VGA Monitor with odd sync problem

[nestor]

Today my backup PS/1 monitor has started to fail. All video modes except 640x480 have a sync problem that makes the monitor unusable. But in 640x480 the image is sharp, clean and free of any sync problems. I'm totally lost, if the VGA cord was in bad state the sync problem would affect all video modes. Here is a video (it doesn't really show the unsynchronised image as it is, the image rolls very fast but it seems that the videocamera and youtube have lost a lot of frames):

Any idea?

 

[133MHz]

Could it somehow be related to the vertical sync polarity?

 

[nestor]

Could it somehow be related to the vertical sync polarity?

I don't think so, because 640x480 and 640x350 have the same vsync polarity, and 640x350 shows the sync problem.
After one hour powered on, the issue disappears. If I let it powered off a while, the issue apears again.

Maybe a heat problem? Or a capacitor in bad state?

 

[Chuck(G)]

Could be. Hang a meter on each of the power supply outputs and watch for variations. It could also be a power resistor misbehaving--or any number of other head-sensitive components. If the power supply output looks stable, then it's time to get out the can of cooling spray and start squirting components.

 

[nestor]

I have found a suspicious electrolytic capacitor, looks a bit blown... and seems the main filter (220 uF, 400v)

 

[modem7]

I have found a suspicious electrolytic capacitor, looks a bit blown... and seems the main filter (220 uF, 400v)

Large capacitors like that were often secured to the PCB via glue. That is what I see when I look at the photo you have posted.