David Peters wrote:
On 11 Apr 2006,
wrote:


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)

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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...



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