CRT discharge

jason vaughn · Mar 30, 2004

I have read in previous subs.. that the CRT can be discharged by
shorting the anode to the crt metal frame. My questions are?

1)Is the metal frame that of the ciruit attached to the neck of the
CRT or the mainboard.?

2)Can I run the ground to an outdoor ground instead or must it be a
shorting to chasis to discharge the circuit?

Jason

Patch · Mar 30, 2004

"jason vaughn" <jsadv@yahoo.com> wrote in message
news:721b60dd.0403300753.468db97b@posting.google.com...

I have read in previous subs.. that the CRT can be discharged by
shorting the anode to the crt metal frame. My questions are?

1)Is the metal frame that of the ciruit attached to the neck of the
CRT or the mainboard.?

2)Can I run the ground to an outdoor ground instead or must it be a
shorting to chasis to discharge the circuit?

Jason

Usually there is a grounding wire that goes around the CRT. It may have some
springs tied to it to keep it tight against the CRT. Put your ground wire on
that & the other end to the anode. I like to clip a skinny screwdriver to
the grounding wire & slip it under rubber anode cover to discharge the tube.
(make sure the other end is grounded. Not earth ground)

Joe Rongen · Mar 30, 2004

"jason vaughn" <jsadv@yahoo.com> wrote in message
news:721b60dd.0403300753.468db97b@posting.google.com...

I have read in previous subs.. that the CRT can be discharged by
shorting the anode to the crt metal frame. My questions are?

1)Is the metal frame that of the ciruit attached to the neck of the
CRT or the mainboard.?

2)Can I run the ground to an outdoor ground instead or must it be a
shorting to chasis to discharge the circuit?

Keep in mind that the Hi Voltage charge is located between
the inside and the outside of the CRT's glass envelope.

In order to discharge the CRT one has to shorted that path.

Basically that means that one shorts the CRT's outside
glass coating / grounding strap against the HV connector.

To avoid ( possible major ) problems one should - never -
discharge the CRT to ANY other ground point.

Regards Joe

Prediction is very difficult, especially about the future.

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LASERandDVDfan · Mar 30, 2004

I have read in previous subs.. that the CRT can be discharged by
shorting the anode to the crt metal frame. My questions are?

The proper method of discharging the CRT is by arcing the anode connection to
the DAG ground. The DAG will be the metal braid that is strapped around the
picture tube and is held in place by springs on the corners of the picture
tube.

Be careful when you do this. Although the charge may not be lethal, it is very
painful if it happens to zap you. - Reinhart

ajb · Mar 30, 2004

On 30 Mar 2004 17:46:15 GMT, laseranddvdfan@aol.com (LASERandDVDfan)
wrote:

I have read in previous subs.. that the CRT can be discharged by
shorting the anode to the crt metal frame. My questions are?

The proper method of discharging the CRT is by arcing the anode connection to
the DAG ground. The DAG will be the metal braid that is strapped around the
picture tube and is held in place by springs on the corners of the picture
tube.

Be careful when you do this. Although the charge may not be lethal, it is very
painful if it happens to zap you. - Reinhart

When the first all-transistor colour TV sets appeared [1] the
instructor on the original servicing course said _never_ to discharge
the tube with a direct short, but _always_ to use a resistor of at
least 10 megohms in circuit. (I should have to go out to the shed to
look it up, but I rather think that the service manual says this too.)
In this design, at any rate, there seems to have been the possibility
of stray surges and electric fields causing some sort of stress damage
elsewhere if you made too much of a spark. (These CRTs ran at 25KV on
the final anode.) Discharge path exactly as Reinhart says above, of
course.

Also, in the old days it used to be not unknown for a picture tube to
be discharged and disconnected, only to accumulate a lesser charge
(but still enough to make you drop it!) minutes later. I don't know if
modern CRTs do this still.

[1] This was the Thorn 2000 chassis, in England, and at the time it
was said to be the most complicated piece of equipment ever to appear
in the home. I believe it was also the first completely solid-state
production colour TV in the world. The sets were, on the whole,
surprisingly reliable and fairly easy to service. They were of course
a switchable dual-standard circuit, that is designed to operate both
on the old VHF channels and B&W standards at 405 lines, and also the
new UHF system with 625 lines and completely different everything else
as well.
--

ajb

My hovercraft is full of eels

RonKZ650 · Mar 30, 2004

Also, in the old days it used to be not unknown for a picture tube to
be discharged and disconnected, only to accumulate a lesser charge
(but still enough to make you drop it!) minutes later. I don't know if
modern CRTs do this still.

