers - dropper resistors were much
cheaper and lighter.

The new innovation of efficient flyback type scan generators and flyback
generated EHT had a fair bit to do with it - EHT had previously come from a
winding on the mains transformer and was downright lethal!


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