Toshiba TV29C90 problem; Image fades to black...

[Alan Harriman]

On 13 Aug 2003 16:42:46 -0700, deaduser@hotmail.com (Jeffrey Ellin) wrote:

I have a 27" Panasonic TV which is about 5years old that has developed
a problem where periodically there are a greenish yellow splotches on
the screen in the left side and lower right corner. There aren't there
all the time. They almost look like there is a speaker near by which
there isn't. I was going to pitch this set until a tv repair tech from
Toshiba was at my house to work on another tv mentioned that it might
be a bad degausing circuit. He couldn't be sure because it wasn't
exhibiting the problem at the time. I tried to press him for some
information, but he was behind schedule and in a hurry to leave and of
course it wasn't a Toshiba set. It got me thinking though, how much
would it cost me to repair a degausing circuit and what is the
likelyhood that is the problem. I could take it in for an estimate,
but the set is dificult for me to move.

Could be a degauss issue, but your description sounds more like a CRT problem
called "doming". Doming generally occurs during times of prolonged bright white
scenes (such as a during a hockey game). As I understand, the CRT beam current
actually warms the shadow mask, causing it to distort slightly, resulting in a
misalignment of the beam landing point. Usually turning the brightness and/or
contrast down a bit will reduce the effect.

Alan Harriman

 

[David]

I have also found that in minor near edge cases of doming, a careful slight
purity adjustment while duplicating the doming problem can
sometimes/manytimes minimize the doming effect. The key is to only
partially compensate for the doming problem with the magnets, but not enough
to effect the crt purity during non-doming times.

As well as reducing the contrast and brightness. It has been my experience
that once a crt shadow mask develops a weak spot where doming starts, it
becomes more and more susceptable over time. They do usually make decent tv
sets for the kids to play video games on as long as the game is a mostly
dark image.

David

Alan Harriman <vtech@usol.com> wrote in message
news:10pljvo03454lktbdtk18iell2pv1osh9k@4ax.com...

On 13 Aug 2003 16:42:46 -0700, deaduser@hotmail.com (Jeffrey Ellin) wrote:

I have a 27" Panasonic TV which is about 5years old that has developed
a problem where periodically there are a greenish yellow splotches on
the screen in the left side and lower right corner. There aren't there
all the time. They almost look like there is a speaker near by which
there isn't. I was going to pitch this set until a tv repair tech from
Toshiba was at my house to work on another tv mentioned that it might
be a bad degausing circuit. He couldn't be sure because it wasn't
exhibiting the problem at the time. I tried to press him for some
information, but he was behind schedule and in a hurry to leave and of
course it wasn't a Toshiba set. It got me thinking though, how much
would it cost me to repair a degausing circuit and what is the
likelyhood that is the problem. I could take it in for an estimate,
but the set is dificult for me to move.

Could be a degauss issue, but your description sounds more like a CRT
problem
called "doming". Doming generally occurs during times of prolonged bright
white
scenes (such as a during a hockey game). As I understand, the CRT beam
current
actually warms the shadow mask, causing it to distort slightly, resulting
in a
misalignment of the beam landing point. Usually turning the brightness
and/or
contrast down a bit will reduce the effect.

Alan Harriman

 

[Jim Yanik]

Lizard Blizzard <NOSPAM@rsccd.org> wrote in
news:bhej1r$3nejd$1@hades.csu.net:

Bill Janssen wrote:

Alan Horowitz wrote:


any of these new-fangled battery chemistries such that, buying an
average one off the shelf, it has a voltage that's within 1% of it's
nominal?


Check the spec.'s on coin size Mercury cells. As I remember a new
one has a well defined voltage.

Mercury cells cannot be sold because of the mercury in them. So the
battery could not be purchased, as the OP requested.



Good luck.
Bill K7NOM

Perhaps new silver oxide cells would suffice.
1.55V IIRC.
Better to use a band-gap reference diode,they come in <100 PPM accuracies.

--
Jim Yanik,NRA member
remove null to contact me

 

[Rufus V. Smith]

"Doug Taylor" <techno2nospam@videotron.ca> wrote in message
news:slrnbjf5tj.ln.techno2nospam@localhost.localdomain...

Have we completely lost our ability to express ourselves?

