Slight Drop in Brightness

[TinCanAlley]

I fired up the Avante yesterday and noticed that the brightness had changed a bit. The level I had set weeks ago where the blacks were set to the point just below washing out. For an example, in scenes I watched earlier, facial details were lit properly and detail in dark hair were there. Yesterday the faces were much darker with less detail and their hair was jet black with less detail as well. I had to bump up the brightness control just to restore the image. I wasn't a big adjustment to get it back. The contrast was fine. I retested the tube and it was good and I have a large range on the brightness and contrast controls.

So why would the brightness level change?

[old_tv_nut]

Can you post the schematic of the video section? What model year is this set?
Did you watch the same exact source material both times?

[Electronic M]

I've seen CRTs get sleeping sickness from sitting a few weeks. Some sets are more prone to it than others.

[vol.2]

1- sources are typically different black levels
2- sometimes takes a bit for a tube to wake up after leaving it off for awhile

In addition to both of those suggestions from tv nut and electric m:

3- even from the same source, the black levels are often different from the point of origin. For example, if you're watching some cable TV, the exact black level (the point at which black is true black) can easily be different on different channels, and even within the programming from the same channel. The copy of Gilligan's Island they are using could have a different black level from the rip they are using of the Rockford Files or whatever.

[old_tv_nut]

I asked for the schematic and year, because if this set is before the era of video procesing IC's with feedback stabilization of CRT gun current ("automatic tracking"), there will be some drift possible. Prior to those chassis, black level stability was totally dependent on stability of the CRT guns and all the components in the video signal chain.