Electrical Engineering - Loose connect makes picture move on CRT

Author

Loose connect makes picture move on CRT

David Peters

2006-04-10, 11:21 pm

Can you help me locate the area on the circuit board which is
causing an intermittent fault on my CRT monitor. I would welcome
some help to work out which part of the circuit board is a
candidate to specifically try the next time it happens.

It is a Tatung Mitsubishi V70. Model VM71RDA. (17inch)

------

HERE ARE THE DETAILS

This fault causes the picture to move vertically. This might
happen only once every two days or it may happen maybe four times
in a day.

The bottom of the picture will go up by about 1 cm and the top
will correspondingly come down by approximately 1 cm.

I notice that the picture at the top and bottom edge is no longer
"linear" but appears squashed: I can see the small icons in XP's
Quick Launch bar looking all squeezed up.

This effect may not be stable and it may move into and out of this
squashed state every half second or so.

I can't help but think that the effect is partly linked to what is
on the screen as (I may be hallucinating) there seems to be a co-
incidence sometimes between the start of some screen activity and
this fault starting.

---

I have removed the rear cover and settled the monitor on the desk.
There is a single large circuit board. I find that if I randomly
(!) press the circuit board (with a length of plastic for safety)
then I can get usually this fault to settle down and go.

I move the board at random because I can not tell where this
problem is located. I imagine there is a crack or bad joint or
something. Sometimes I fancy that moveing the heavy thick signal
cable is helping but I think doing this is really flexing the
board.

Can you help me identify which component or which area of the
circuit board is a candidate to specifically try the next time it
happens.

Newfdog

2006-04-11, 12:21 am

Palindr☻me

2006-04-11, 5:21 am

David Peters wrote:
> Can you help me locate the area on the circuit board which is
> causing an intermittent fault on my CRT monitor. I would welcome
> some help to work out which part of the circuit board is a
> candidate to specifically try the next time it happens.
>
> It is a Tatung Mitsubishi V70. Model VM71RDA. (17inch)
>
> ------
>
> HERE ARE THE DETAILS
>
> This fault causes the picture to move vertically. This might
> happen only once every two days or it may happen maybe four times
> in a day.
>
> The bottom of the picture will go up by about 1 cm and the top
> will correspondingly come down by approximately 1 cm.
>
> I notice that the picture at the top and bottom edge is no longer
> "linear" but appears squashed: I can see the small icons in XP's
> Quick Launch bar looking all squeezed up.
>
> This effect may not be stable and it may move into and out of this
> squashed state every half second or so.
>
> I can't help but think that the effect is partly linked to what is
> on the screen as (I may be hallucinating) there seems to be a co-
> incidence sometimes between the start of some screen activity and
> this fault starting.
>
> ---
>
> I have removed the rear cover and settled the monitor on the desk.
> There is a single large circuit board. I find that if I randomly
> (!) press the circuit board (with a length of plastic for safety)
> then I can get usually this fault to settle down and go.
>
> I move the board at random because I can not tell where this
> problem is located. I imagine there is a crack or bad joint or
> something. Sometimes I fancy that moveing the heavy thick signal
> cable is helping but I think doing this is really flexing the
> board.
>
> Can you help me identify which component or which area of the
> circuit board is a candidate to specifically try the next time it
> happens.

Well, if I told you, I'd have to kill you - or at least encourage you to
run the risk of killing yourself. CRT circuitry is well capable of
killing people, even with the mains plug removed.

The symptoms are of symettrical clipping of the vertical deflection
signals - which pins it down pretty closely as most faults in that area
won't produce symettrical distortion. Simply by following the wires back
from the yoke, that heap of coils around the tube, will probably lead
you to the area of the board used for the horizontal and vertical
amplifier circuitry.

Now it may be that it is simply a little trimming veriable potentiometer
in that area that has a bad connection on its wiper. So, with the power
off and left off for a few hours, carefully noting the current setting,
changing the setting and then very carefully putting it back where it
was, may do the trick. Only do one at a time! Check the picture
afterwards...be very careful to put it back exactly where it was. Don't
do this with power on...never, ever, do this to more than one at once.

Or it could, indeed, be a bad joint or hairline crack. Which could be
/inside/ a component, such as an electrolytic capacitor. No disrespect
intended, but, it is a very skilled job repairing such things -
especially if the components are surface-mounted. It also needs special
tools and a patience and care more commonly found in saints..

Unfortunately, with 17" CRTs now selling for a pittance and second hand
ones even more so, in most places that have them, it isn't economic to
repair them. Unless you are in one of those parts of the World where
this isn't true?... It certainly isn't worth the risk of electrocuting
yourself over something that would only be worth 25GBP - fully working...

