K7400 / WGM2574 partial vertical collapse

[cyberbaud]

I have a K7400 that has a partial vertical collapse issue. Bright line in the middle and about one third of the video is showing fine below the line. Picture attached.

I have done a full re-cap on the P793 board and it made no difference. Measured the pots on the remote board, especially the 200K height pot. It seemed OK through the 200K range and shows height adjustments in the visible part of the video.

Also tried replacing D302 and Q601 with no difference. I measured R402, R303, R627, R618 in circuit and they seem to be OK. B+ voltage on TP202 seems OK (117.2V). I also do get 24.75V on pin 9 of U601.

I am running out of ideas. What would be my next best bet to track this one down? Does the screenshot look like it could be a defective U601?

Thanks

[lilshawn]

U601 or U701. partial faults in either chip can cause vertical problems.

double check the value of R619 it feeds juice to U601. if it's been strained by a bad C613 it may have drifted out of spec and not delivering enough voltage to U601.

[cyberbaud]

Replacing U601 did the trick. It was complicated by the fact that the first replacement TDA1771 I received was defective. Now with a good one installed the board works fine again.