Hi All,

The strangest thing happened the other day. I had just made a minor
repair (bad solder joint on the tuner) on an RCA Proscan PS27112/
CTC169CB5 set. With the back cover still off, I unplugged the set (these
ones use IEC line cords minus the ground connection), pulling the cord
from the socket on the back of the set. I moved some loose wires to
their correct locations, then began reassembling the case. With the
cover back on, I was reaching for one of the screws next to the AC line
connector, nutdriver in hand. I was shocked, literally, as I drew a 1/2
inch arc from the A/C connector to the nutdriver, and through me!

I brought the nutdriver back over to the the line socket and got another
(smaller) zap! This happened despite the fact that I was holding the
nutdriver by its insulated grip. It felt like a static shock, only
stronger than the usual type you get touching an appliance or doorknob
after walking across the carpet.

The set was sitting on a wooden table with nothing else on it (except a
couple of tools), and I was sitting on a wooden chair. The set was not
connected to anything, via coax or any other cables. I happened to have
my shoes off, and one of my sock-clad feet was on top of the insulated
line cord I had just unplugged.

Given these conditions, it seems the only explanation is that there was
an excess of static charge on the hot side of the power supply. Yet, it
had to accumulate AFTER I unplugged the set, otherwise it would have
found its way to ground, through the line cord.

My only guess to the cause of this phenomenon is that a wire from the
primary side of the power supply was close to the anode wire, and this
drew some electrons toward it, acting as a tiny capacitor. Then, when I
pulled some of the wires back (I do remember having moved some wires
away from the anode wire), the capacitance decreased to nearly zero, and
the net result was a large number of electrons left behind on the
primary part of the supply.

There is a physics experiment where you can show that for an air
capacitor with two parallel plates Q = CV. If you charge this capacitor
and move the plates apart, the charge should stay constant, and the
voltage should rise as the capacitance decreases. I guess as you move
them far enough apart, you would end up with one plate with positive
charge, and one with negative change.

For this explanation to make sense, my foot must have not been close to
the line cord on the floor while I was moving the wire. I guess I moved
my foot close to the cord sometime just before reaching for the screw
with that nutdriver.

Has this happened to anyone else? Does this explanation make sense to
anyone? Is there an alternate explanation?

This set had no history of any arcing or horizontal problems whatsoever.
Plugging and unplugging the line cord now with the cover on does not
reproduce the problem, and I wouldn't expect it to. In fact, it works
perfectly right now. I think I'm more curious about this more for
intellectual reasons than any concern about the set.

Thanks,

--
Stephen Santarelli

stephen
'AT'
nii.net

"stephen" a écrit le/schreef op/wrote on 20/11/2003
Hi All,

Hi, Stephen,

I did experience this, a few times and I think that I can reproduce
this...

In the shack, plugging a working tv set out of the mains outlet and
then carrying it the the Van for transportation back to the customer,
when you carry the set, with the tube against your chest, if at that
moment you touch the mains plug you could have the symptom you
describe.

I think that you statically load up from the tube in touch with your
body, and discharge via the main plug?

Another theory? :-?

Have a nice day,

Alain

--
albeguin @ village . uunet . (+ Belgium Country Code)
Free NL/FR/ENG Newsreader "MesNews" http://mesnews.no-ip.com/

The set should have been properly grounded to an external ground.
Most likely there was a build up of charge from the stored charge in
the HV section. The complete chasis can come to a type of equilization
from this charge, if it is floating in relation to a true ground. I
have seen this mostly in large screen CRT sets. Having a clip lead
connected from the main ground of the set to an external electrical
ground will stop this problem.

If there is a build up static charge from the HV to the main chasis of
the set, and you were to touch a part of the circuit board, and cause
a discharge, many components on the board can be damaged.


Jerry Greenberg

--





stephen wrote in message news:dFVub.53715$Dw6.275973@attbi_s02...
Hi All,

The strangest thing happened the other day. I had just made a minor
repair (bad solder joint on the tuner) on an RCA Proscan PS27112/
CTC169CB5 set. With the back cover still off, I unplugged the set (these
ones use IEC line cords minus the ground connection), pulling the cord
from the socket on the back of the set. I moved some loose wires to
their correct locations, then began reassembling the case. With the
cover back on, I was reaching for one of the screws next to the AC line
connector, nutdriver in hand. I was shocked, literally, as I drew a 1/2
inch arc from the A/C connector to the nutdriver, and through me!

I brought the nutdriver back over to the the line socket and got another
(smaller) zap! This happened despite the fact that I was holding the
nutdriver by its insulated grip. It felt like a static shock, only
stronger than the usual type you get touching an appliance or doorknob
after walking across the carpet.

The set was sitting on a wooden table with nothing else on it (except a
couple of tools), and I was sitting on a wooden chair. The set was not
connected to anything, via coax or any other cables. I happened to have
my shoes off, and one of my sock-clad feet was on top of the insulated
line cord I had just unplugged.