--
Sue

David Peters

2006-04-11, 9:21 pm

On 11 Apr 2006, =?UTF-8?B?UGFsaW5kcuKYu21l?=<me9@privacy.net>
wrote:

> David Peters wrote:
>
> Well, if I told you, I'd have to kill you - or at least
> encourage you to run the risk of killing yourself. CRT circuitry
> is well capable of killing people, even with the mains plug
> removed.
>
> The symptoms are of symettrical clipping of the vertical
> deflection signals - which pins it down pretty closely as most
> faults in that area won't produce symettrical distortion. Simply
> by following the wires back from the yoke, that heap of coils
> around the tube, will probably lead you to the area of the board
> used for the horizontal and vertical amplifier circuitry.
>
> Now it may be that it is simply a little trimming veriable
> potentiometer in that area that has a bad connection on its
> wiper. So, with the power off and left off for a few hours,
> carefully noting the current setting, changing the setting and
> then very carefully putting it back where it was, may do the
> trick. Only do one at a time! Check the picture afterwards...be
> very careful to put it back exactly where it was. Don't do this
> with power on...never, ever, do this to more than one at once.
>
> Or it could, indeed, be a bad joint or hairline crack. Which
> could be /inside/ a component, such as an electrolytic
> capacitor. No disrespect intended, but, it is a very skilled job
> repairing such things - especially if the components are
> surface-mounted. It also needs special tools and a patience and
> care more commonly found in saints..
>
> Unfortunately, with 17" CRTs now selling for a pittance and
> second hand ones even more so, in most places that have them, it
> isn't economic to repair them. Unless you are in one of those
> parts of the World where this isn't true?... It certainly isn't
> worth the risk of electrocuting yourself over something that
> would only be worth 25GBP - fully working...
>

Thank you for the reply. I will have a look at the area you
suggest. I need to switch the minitor off and perhaps leave it
for a while to permit any retained HT charges to drain away.

I might add one extra observation -- at the same time as the other
symptoms I get one, two, maybe three horiszontal lines across the
screen which seem to be brighter/lighter than the image on the
screen.

(I understand what you mean about cost etc. In fairness to me,
these monitors may be cheap but one might have to get more than
one to make sure there was a decent one among them all!)

Palindr☻me

2006-04-12, 7:21 am

David Peters wrote:
> On 11 Apr 2006, =?UTF-8?B?UGFsaW5kcuKYu21l?=<me9@privacy.net>
> wrote:
>
>
>
>
> Thank you for the reply. I will have a look at the area you
> suggest. I need to switch the minitor off and perhaps leave it
> for a while to permit any retained HT charges to drain away.

There are design rules about fitting bleed resistors to high voltage
capacitors but, if you are suspecting a board may have the odd dry joint
or two, Murphy suggests that one of them is going to be on a bleed
resistor.. So I wouldn't go touching anything with bare fingers, if I
were you, even after "a while".

>
> I might add one extra observation -- at the same time as the other
> symptoms I get one, two, maybe three horiszontal lines across the
> screen which seem to be brighter/lighter than the image on the
> screen.
What make tube? Every single Trinitron (or Diamondtron) monitor ever
made has at least one and, for bigger screens, two or more, faint dark
lines running horizontally across the screen. They're most visible on a
plain white screen, but once you know they're there you can't miss them
on any reasonably bright colour. They are caused by damper wires built
into the screen.

>
> (I understand what you mean about cost etc. In fairness to me,
> these monitors may be cheap but one might have to get more than
> one to make sure there was a decent one among them all!)

Oh, I hated writing that! I really hate throwing stuff away that /is/
repairable, simply because it is uneconomic to repair it. Particularly
as there are places in the World which would love to have it, faults and
all.

The present system looks to be sheer nonsense - at the very least it
would cost so little to throw in a schematic with every electronic
gubbins, together with the waverforms at various testing points. But no,
why make it more economic to repair and lose the sale of the replacement?

--
Sue

JohnR66

2006-04-12, 11:21 pm

"David Peters" <no-email@mail.com> wrote in message
news:Xns97A2184E46A46351D7E@127.0.0.1...
> Can you help me locate the area on the circuit board which is
> causing an intermittent fault on my CRT monitor. I would welcome
> some help to work out which part of the circuit board is a
> candidate to specifically try the next time it happens.
>
<snip>
If you can't find a schematic for the monitor, you will have to figure out
what wire connects to the vertical deflection coil and trace it back to the
board. The vertical deflection circuit is comprised of an IC mounted to a
small heat sink and a few other components. Any of these could have a bad
solder joint. In some cases, it can be a contact pad inside the IC.

I've rescued a number of TVs and monitors from the dumper because of
bad/cold solder joints. Seems to be a common problem.
John

Andy

2006-04-27, 10:21 am

On 12 Apr 2006, =?UTF-8?B?UGFsaW5kcuKYu21l?=<me9@privacy.net>
wrote:

> David Peters wrote:
>
> There are design rules about fitting bleed resistors to high
> voltage capacitors but, if you are suspecting a board may have
> the odd dry joint or two, Murphy suggests that one of them is
> going to be on a bleed resistor.. So I wouldn't go touching
> anything with bare fingers, if I were you, even after "a while".
>
> What make tube? Every single Trinitron (or Diamondtron) monitor
> ever made has at least one and, for bigger screens, two or more,
> faint dark lines running horizontally across the screen. They're
> most visible on a plain white screen, but once you know they're
> there you can't miss them on any reasonably bright colour. They
> are caused by damper wires built into the screen.
>
>

Well, I thought I had this fixed but it is still there. Less
frequent now. I need to keep looking for a hairline crack or loose
components or the mistracking pot slider.

BTW I don't know if I have a Trinitron (or Diamondtron) tube with the
two wires going across the screen. Certainly the tube curvature is
NOT at all like a Triniton tube (a segment of an upright cylinder).
Sometimes the two lines on screen when my problem appears become
three and they seem to me to move up and down quite quickly.
Sometimes they seem bright and then they jump for a moment and are
dark and then jump again, you know what I mean.

There is one thing I can't help but "notice" and that is the CONTENT
of the picture seems to have some connection with some connection
with the intermmittent fault appearing. I can not say that this
makes much sense to me but someone may be able to interpret such a
thing.

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