I love reading this Newsgroup, but I am astounded by how many
people can't articulate their technical problem in plain English.

" i got atv don't work. its a ge. has a black screen no picture
whaatz the problem"

They don't give Model Numbers, or explain clearly what the problem
is exactly, (like it stopped working after a lighting strike)
but they expect instant solutions with no effort or expense.

"Fix my VCR for me, as I don't have the time or skill to do it myself."
"Nor do I want to pay a REAL technician to repair it for me."

What do you think?

Doug Taylor

Not everyone in newsgroup-land is a native english speaker (or typist).

But many english speakers just aren't that bright (or maybe they're simply
ignorant). One of the problems with bringing computer power to the
unwashed masses. They want to mingle...

I certainly agree that questions like: "I turn the TV on and get no picture,
what's wrong?" Should instantly terminate newsgroup access for the poster,
at
a minimum.

Too bad there's no short message to effectively say: Read the manual, tell
us
model numbers, tell us unusual circumstances prior to the failure, etc...

It calls for a FAQ on problem posting, but anyone who could find the FAQ
probably already knows how to post a problem.

I tend to ignore questions where the poster hasn't bother to think a little.

Rufus






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[Uns Lider]

In article <3f36669d.10919005@news.freeserve.net>, FiveRings wrote:

The rattle was due to the top of a 8 pin DIL microchip from the PSU board.
(A.C. side). It had blown right off due to a voltage spike or i don't know
what.

Probably something else went bad and smoked it.

The PSU does not use a transformer to step down the mains supply,
only electronic parts and uses a small transformer and opto isolator?

Switching power supply. They're very popular (and also dangerous if you don't
know what you're doing). See www.repairfaq.org.

12VS
Probably 12-volt standby
CHGND
Probably chassis ground
GNDD
Probably digital ground

-- uns

 

[Arthur Jernberg]

Sony and Panasonic tend to have a problem with the degauss thermistor they
incorporate in their sets. Have that device replaced and it probably will
eliminate the problem. Seen it too many times.
"Jeffrey Ellin" <deaduser@hotmail.com> wrote in message
news:e8a45386.0308131542.5c01e7bb@posting.google.com...

I have a 27" Panasonic TV which is about 5years old that has developed
a problem where periodically there are a greenish yellow splotches on
the screen in the left side and lower right corner. There aren't there
all the time. They almost look like there is a speaker near by which
there isn't. I was going to pitch this set until a tv repair tech from
Toshiba was at my house to work on another tv mentioned that it might
be a bad degausing circuit. He couldn't be sure because it wasn't
exhibiting the problem at the time. I tried to press him for some
information, but he was behind schedule and in a hurry to leave and of
course it wasn't a Toshiba set. It got me thinking though, how much
would it cost me to repair a degausing circuit and what is the
likelyhood that is the problem. I could take it in for an estimate,
but the set is dificult for me to move.

 

[Leonard Caillouet]

I see bad solder joints and bad thermistors in the degaussing circuits on
Panasonics all the time. Much more likely than shadow mask doming and easy
to fix/try.

Leonard Caillouet

"Alan Harriman" <vtech@usol.com> wrote in message
news:10pljvo03454lktbdtk18iell2pv1osh9k@4ax.com...

On 13 Aug 2003 16:42:46 -0700, deaduser@hotmail.com (Jeffrey Ellin) wrote:

I have a 27" Panasonic TV which is about 5years old that has developed
a problem where periodically there are a greenish yellow splotches on
the screen in the left side and lower right corner. There aren't there
all the time. They almost look like there is a speaker near by which
there isn't. I was going to pitch this set until a tv repair tech from
Toshiba was at my house to work on another tv mentioned that it might
be a bad degausing circuit. He couldn't be sure because it wasn't
exhibiting the problem at the time. I tried to press him for some
information, but he was behind schedule and in a hurry to leave and of
course it wasn't a Toshiba set. It got me thinking though, how much
would it cost me to repair a degausing circuit and what is the
likelyhood that is the problem. I could take it in for an estimate,
but the set is dificult for me to move.

