Loose connect makes picture move on CRT

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.

Reply to
David Peters
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check in and around the vertical deflection circuitry.

Newfdog

Reply to
Newfdog

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...

Reply to
Palindr☻me

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!)

Reply to
David Peters

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.

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?

Reply to
Palindr☻me

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

Reply to
JohnR66

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.

Reply to
Andy

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