Did you check the logic/power connection on both ends? Often there is enough for the board to pass the test but still a bad enough connection for no video. I always look over every single joint on the analog and the connector on the motherboard. Solder joint can even look okay but have a fine enough crack to cause issues.

In fact, I am doing the same thing you are right now on a 128k. For me I either missed a bad solder or I have another issue. Screen works for a while then nothing.

This process can be frustrating in hopes that its always something simple and it turns out to be more. But when it finally works, its so worth it!

edit: If you find its your flyback, I may have replacements. I have a pile of compact analog boards but they are all untested and many are missing caps. However, they all have a transformer.