CRT Trouble? Danger Will Robinson!

Danger Will Robinson! High Voltage!

In order to work, a TV or monitor uses high-voltage currents which can electrocute you!

For electrons to be attracted enough to the anode to leave the cathode in a CRT, a very high level of charge, or voltage, needs to be generated. This voltage is created by a component called a “flyback transformer”, which can generate between 20,000 and 50,000 volts!

This voltage can linger for days or even weeks in both the CRT and the flyback transformer after the TV is turned off and unplugged. You don’t even need to fully touch them or be grounded to get electrocuted by either! Very dangerous.

Also, while the exterior screen surface is reinforced, the CRT is very fragile and can implode!

“Implode? Well, that doesn’t sound so bad! Doesn’t that mean it just collapses and winks out of existence? That’s what it means in science-fiction cartoons anyway…”

Um, no. The CRT is a vacuum tube – it has no air in it, because any matter (molecules, atoms, etc.) would interfere with the electron stream. But the air around it wants in, to equalise the pressure, and if the smallest hole or crack is made in the CRT, the outside air will push in on it, crushing it.

But once all the glass piles into the centre, it’s not like it just stops there. All that kinetic energy has to go somewhere, and no longer confined by the equalising air pressure, all those glass fragments then just go…well, everywhere, like shrapnel from a hand grenade!

More modern CRTs have “implosion protection” which is meant to reduce the risk of the tube crushing, but it’s not 100% effective. So do you really want to take that chance?

Both high voltage and implosion risks should make you want to be very careful around CRTs!

“So what exactly can I do to try to fix a broken CRT TV or computer monitor?”

Well, firstly don’t bother acquiring a non-working CRT if a working one is available. But here’s some tips on how to tell how much trouble it will be to fix… 

1. It doesn’t turn on or there’s no screen.

Does anything happen? Does it have a pilot light that comes on when you plug it in? If it does, then does that light change colour when you press the power button? Does it have a remote? Have you tried the power button on the remote?

Sometimes a defective power switch or disconnected control panel can prevent a TV or monitor from turning “on”. Also listen for a “zap” when you press the power switch. If you hear that, but the CRT doesn’t brighten up, you could have a bad flyback transformer. These are typically straightforward repairs (when done by a professional!) Would be worth looking into.

2. There’s no colour, or the screen is “rolling”.

Have you adjusted the colour / vertical hold (vhold) controls? (Sometimes they’re hidden under a panel on the bottom or side of the monitor or TV, or may be inside an on-screen menu somewhere) Do the on-screen menus have colour? Do they roll? Have you tried another signal source, e.g. another console or computer?

Don’t forget that NTSC and PAL signals are incompatible – so if you use a PAL console on an NTSC TV, there won’t be colour and it (might) roll, and vice versa. Also, some computer monitors used in PAL countries are actually NTSC (for example Apple composite monitors). If you find mechanical controls and there’s still no colour or you can’t stabilise the picture, it could be a bad control which might be fixed using contact cleaner, but otherwise it could be a serious problem with the video decoding circuitry which could be very time consuming (and expensive) to fix. Pass.

3. Picture is dim, blurry or too bright.

Once again, check for mechanical or “soft” brightness / contrast controls and adjust them. If you see flickering on the screen when you adjust mechanical controls, they could need cleaning. Give it a minute or two and see if the picture improves.

CRTs do have a finite lifespan and a dim picture could be a sign its time is almost up. Pre-1994 CRTs can often be “rejuvenated” with special equipment, but this may only improve the picture for a short amount of time. There are other methods to brighten up (very) old TVs, but they are a gamble.

Focus can be adjusted (by a technician via an internal control) to sharpen up a blurry image – but not if the voltage has been increased to overcome a dim CRT! Check if there’s “bloom” (the picture looks like it’s ballooning out to the edges) – if so, pass.

Other issues can include “rainbow” fringes on the edges of colours (can often be adjusted on older pre-1990 CRTs) and areas of weird colour or brightness (can usually be fixed by “degaussing” or demagnetising the CRT.)  More complex issues are typically expensive to repair. Caveat emptor!