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One of the many bits of vintage computer equipment I have lying around from when refurbishing old PCs acquired from Freecycle and selling them on Craigslist seemed like a viable business model is the monitor from a ten year-old IBM Aptiva. For reasons not worth going into it's currently the only working monitor I have, and is going to have to suffice until the January sales.
Unfortunately, it's beginning to play up. Sometimes the screen will fade to black or suddenly go a vivid shade of yellow, which can only be remedied by a firm wallop to the top of its casing.

The fact that percussive maintenance actually works gives me some hope that it's actually repairable, which would be nice as it's part of a complete system that might be a bit of a collector's item by now. I'm also acutely aware that opening up a CRT monitor can result in death or serious injury if I make a mistake, not to mention the fact that putting the damn thing back together again afterwards will inevitably be four times harder, so this isn't something I want to attempt unless there's a high probability I'll be able to correct the fault.

Any insights from more experienced techs?

It sounds like it could be a matter of loose or damaged solder on a part, but personally I wouldn't risk opening it up. I'm pretty confident about fiddling around with the innards of a PC but I'd be daft to screw with the monitor.

The monitor's dying and at best, can be used for spare parts. I'm guessing that the parts that would need fixing are too deep for anyone not experienced in such tech to repair. That, or you would need to replace a specialized part.

If I were you, I'd give a try with looking up its drivers or use lower resolution.

If nothing else, see whether you can clean some dust out of it or help it cool. Electronics at a lower temperature may function better and you might get some life out of it.

I'd be looking to buying a used monitor if I were you. Especially if you use the computer for work.

I had one of those - a CRT that would suddenly forget what the color "blue" was and would respond (usually) to percussive maintenance. I replaced it. My Other Half, who has a vastly superior knowledge to mine of all things electronic, who does not hesitate to create his own devices, did not want to open it up for repairs. I'm going to chime in with the others saying just replace it, but I am also aware that saying that is one thing and doing it is another - it was over a year before I replaced mine.

@General Zod: I've done it once in my A+ Certification class, and I know a reasonably safe way to discharge any residual static charge, but I hear you.

@Zixinus: I'd rather save up until I can afford a really good flatscreen, though if it gives up the ghost in the meantime I can get a reasonable used one for around £30. I'd mostly like to keep it functional as it's part of a complete system, and a fully working Pentium 1 with its original monitor and keyboard (the mouse isn't original equipment but only a year or two newer) might be quite valuable in twenty years; after all, an Apple 1 sold for twelve grand at auction last week.

Working inside CRTs is pretty dangerous.

An Apple 1 isn't a mass-produced hodge-podge of off-the-shelf hardware, either.

Eh, I can hope. Besides, even if it won't put my first kid through college, it might make a nice gift to a science museum someday.

Still, unless amusing YouTube video footage of me getting it horribly wrong would improve my chances of not getting an unflattering custom title, I think I'll stick with what Zixinus suggested and just clean out the innards.

I take apart a lot of CRT monitors to steal their mosfets and flyback transformers [which is the part that can fry you] and it's not especially hard to avoid zapping yourself. However, I've never actually managed to fix one that's in the 'whack really hard to restore picture' state. The only thing I've seen that sort of works is going over the whole motherboard with a heat gun to possibly rejoin any cold solder joints.

Keep in mind that what I mean "cleaning out" is using some compressed air to push out the dust and/or a brush to get it off. I didn't mean to clean the innards.

Is there any sort of internet classifieds that serves the UK? I know if I look up "monitor" in the Free section of Craigslist, there are several entries for CRT monitors, maybe someone near you has one they want to get rid of.

Fixing a CRT monitor is not something you'd want to try unless you have a multimeter, oscilloscope, signal generator, and a fair amount of experience in working with high voltage electronics. If you can't see the fault right away (exploded capacitor, broken solder joint) you'll then have to trace out the circuit and figure out which part(s) are broken or out of spec. Could be a bad transistor, could be a resistor that's drifted and gone off-spec, could be a coil that's lost some insulation and done the same. It's a major pain in the ass, not to mention pretty dangerous if you don't know what you're doing, strange things happen once you get above 1000V and a CRT is at something like 10-20kV. I've done it and succeeded, but I have about half of an Electrical Engineering degree and over 100 hours of formal instruction in electronics labwork plus 10X that many hours doing the same in industry. Even then I'm still rather hesitant in fiddling with CRTs.

Erh... Have you tried to see if the cable connector at both ends of the cable a bit loose or not ? I had a similar problem before (a primary color vanishing then coming bak) and it happened to be the cause of it.

Check connections, maybe look for broken solder joints or blown capacitors. Beyond that, as Aerius indicated, will require electrical engineering knowledge to attempt a repair.