I had an issue with the newest set of the bunch, the "Ultravision Digital" one from 1999. Despite a great picture, the set suffered from a very aggressive "breathing" on bright images that caused the image to "squeeze in" from the sides about 1 cm when going from say a full red screen to a full white screen. Also very noticeable when opening mostly white windows in Microsoft Windows.
It really bugged the hell out of me, so the first thing I thought was failing caps or resistors in the power supply and horizontal output areas. Checked a bunch of values vs a set that didnt have the the issue and came up with nothing. Then I changed main power supply regulator and the large cap that is right near the HOT. Again, no change.
The next thing I looked at was the way ABL is handled. ABL is a feedback loop used in modern CRTs to regulate flyback B+ voltage to prevent drastic size changes in the picture when going from dark to bright images. Without it, bright images become larger and dark images become smaller, which is very noticeable. I first changed out the jungle /video board, because the jungle does have an ABL pin on it. The service manual is not very helpful in describing how or if its used though. Changing the board made no difference.
So it turns out that the PA1 SIZE PWB handles a variety of functions, such as horizontal size, position, pin, and ABL. So naturally I swapped it with another sets to see if the problem followed. No change. So I continued looking and saw how they are taking the ABL signal from the flyback and using it to drive a couple of transistors in series with a +12V rail and the primary winding of T771, which causes inductance changes in the secondary winding through which the B+ current flows, thus regulating the B+ coming out of the flyback.
I started looking at what modifications I could do that could potentially affect the ABL agressiveness and the easiest thing to try was to increase the resistance of the circuit that drives the transistors and transformer primary above. R781 is the initial resistor in the ABL signal path on the card, and its a 150R resistor. Normally, increased driving voltage/current happens when images go bright, which makes the secondary of the transformer conduct less and reduce B+, so an increase in resistance should theoretically tone down the ABL changes. I went with a 220R resistor put in series with R781 to start, and to my surprise, the issue was GREATLY improved. It improved so much that the ABL handling looked very much like all the other sets. It may or may not be able to be further improved, I may try a bit more resistance to see if theres any change, but Im pretty satisfied. I wish I could have located the actual culprit causing the issue instead of doing this hack, but it worked so well I decided to post it here, in case anyone else runs across a similar issue on their sets.
Full Size:
https://i.imgur.com/ZgyGRtj.jpg