Given these conditions, it seems the only explanation is that there was
an excess of static charge on the hot side of the power supply. Yet, it
had to accumulate AFTER I unplugged the set, otherwise it would have
found its way to ground, through the line cord.

My only guess to the cause of this phenomenon is that a wire from the
primary side of the power supply was close to the anode wire, and this
drew some electrons toward it, acting as a tiny capacitor. Then, when I
pulled some of the wires back (I do remember having moved some wires
away from the anode wire), the capacitance decreased to nearly zero, and
the net result was a large number of electrons left behind on the
primary part of the supply.

There is a physics experiment where you can show that for an air
capacitor with two parallel plates Q = CV. If you charge this capacitor
and move the plates apart, the charge should stay constant, and the
voltage should rise as the capacitance decreases. I guess as you move
them far enough apart, you would end up with one plate with positive
charge, and one with negative change.

For this explanation to make sense, my foot must have not been close to
the line cord on the floor while I was moving the wire. I guess I moved
my foot close to the cord sometime just before reaching for the screw
with that nutdriver.

Has this happened to anyone else? Does this explanation make sense to
anyone? Is there an alternate explanation?

This set had no history of any arcing or horizontal problems whatsoever.
Plugging and unplugging the line cord now with the cover on does not
reproduce the problem, and I wouldn't expect it to. In fact, it works
perfectly right now. I think I'm more curious about this more for
intellectual reasons than any concern about the set.

Thanks,

Hi Jerry,

I will certainly ground any televisions I reassemble in the future.

I'm still curious about the exact mechanism for this, but I'm not going
to experiment with this set to find out. You're right, if the charge
had built up on the secondary side of the supply, and I had discharged
that, the set would be dead right now.

Thanks,

Stephen



Jerry Greenberg wrote:

The set should have been properly grounded to an external ground.
Most likely there was a build up of charge from the stored charge in
the HV section. The complete chasis can come to a type of equilization
from this charge, if it is floating in relation to a true ground. I
have seen this mostly in large screen CRT sets. Having a clip lead
connected from the main ground of the set to an external electrical
ground will stop this problem.

If there is a build up static charge from the HV to the main chasis of
the set, and you were to touch a part of the circuit board, and cause
a discharge, many components on the board can be damaged.


Jerry Greenberg

I did experience this, a few times and I think that I can reproduce this...

In the shack, plugging a working tv set out of the mains outlet and then
carrying it the the Van for transportation back to the customer, when
you carry the set, with the tube against your chest, if at that moment
you touch the mains plug you could have the symptom you describe.

I think that you statically load up from the tube in touch with your
body, and discharge via the main plug?

I think it was the primary part of the supply that had built up the
charge, or I probably would have "discharged" when my foot came into
contact with the insulated power cord on the floor, but I'm not 100%
sure that my theory makes sense either. I'll be sure to ground any sets
under similar situations though in the future.





Another theory? :-?

Have a nice day,

Alain



--
Stephen Santarelli

stephen
'AT'
nii.net

"stephen" wrote in message
news:Qjyvb.266984$HS4.2381383@attbi_s01...
Hi Jerry,

I will certainly ground any televisions I reassemble in the future.

I'm still curious about the exact mechanism for this, but I'm not going
to experiment with this set to find out. You're right, if the charge
had built up on the secondary side of the supply, and I had discharged
that, the set would be dead right now.

Thanks,

Stephen



It's probably just internal parts of the tube that bleed back to the face, I
always discharge the tube again before I put things back together as the
charge creeps back up a bit.

This shck thing is worse on newer tv sets .
i work on new tv sets at a return warehouse replacing cable jacks or
fixing cracked circuit boards e.t.c. i have been shocked pretty good
just by rubbing my hand across the plastic cabinet while tipping the set
to see the cable jack then get bit real good off of the cable jack .
Too much plastic on these things .

"Alain Beguin" bravely wrote to "All" (20 Nov 03 10:19:39)
--- on the heady topic of " TV builds up huge static charge during =
reassembly, anybody seen th"

AB From: Alain Beguin

AB "stephen" a icrit le/schreef op/wrote on 20/11/2003
Hi All,

AB Hi, Stephen,

AB I did experience this, a few times and I think that I can reproduce
AB this...

AB In the shack, plugging a working tv set out of the mains outlet and
AB then carrying it the the Van for transportation back to the customer,
AB when you carry the set, with the tube against your chest, if at that
AB moment you touch the mains plug you could have the symptom you
AB describe.

AB I think that you statically load up from the tube in touch with your
AB body, and discharge via the main plug?

AB Another theory? :-?

Since obviously the dag coating doesn't cover the front of the crt some
charges will naturally tend to drift over and collect on the tube face.
In the old days crts had metal frames that covered slightly over thew
edge and were grounded. Today's tvs and crts constructed with all
plastic and less strapping there is more of a static problem. Perhaps a
strip of grounded antistatic plastic or an aluminium adhesive tape
around the edge of the crt might help? An antistatic spray on the crt
face might help too?

.... Dunno if we'll get that past the CSA und UL 'owever.