Yes, they do. Best thing is to just wait overnight and the tube will always be
discharged on it's own.
Kind of a funny, (or stupid) or just crazy guy I used to know would just for
fun light a picture tube on a TV set running the HV through his body. Yes, he'd
hold the anode lead in one hand, place his finger in the anode hole in the
picture tube then someone else would turn on the TV. You could watch the
picture with him standing there.
I know this story has nothing to do with anything, but it's true. (don't try
this at home).
Ron

Sam Goldwasser · Mar 30, 2004

ronkz650@aol.comnospam (RonKZ650) writes:

Also, in the old days it used to be not unknown for a picture tube to
be discharged and disconnected, only to accumulate a lesser charge
(but still enough to make you drop it!) minutes later. I don't know if
modern CRTs do this still.

Yes, they do. Best thing is to just wait overnight and the tube will always be
discharged on it's own.
Kind of a funny, (or stupid) or just crazy guy I used to know would just for
fun light a picture tube on a TV set running the HV through his body. Yes, he'd
hold the anode lead in one hand, place his finger in the anode hole in the
picture tube then someone else would turn on the TV. You could watch the
picture with him standing there.
I know this story has nothing to do with anything, but it's true. (don't try
this at home).

Yes, don't try this at home but it is quite reasonable. Typical current
is 1 mA or so for a bright picture. As long as the connections on both
sides of his head or arms are tight so the skin resistance is relatively
low, won't feel a thing as long as he is insulated from ground unless he
tries to disconnect himself with the TV on!

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

Bill · Mar 30, 2004

Why not just stick a HV probe under the anode cup and wait for it to drain?
Saves "flaking" the inside of the tube. Bill
"Sam Goldwasser" <sam@saul.cis.upenn.edu> wrote in message
news:6wad1xx48w.fsf@saul.cis.upenn.edu...

ronkz650@aol.comnospam (RonKZ650) writes:

Also, in the old days it used to be not unknown for a picture tube to
be discharged and disconnected, only to accumulate a lesser charge
(but still enough to make you drop it!) minutes later. I don't know if
modern CRTs do this still.

Yes, they do. Best thing is to just wait overnight and the tube will
always be
discharged on it's own.
Kind of a funny, (or stupid) or just crazy guy I used to know would just
for
fun light a picture tube on a TV set running the HV through his body.
Yes, he'd
hold the anode lead in one hand, place his finger in the anode hole in
the
picture tube then someone else would turn on the TV. You could watch the
picture with him standing there.
I know this story has nothing to do with anything, but it's true. (don't
try
this at home).

Yes, don't try this at home but it is quite reasonable. Typical current
is 1 mA or so for a bright picture. As long as the connections on both
sides of his head or arms are tight so the skin resistance is relatively
low, won't feel a thing as long as he is insulated from ground unless he
tries to disconnect himself with the TV on!

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work.
To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

Sam Goldwasser · Mar 31, 2004

"Bill" <fusebhub@ver.net> writes:

Why not just stick a HV probe under the anode cup and wait for it to drain?
Saves "flaking" the inside of the tube. Bill

Yes, that is probably the best way to do it. The alternative preferred
over just a wire is, as has been noted, to use a high value resistor
that is rated for 30 kV or more.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

Andre · Mar 31, 2004

Sam Goldwasser <sam@saul.cis.upenn.edu> wrote in message news:<6wad1xx48w.fsf@saul.cis.upenn.edu>...

ronkz650@aol.comnospam (RonKZ650) writes:

Also, in the old days it used to be not unknown for a picture tube to
be discharged and disconnected, only to accumulate a lesser charge
(but still enough to make you drop it!) minutes later. I don't know if
modern CRTs do this still.

Yes, they do. Best thing is to just wait overnight and the tube will always be
discharged on it's own.
Kind of a funny, (or stupid) or just crazy guy I used to know would just for
fun light a picture tube on a TV set running the HV through his body. Yes, he'd
hold the anode lead in one hand, place his finger in the anode hole in the
picture tube then someone else would turn on the TV. You could watch the
picture with him standing there.
I know this story has nothing to do with anything, but it's true. (don't try
this at home).

Yes, don't try this at home but it is quite reasonable. Typical current
is 1 mA or so for a bright picture. As long as the connections on both
sides of his head or arms are tight so the skin resistance is relatively
low, won't feel a thing as long as he is insulated from ground unless he
tries to disconnect himself with the TV on!

I also heard the one about this TV repairman who used to arc the HV
from the anode cup to his finger, with his thumb placed on the
shielding for about a second. Sure scared the heck out of anyone who
watched

(BTW, this was presumably the old B&W chassis without an HV
multiplier)

-A

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

Jerry G. · Mar 31, 2004

In my working life on these sets, I have changed probably over several
hundred CRT's. I was working in large warranty contracts for several large
companies. I always discharged the anode directly to the DAG ground of the
tube. I never had any fault from this. Infact, this was the standard used
for all of the sets I have worked on. Make sure that the tool used to do the
grounding is properly insulated, and is connected to the ground point before
going under the anode cap with it. Some of these tube discharges can really
hurt someone very badly.

