To Study circuitry of monitor and some fault in circuitry.

Objective: To Study circuitry of monitor and some fault in circuitry.

 

The cathode ray tube (CRT) is a vacuum tube containing an electron gun (a source of electrons) and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets and others.

The CRT uses an evacuated glass envelope, which is large, deep, heavy, and relatively fragile. Display technologies without these disadvantages, such as flat plasma screens, liquid crystal displays, DLP, OLED displays have replaced CRTs in many applications and are becoming increasingly common as costs decline.

 

                                                               An Internal Structure of CRT

Faults and Repair:

Specifications for Philips CRTs can be found in the regular series of data books from Philips Components. Companies and universities usually have them. Usually the data sheets show typical Ik/Vk characteristics. They also list the spread on cutoff voltage and cathode gain, and this spread is quite large even on new CRTs. They also list phosphor sensitivity (Lum/Ik), this too has a large spread. But they almost never list anything about the aging process.

Here are some of the effects:

Phosphor ages due to burn-in, particularly on static pictures, this is immediately obvious on visual inspection. If the aging is even (no pattern) then at least the efficiency is reduced.

Cathodes age due to loss of emission material, particularly for oxide cathodes. The central part of the cathode surface has carried the most current density and will wear out first. The surrounding area takes over, this will lead to an unsharp picture. Adjusting the focus voltage will not really improve it. The tube is worn out.

Also poisoning of cathode surface may occur. This can be cured temporarily by short-time overheating ("re-conditioning").

The cathode that wears out first (often the red one) also loses gain, so the white point of the image will shift (to cyan). The white point can be re-adjusted with the gain potentiometers and the contrast, but peak brightness will not be as high as new.

The cutoff voltages of all cathodes will drift. Common drift is adjusted by the user by controlling the brightness. Different drift leads to a coloration of the black background level. In extreme cases vertical flyback lines will appear. Cutoff voltage can be adjusted with potentiometers, or there is automatic stabilisation. Still, the VG2 (screen) may need periodic adjustment too.

Leakage currents may disturb VG2 and focus voltage, re-adjustment has only a temporary effect.

VG2 and focus potentiometers may wear out due to electromigration etc. A hole may form under the wiper, re-adjustment is then impossible.

Some types of cathode wear (according to a friend in Philips Semiconductors) can cause the Ik/Vk transfer characteristic to divert so much from an ideal gamma function that no adjustment can compensate for it. Then the tube is really worn out.

I hope that this helps you to distinguish between a really worn out tube and one that still has some life in it after re-adjustment.

CRT Age Resulting in Dark Picture

Where circuit problems have been ruled out:

Most probably the cathodes have worn out. The emission material on the surface slowly becomes inactive. Usually you see one colour go first, then the others. At the same time you will observe a loss in sharpness, because a larger cathode area is being used, giving a bigger spot.

Rejuvenating is done by applying a (too) high filament voltage, in order to bring new emission material to the surface. It will not work for long and there is the risk of burning the filament wire for good. It may be worth a try, though.

Other wear mechanisms are:

Glass browning (generally only for projection tubes).

Phosphor aging (life time is defined by efficiency < 50%).

Vacuum leaks (generally cause EHT flashover, audible).

Then again, it may also be that for some mysterious reason your VG2 voltage has dropped below spec. A too high VG2 voltage will cause a smaller cathode area to be used, leading to a sharper picture but accelerated cathode wear.

Brightening an Old CRT

If performing adjustments of the internal background and/or screen controls still results in a dark picture even after a long warmup period (and the controls are having an effect - they are not faulty), the CRT may simply be near the end of its useful life. In the old days of TVs with short lived CRTs, the CRT brightener was a common item (sold in every corner drugstore, it seemed!).

First confirm that the filaments are running at the correct voltage - there could be a marginal connection or bad resistor or capacitor in the filament power supply. Since this is usually derived from the flyback, it may not be possible to measure the (pulsed high frequency) voltage with a DMM but a service manual will probably have a waveform or other test. A visual examination is not a bad way to determine if the filaments are hot enough. They should be a fairly bright orange to yellow color. A dim red or almost dark filament is probably not getting its quota of electrons. It is not be the CRT since all three filaments are wired in parallel and for all three to be defective is very unlikely.

If possible, confirm that the video output levels are correct. For cathode driven CRTs, too high a bias voltage will result in a darker than normal picture.

CRT brighteners are available from parts suppliers like MCM Electronics. Some of these are designed as isolation transformers as well to deal with heater-to-cathode shorts.

You can try a making a brightener. Caution: this may shorten the life of the CRT - possibly quite dramatically (like it will blow in a couple of seconds or minutes). However, if the monitor or TV is otherwise destined for the scrap heap, it is worth a try.

The approach is simple: you are going to increase the voltage to the filaments of the electron guns making them run hotter. Hopefully, just hotter enough to increase the brightness without blowing them out.

Voltage for the CRT filament is usually obtained from a couple of turns on the flyback transformer. Adding an extra turn will increase the voltage and thus the current making the filaments run hotter. This will also shorten the CRT life - perhaps rather drastically. However, if the monitor was headed for the dumpster anyhow, you have nothing to lose. You can just add a turn to an existing winding or make your own separate filament winding as outlined in the section: Providing Isolation for a CRT H-K short.

In some monitors, there is a separate filament supply on the mainboard - this should be obvious once you trace the filament wires from the video driver board). In this case, it still may be possible to increase this output or substitute another supply but a schematic will be required.

There are also commercial CRT rejuvenators that supposedly zap the cathodes of the electron guns. A TV or monitor service center may be able to provide this service, though it is, at best, a short term fix