CRT monitor getting brighter over time

Some

The ViewSonic I have is unbelievably good. It uses a Mitsubishi Diamondtron tube, and the convergence and purity are "perfect" right to the corners. I used to own Sonys, and they were never that good.

The HV multiplier is more complicated than just wires. It consists of a number of high voltage capacitors, diodes and resistor combinations to multiply the voltage from the flyback. There are tap-offs for the focus supply, screen supply, and the high voltage for the CRT anode. Because feedback regulation is required, the HV multiplier in your TV set can be fairly sophisticated. There can be a fair number of components involved.

The flyback is really a high voltage; high speed switching transformer with a number of secondaries and usually a single primary. The HV multiplier is usually combined together as part of the flyback assembly.

A very basic idea of how a high voltage multiplier is constructed:

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JANA _____

In these monitors, what exactly is going bad? The HV multiplier is nothing but a bunch of copperwire wrapped around some core, right? How can that wear down...?

No flame intended, but I really don't understand why people are advising me to get a new TFT screen. The issue I described is a very minor. The drift has been so slow that every few months the colors needed a bit of calibration. The colors are still extremely pure, black is pitch black, white is brightly white, the image is sharp, high voltage regulation is perfect (so the image is stable under all brightness conditions, RGB convergence is good, etc. In other words, it displays a perfect picture.

Even if you didn't dispise TFT's like I do (see my other posts), you would be mad to replace this screen only because the HV bias needed a minor adjustment... If it were 10 or 15 years old, perhaps there would be some logic to it, but after 4 years of use and the MTBF not reached by a long shot...?

I detect some cognitive disconance on my part. I work a lot with audio and other low voltage electronics, and the full wave and half wave rectifiers are quite common, and I can see it's the same thing. That is, as far as the basics go.

Potential difference between a cathode and final anode increases when ;

1) Final anode voltage goes up OR 2) Cathode voltage (normally held at 180 or 200) is decreased Cheers!

I have a Panasonic CRT TV with a similar problem. It comes on dark and defocussed, but comes right after about half an hour. The effect can be corrected by adjusting the screen and focus pots, but after warmup it's too bright and defocussed. The screen and focus controls are tapped off a potential divider from the EHT, inside the output transformer assembly. The resistance to earth at the screen output (probably about 500V) is 17M ohms, so the resistance at the top (25kV EHT) must be in the region of 800M. My theory is that this resistance is unstable. I would need a new output assembly costing about =A325 to repair it.

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I suspect bad capacitors, that's the usual cause of things changing as they warm up. It could be affecting power supply voltage.

On Dec 28 2007, 6:58 pm, "James Sweet"

in both of these cases i would start by changing the cap in the RGB output and b+ stages. i have cured many faults this way. the caps are usually rated 160 v or more and has a low value - between 4.7 and 47 uF in my experience.

For now, the monitor is working perfectly, but I'll keep it in mind.

Are there others ways I can recognize those caps, by where they are situated perhaps?