Could be a degauss issue, but your description sounds more like a CRT
problem
called "doming". Doming generally occurs during times of prolonged bright
white
scenes (such as a during a hockey game). As I understand, the CRT beam
current
actually warms the shadow mask, causing it to distort slightly, resulting
in a
misalignment of the beam landing point. Usually turning the brightness
and/or
contrast down a bit will reduce the effect.

Alan Harriman

 

[b]

try copious amounts of glue and string. works every time, machine
turns out like new.


John Pigye <idiot@failed.isp.besthost1.com> wrote in message news:<200308140617.CAA30348@orion.besthost1.com>...
> I got into a fight with Todd Wonderson so I threw my boombox at him and he caught it and slammed it hard into the desk and broke it in half. How do I fix it?

 

[Asimov]

"John Pigye" wrote to "All" (14 Aug 03 02:17:29)
--- on the topic of "Broken Boombox"

JP> From: John Pigye <idiot@failed.isp.besthost1.com>

JP> I got into a fight with Todd Wonderson so I threw my boombox at him
JP> and he caught it and slammed it hard into the desk and broke it in
JP> half. How do I fix it?

The handman's handy assistant of course.... Duct tape!!!
(If it ain't broke, you're not trying - Red Green)

.... You may be a tech if you're entertained by a 6-pack and sparking HV.

 

[Bill Janssen]

Lizard Blizzard wrote:

Bill Janssen wrote:

Alan Horowitz wrote:


any of these new-fangled battery chemistries such that, buying an
average one off the shelf, it has a voltage that's within 1% of it's
nominal?


Check the spec.'s on coin size Mercury cells. As I remember a new
one has a well defined voltage.

Mercury cells cannot be sold because of the mercury in them. So the
battery could not be purchased, as the OP requested.

Oh, I wasn't aware that Mecury cells were no longer available.

Good luck.
Bill K7NOM

 

[Mike Berger]

You can still get them at some camera stores -- they're imported
from Poland.

Bill Janssen wrote:

Oh, I wasn't aware that Mecury cells were no longer available.

 

[Jerry G.]

It would take a very large lightning hit to magnetize or damage a CRT.
The set would have been destroyed first! I think the screen became
magnetized, and should be able to be demagnetized. But, if he dropped
the set, banged it very hard, or put a very strong magnet to the mask of
the screen, this would do permanent damage to the tube.

After thunder storms sometimes we have seen people try to make
questionable insurance claims for equipment that is defective. This is
something that cannot be indicated openly without proper evidence.
Insurance companies have many claims from people for damages that are
not even relative to the event.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
==============================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
Instruments http://www.zoom-one.com/glgtech.htm
==============================================
"Bill" <elect21st@aol.com> wrote in message
news:8fb59540.0308132109.25d43d7f@posting.google.com...
Hello folks- I have an RCA TV chassis # CTC185AB. Customer said
after a severe storm he started getting weird colors on the screen.
Picture is perfect. I checked for blown parts and found none. I
resoldered some joints and replaced the deg. thermister just to be
sure. I also tried to clean these patches of color externally via a
deg. coil. I made some progress but the corners are still messed up.
I'm starting to believe he dropped it. Can storms do so much damage
that the screen can't be completely demagnetized ? What other
solutions are there in this case other than placing magnets on the CRT
? Thanks

 

[Jerry G.]

There may be a few issues here. The degaussed circuit may not be
working properly. Sometimes when turning on the set, the screen may not
be properly degaussed. The thermosistor, and all the associated solder
connections should be checked. An experienced TV tech can service this
very easily.

There is the possibility that there is a magnetic field near to the set
that you are not aware of. If there are power wires with a heavy load on
them, that are within about 5 feet to the set, this can cause this
effect. There may be something on the other side of the wall from the
set that can cause problems, such as a large speaker, or device that has
a magnetic field.

As for the CRT having a defective mask, I sort of would rule this out
since you are indicating that the fault is not there all the time. This
means that it is clearable. I have seen defective masks in the CRT
cause this type of fault, but the fault could not be cleared.

There are rare occasions where for some reason the mask starts to have a
doming effect, especial when it gets warm. This means that something
became weak with the mask, and as it raises in temperature, it would
warp slightly. If this was the case, it should be very consistent, and
not go away.

