I was in the local superstore looking at a range of TVs, which were all playing the same music videos. The name of the track was displayed along the bottom of each screen.
Now as the scene changed in the video (light to dark, dark to light etc) the words in the display seemed to jump about (unstable). What is going on here?

[DRGL]

poor power regulation-a lot of TV's do it. my Sony HQ100 doesn't though thankfully(but does have other issues!)

[Nick_UK]

When the brightness changes in the picture, the "beam current" changes too, and this causes the HV supply (about 25,000 volts) to the CRT to vary. The variations in the HV supply cause the picture size to change slightly in sympathy with the voltage variations. You only see this effect on CRT screens.

It will vary depending on the design though. Generally the rule of thumb is the cheaper the set the worse the effect. All the 36" CRT's I've seen are pretty good and only change slightly. My mate's cheapo (goodmans) 32" is terrible and even bends around the sky tv bar at the bottom of the screen. My 51WH is excellent in this area.

[Nick_UK]

A huge amount depends on the design. Early colour TV's used a big valve (tube) to generate the EHT (HV), and they had another valve which regulated the EHT supply. These babies generated so much power that you could draw a spark fat enough to light a cigarette, but you had to be careful about X-Rays, though No "breathing" of the picture there, though !

Modern designs use a much lower power transistor circuit, which generates about 8,000 volts which is "tripled up" by a diode circuit to give around 25,000 volts. Very little regulation, though. Most only rely on the graphite coating on the CRT and an inner silvering coat inside the CRT to produce a big capacitor for smoothing the EHT supply.

Thanks Guys
Could this problem heave been worsened in the shop by having all the TVs with the same picture, and therefore all changing the power supply at the same point from the same mains supply?
Cheers Chris

[LV426]

No, it's not the mains supply that's affected - that would be illegal. It's what's going on inside each set.

One of the many ways in which CRTs do not produce the best picture.

Originally Posted by LV426

No, it's not the mains supply that's affected - that would be illegal. It's what's going on inside each set.

I'm no expert but bare with me while I make an observation. When I turn on my Hi-Fi amplifier it dims the house lights momentarily (and this is a new house and wiring). Why could this effect not be happening with numerous TVs on the same supply.

Cheers Chris

OK, when you turn on your amp it's momentarily a complete short circuit to the mains while the electromagnetic field on the transformer establishes itself. During this brief phase it will draw a lot of mains current so, for a brief moment, the lights will dim.

This does not happen when you power up a TV because it's power supply is totally different.

OK thanks for the explanation.
Cheers Chris