Any good thin flat blade screwdriver, with a nice thick high voltage
insulated handle will work well for this type of application. Just make sure
it is properly grounded to the CRT ground wire that goes to the DAG before
doing the discharge, or you may get it instead of the ground!

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================

"ajb" <newsG@sphynx.demon.co.uk> wrote in message
news:h7lj60tnbfrshmnq7ft75ijrcla8sf0i13@4ax.com...
On 30 Mar 2004 17:46:15 GMT, laseranddvdfan@aol.com (LASERandDVDfan)
wrote:

I have read in previous subs.. that the CRT can be discharged by
shorting the anode to the crt metal frame. My questions are?

The proper method of discharging the CRT is by arcing the anode connection
to
the DAG ground. The DAG will be the metal braid that is strapped around
the
picture tube and is held in place by springs on the corners of the picture
tube.

Be careful when you do this. Although the charge may not be lethal, it is
very
painful if it happens to zap you. - Reinhart

When the first all-transistor colour TV sets appeared [1] the
instructor on the original servicing course said _never_ to discharge
the tube with a direct short, but _always_ to use a resistor of at
least 10 megohms in circuit. (I should have to go out to the shed to
look it up, but I rather think that the service manual says this too.)
In this design, at any rate, there seems to have been the possibility
of stray surges and electric fields causing some sort of stress damage
elsewhere if you made too much of a spark. (These CRTs ran at 25KV on
the final anode.) Discharge path exactly as Reinhart says above, of
course.

Also, in the old days it used to be not unknown for a picture tube to
be discharged and disconnected, only to accumulate a lesser charge
(but still enough to make you drop it!) minutes later. I don't know if
modern CRTs do this still.

[1] This was the Thorn 2000 chassis, in England, and at the time it
was said to be the most complicated piece of equipment ever to appear
in the home. I believe it was also the first completely solid-state
production colour TV in the world. The sets were, on the whole,
surprisingly reliable and fairly easy to service. They were of course
a switchable dual-standard circuit, that is designed to operate both
on the old VHF channels and B&W standards at 405 lines, and also the
new UHF system with 625 lines and completely different everything else
as well.
--

ajb

My hovercraft is full of eels

Steve Bell · Apr 1, 2004

On 30/3/04 9:42 pm, in article h7lj60tnbfrshmnq7ft75ijrcla8sf0i13@4ax.com,
"ajb" <newsG@sphynx.demon.co.uk> wrote:

<snip>

Also, in the old days it used to be not unknown for a picture tube to
be discharged and disconnected, only to accumulate a lesser charge
(but still enough to make you drop it!) minutes later. I don't know if
modern CRTs do this still.

Yes

[1] This was the Thorn 2000 chassis, in England, and at the time it
was said to be the most complicated piece of equipment ever to appear
in the home. I believe it was also the first completely solid-state
production colour TV in the world. The sets were, on the whole,
surprisingly reliable and fairly easy to service. They were of course
a switchable dual-standard circuit, that is designed to operate both
on the old VHF channels and B&W standards at 405 lines, and also the
new UHF system with 625 lines and completely different everything else
as well.

I remember these. All other chassis at the time used valves in many areas,
typically PL509, GY501 and PY500 in the line output/EHT area. The
semiconductor technology used then was more sensitive, so I can understand
the concerns. A Pye later hybrid chassis I used to work on around 1971 was
very sensitive to arcing. In a Visionhire refurb depot we used lots of re
gunned CRT's, these would often flash over for a while when first fitted.
This would often take out the BD124's in the vertical output stage. I've
never had this sort of problem with newer technology, since working on
monitors since 1985/6 time. Either an insulated screwdriver with a link wire
and croc clip attached, or 2 screwdrivers, has always worked and never
caused problems.

Steve Bell

ajb · Apr 1, 2004

On Thu, 01 Apr 2004 03:35:34 +0100, Steve Bell
<i-hate-spam@no-replies.fsnet.co.uk> wrote:

On 30/3/04 9:42 pm, in article h7lj60tnbfrshmnq7ft75ijrcla8sf0i13@4ax.com,
"ajb" <newsG@sphynx.demon.co.uk> wrote:

snip

Also, in the old days it used to be not unknown for a picture tube to
be discharged and disconnected, only to accumulate a lesser charge
(but still enough to make you drop it!) minutes later. I don't know if
modern CRTs do this still.