Have the set properly checked by someone who really knows what they are
doing.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
==============================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
Instruments http://www.zoom-one.com/glgtech.htm
==============================================
"Jeffrey Ellin" <deaduser@hotmail.com> wrote in message
news:e8a45386.0308131542.5c01e7bb@posting.google.com...
I have a 27" Panasonic TV which is about 5years old that has developed
a problem where periodically there are a greenish yellow splotches on
the screen in the left side and lower right corner. There aren't there
all the time. They almost look like there is a speaker near by which
there isn't. I was going to pitch this set until a tv repair tech from
Toshiba was at my house to work on another tv mentioned that it might
be a bad degausing circuit. He couldn't be sure because it wasn't
exhibiting the problem at the time. I tried to press him for some
information, but he was behind schedule and in a hurry to leave and of
course it wasn't a Toshiba set. It got me thinking though, how much
would it cost me to repair a degausing circuit and what is the
likelyhood that is the problem. I could take it in for an estimate,
but the set is dificult for me to move.

 

[Alan Harriman]

On Thu, 14 Aug 2003 12:42:48 -0400, "Jerry G." <jerryg@total.net>
wrote:

There are rare occasions where for some reason the mask starts to have a
doming effect, especial when it gets warm. This means that something
became weak with the mask, and as it raises in temperature, it would
warp slightly. If this was the case, it should be very consistent, and
not go away.

Samuel M. Goldwasser has an excellent article on impurity, doming and
other issues relating to purity problems:
http://www.repairfaq.org/REPAIR/F_crtfaq.html#CRTFAQ_027

Alan Harriman

Have the set properly checked by someone who really knows what they are
doing.

 

[Arnold stewart]

The cap is a regular electrolytic. The zener is connected anode to
negative side of cap (and -6V at pin 6 of the regulator) with the
cathode to the positive side of the cap (0V at pin 4). By the way, the
trouble with the TV set was no power and the solution was the cap
measured 0 mfd and the zener was leaky (126 ohms both ways) Replacing
the cap got the set going. I did replace the 30V zener, but was
wondering about its usefulness.

Mike wrote:

This is in reply to message of Tue, 12 Aug 2003 14:33:31 GMT,
chris@nospam.com which said:


On Mon, 11 Aug 2003 00:50:44 -0400, Arnold stewart
arnold.stewart@sympatico.ca> wrote:


But this zener is connected between two pins that have only a 6 to 7
volt potential in normal operation. If the voltage were to climb to 30
volts, wouldn't the 25v cap be most likely to get damaged, probably
even short out? In that case the zener is not really helping out.

A cap rated for 25v will easily handle 30volts for a short duration.


My guess is that it is there for reverse transient protection, is the
capacitor tantalum and is the diode reverse polarity to the capacitor?


Mike
Please remove DE SPAM to e-mail

 

[jakdedert]

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

"Clifton T. Sharp Jr." <clifto@clifto.com> writes:

Jerry G. wrote:
It would take a very large lightning hit to magnetize or damage a CRT.
The set would have been destroyed first! I think the screen became
magnetized, and should be able to be demagnetized.

Every time I drive past that house, I remember the first time I ran into
a set magnetized by a close lightning strike. It was extremely weird.
The red purity was fine, so were blue and green. But the color picture
looked like severe purity problems. I called the head tech, who told me
to degauss; I was amazed when it fixed the problem.

Yes, the effects of a close lightning strike and nuclear EMP might be very
similar (at least in terms of magnetization of a CRT!).

--- 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 at repairfaq.org. Thanks.

I had a strike around eight years ago which took out many, many electronic

devices in my home. Partial list includes vcr (no audio...video fine),
several telephones, one PC and most everything attached to it (even blew out
one of the tweeters on the speakers attached, along with the parallel port
of the printer), another pc which I was able to partially resurrect (blew
out all onboard external ports and HD interface...replaced with ISA/PCI
cards, worked like that for many years until I finally scrapped it).

It literally blew my neighbors telco interface box off the wall of his
house...blew the bark off a stately old oak tree in my back yard (3" strip
running radially from the ground up to about 20'...mostly healed over now,
but I wonder if it will ultimately kill the tree).

The interesting thing is that other many devices survived, including the fax
machine connected to the same line as one of the dead phones, a couple of
other phones, a couple of other vcr's and all of the TV's/monitors in the
house (including the ones connected to the above pc's).

