CRT colour screen with gray background and pale colours

[Rauli]

Yesterday I was doing some research on my Amstrad 5086 XT, on DOS, and suddenly something happened on my screen and the black background turned gray (mid-gray, about 50%), and there's no way to make it black again. And all the colours look very pale. The only part in black is the border.

It happened without producing any noise. There was no smoke but it smelled like some component got burned. I know that by the smelling, some experts can tell if the burned component is a capacitor, a resistor or a semiconductor, but my nose is not so well trained.

The screen has 2 knobs for brightness and contrast. The contrast knob doesn't change the gray, only the white (text foreground). The brightness knob doesn't change anything.

I connected the screen to other computer (to make sure that the problem was in the screen) with the same results.

It seems that it is not a serious problem, but I'm concerned about high voltages inside CRTs and I still haven't opened it. I have left it unplugged but I know some components can keep its charge for months.

So, my questions are:
Which precautions should I take for opening it? Which are the dangerous parts?
and:
Which components are more likely to be damaged by the symptoms I have described?

 

[modem7]

There was no smoke but it smelled like some component got burned.

In a way that is good, because it means that you should be able to visually identify the faulty component.

And hopefully, it is the only faulty component. For example, a transistor that has become short-circuit can cause a resistor to burn open. Replace the resistor, and the replacement also burns open.
If you do not have electronics knowledge, then all you can do is replace the faulty component and then see what happens.

It seems that it is not a serious problem, but I'm concerned about high voltages inside CRTs and I still haven't opened it. I have left it unplugged but I know some components can keep its charge for months.
So, my questions are:
Which precautions should I take for opening it? Which are the dangerous parts?

See [here] and [here].

If you leave your monitor uplugged from mains (house) power overnight (overnight just be extra safe), then the only voltage/charge concern will be the possibility that the CRT (the 'tube') itself still has charge on its anode. From what I have read, there is not enough energy in the CRT to kill you, but if you get a decent shock, the experience is one that you will remember for the rest of your life.

Which components are more likely to be damaged by the symptoms I have described?

No point in speculation - there is a good chance that you will be able to see the faulty component (the one that produced the smell).

 

[NeXT]

It sounds like something in the brightness control circuit failed.

 

[Rauli]

Thanks for feedback.

In a way that is good, because it means that you should be able to visually identify the faulty component.

OK, I will try to identify it. I hope the faulty component is not completely burned and its value is still readable.

From what I have read, there is not enough energy in the CRT to kill you,

Well, maybe it's a bit different when the heart is ill, which is the case... Anyway the power cord is going to be unconnected for at least 5 days because I don't have that screen at home and I will not see it till week-end.

It sounds like something in the brightness control circuit failed.

It makes sense. If can't see anything burned I will pay attention to the place where the brightness control is, and try to follow where it connects.

 

[modem7]

Well, maybe it's a bit different when the heart is ill, which is the case... Anyway the power cord is going to be unconnected for at least 5 days because I don't have that screen at home and I will not see it till week-end.

And even after five days, you could play safe by discharginging the CRT via the procedure shown in the second link that I provided.

 

[Malc]

Yes, Take all precautions when working inside monitors, Especially if you have heart problems, Years ago i was removing a mainboard from a monitor which had been stored for a couple of months, I got distracted and that was it, Not sure what i touched but my arm hurt for a week, I learned my lesson.

And even after five days, you could play safe by discharginging the CRT via the procedure shown in the second link that I provided.

 

[luvit]

I serviced dumb terminals for a year. This symptom was common on our monochrome displays. Replacing the flyback transformers usually fixed that symptom.. hopefully this can work for you.. fwiw

 

[Rauli]

Oh, after reading the CRT safety document I'm even more worried... And according to it, the flyback transformer is one of the things that I should never touch. Great!

 

[luvit]

The first time can be nerve racking.. you'll want to study it, but here is a link to get you in the right direction.
There are ways to ground a screwdriver with a wire and you stick it under the rubber cup.. link study what the anode looks like, because you'll be doing this blindly. modem7's links have good details, too.