Yes

[1] This was the Thorn 2000 chassis, in England, and at the time it
was said to be the most complicated piece of equipment ever to appear
in the home. I believe it was also the first completely solid-state
production colour TV in the world. The sets were, on the whole,
surprisingly reliable and fairly easy to service. They were of course
a switchable dual-standard circuit, that is designed to operate both
on the old VHF channels and B&W standards at 405 lines, and also the
new UHF system with 625 lines and completely different everything else
as well.

I remember these. All other chassis at the time used valves in many areas,
typically PL509, GY501 and PY500 in the line output/EHT area. The
semiconductor technology used then was more sensitive, so I can understand
the concerns. A Pye later hybrid chassis I used to work on around 1971 was
very sensitive to arcing. In a Visionhire refurb depot we used lots of re
gunned CRT's, these would often flash over for a while when first fitted.
This would often take out the BD124's in the vertical output stage. I've
never had this sort of problem with newer technology, since working on
monitors since 1985/6 time. Either an insulated screwdriver with a link wire
and croc clip attached, or 2 screwdrivers, has always worked and never
caused problems.

Steve Bell

That PD500 - a shunt stabiliser triode working at 25kVand 30W anode
dissipation....

If you took the metal shield off the EHT compartment in a Philips G6 chassis and
shorted the interlock switch with your little screwdriver, you could take
interesting X-ray pictures. Arrange your subject and a piece of ordinary Ilford
printing paper in its black envelope carefully, retreat to the _other_ side so
the lead glass CRT was protecting your vital bits, and switch on with the CRT
heaters off (to give zero beam current) for as many seconds as the exposure
took.

I won several prizes in photographic competitions for my radiographs, but never
let on how I did them....

I remember that Pye hybrid chassis. We used to improve the picture stability and
reliability, and colour quality, by a simple modification. The heater current in
the panel with the four video output valves was a little high, and you could
improve things dramatically by the addition of a few more wirewound ohms in the
series chain.

There was another very common surge problem with the Philips G8 chassis, their
first all transistor one. It had a curious power supply using a BT106 thyristor,
and it was common for the customer to have the TV set plugged into a double
adaptor at the mains socket with a standard lamp or table lamp. The bulb
filament fails, with the usual huge momentary internal arc, and before any fuse
could blow the BT106 pops its clogs.
--
ajb

If the 'self' is an illusion, who is being fooled?

Steve Bell · Apr 1, 2004

On 1/4/04 9:37 pm, in article 3nto60l5sn1mmho16pfs8pq0caplkffaep@4ax.com,
"ajb" <newsG@sphynx.demon.co.uk> wrote:

I remember that Pye hybrid chassis. We used to improve the picture stability
and
reliability, and colour quality, by a simple modification. The heater current
in
the panel with the four video output valves was a little high, and you could
improve things dramatically by the addition of a few more wirewound ohms in
the
series chain.

The 3 colour difference amplifiers were PCL84's, and luminance a PL802. Pye
were late changing over to transistors here.

There was another very common surge problem with the Philips G8 chassis, their
first all transistor one. It had a curious power supply using a BT106
thyristor,
and it was common for the customer to have the TV set plugged into a double
adaptor at the mains socket with a standard lamp or table lamp. The bulb
filament fails, with the usual huge momentary internal arc, and before any
fuse
could blow the BT106 pops its clogs.

The G8 was my favorite chassis at that time, and the Swedish Philips in the
teak case with tone and tint controls, was it a K2? I never did like the G6,
somewhere I think I've still got the decoder alignment tools I used on them.
The Decca Bradford chassis was another good performer. Around 1976 I moved
on to another area of electronics.

Steve Bell

jason vaughn · Apr 8, 2004

Many thanks for clearing this issue up for me. This is not a task I
will repeat very often . My initial interrest was get a few free
mosfets from the logic board for an RC electronic speed controller.
Where I live they are either too costly or a rear find.

The CRT didnt yark at all when I shorted it. I used the resistor as
suggested and wore protective gear.

Thank you
Jason
God Bless

LASERandDVDfan · Apr 9, 2004

The CRT didnt yark at all when I shorted it. I used the resistor as
suggested and wore protective gear.

That's good. The CRT holds a big charge, but doesn't hold it for a very long
time.

However, it's always a good idea to assume it's charged anyways and take
appropriate measures to discharge it.

Thank you

You're welcome. - Reinhart

Tom MacIntyre · Apr 9, 2004

On Wed, 31 Mar 2004 00:37:51 GMT, "Bill" <fusebhub@ver.net> wrote:

Why not just stick a HV probe under the anode cup and wait for it to drain?
Saves "flaking" the inside of the tube. Bill

That's the way I always did it.

Tom

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