One common thread, however, was that *every* color crt device connected to
a/c mains at the time--whether switched on or not--had severe purity
problems...most of which worked themselves out over the next several days
with normal on/off switching (before most monitors had on-screen
menus/manual degaussing).

So, for the previous poster (not you Sam), it's *very* possible to induce
enough emf into a crt--by way of a lightning strike--to cause the problem
the OP described...without destroying the device.

The symptoms will likely disappear after a few power on/off cycles. If it's
possible to view as is, just wait a few days. Normal usage will eventually
degauss it. If it's particularly severe, you may have to degauss it
manually (if you have a coil). I wouldn't recommend repeated on/off
switching as a degaussing strategy...could stress other components of the
set.

jak

 

[jakdedert]

"Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> wrote in message
news:MSGID_1=3a167=2f133.0_3f3b831c@fidonet.org...
"John Pigye" wrote to "All" (14 Aug 03 02:17:29)
--- on the topic of "Broken Boombox"

JP> From: John Pigye <idiot@failed.isp.besthost1.com>

JP> I got into a fight with Todd Wonderson so I threw my boombox at him
JP> and he caught it and slammed it hard into the desk and broke it in
JP> half. How do I fix it?

The handman's handy assistant of course.... Duct tape!!!
(If it ain't broke, you're not trying - Red Green)

.... You may be a tech if you're entertained by a 6-pack and sparking HV..

You forgot the superglue and bailing wire.

Works for me....

jak

 

[Bob Minchin]

Razor's Edge wrote:

Hello all you lovely boys and girls,

Here comes some discussion around a vertical output problem that I have.
The set is a Telefunken SR253, an old modell but quite nice anyways.
Features nicam stereo though.
Well, the vertical amp ic TDA8178 is shorted between supply voltage and inv.
input pins. The problem is that it's no longer available.
So I must use a similair ic with no or small modifications.
At first thought, this seems to be easy enough, but the design confuses me.

Take a look:
http://www.elektronikcentrum.com/tda8170.pdf

Here's the datasheet for a standard vertical output ic.
Between pin 6 & 3, the flyback capacitor is located, just like it use to.
But in the Telefunken SR253, that capacitor is placed from pin 3 to ground.
And this cap is charged to +66V by rectification from a separate winding on
the LOT.
The supply-voltage is 24V and is fed to pin 2 and 6. Why are they not using
the internal flyback generator inside the ic?
May it be possible to use a TDA8170 and modify the circuit to look the same
as the datasheet? And just throw that +66V supply away.

Any advice is warmly welcome,

Regards
Stefan

Hi Stefan,

Did you get anywhere with those Donberg links I sent you?

Bob in UK

 

[Jim Adney]

On 13 Aug 2003 13:48:19 -0700 alanh_27@yahoo.com (Alan Horowitz)
wrote:

any of these new-fangled battery chemistries such that, buying an
average one off the shelf, it has a voltage that's within 1% of it's
nominal?

I seem to recall fresh ordinary carbon-zinc batteries being used as
"standards" in an earlier era. I think the voltage was supposed to be
1.56V, but that's a very old memory and could be off a bit.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------

 

[Robert Baer]

Jim Adney wrote:

On 13 Aug 2003 13:48:19 -0700 alanh_27@yahoo.com (Alan Horowitz)
wrote:

any of these new-fangled battery chemistries such that, buying an
average one off the shelf, it has a voltage that's within 1% of it's
nominal?

I seem to recall fresh ordinary carbon-zinc batteries being used as
"standards" in an earlier era. I think the voltage was supposed to be
1.56V, but that's a very old memory and could be off a bit.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------

I have never heard of using carbon-zinc (Leclanche) cells for any kind
of reference, and i have been an electronic technician for over 50
years.
The mercury cells made reasonable inexpensive secondary standards, and
the voltage known to 4 digits (if i remember correctly) and could be
calibrated and used as transfer standards to 5 digits (if i remember
correctly).
It is my guess that the Lithium cells may be the best of the various
voltage sources, for that purpose.
However, one might do better using a 1.25V band-gap reference driven
from a Lithium cell.
There now seems to be an even better reference using a new technology;
see:
http://www.xicor.com/voltage_family.php