A few days ago I rescued a monitor from the dumpster. Very dirty inside and out, and yellowed, but I have left it pristine, inside and out, and quite whitened (a new way, with a scouring pad, 🤣)
It is a HP D8891A 14" monitor in an Acer JD144K chassis (Acer branded).
As it is "small" it is better for me in the workshop, instead of other larger ones that I have of 17" and more.

When testing it I have determined the following:
-The image is only seen in 640x480@75Hz. And sometimes in 640x480@60Hz but then it is lost.
-Sometimes the image is seen when booting the PC, but then it is lost.
-The contrast control must be at minimum.
-The brightness control does not respond.
On the control buttons
-The horizontal zoom does not respond.
-The vertical zoom works but should be at the minimum.
-The pincushion control does not respond.
-The up/down & left/right scrolling works kind of OK.
Others
-The Gamma and Focus on the flyback work OK.

After changing a visibly bad capacitor (C207 on the schematics) the image improved a bit in its shape, but the other problems continue.

Sometimes it makes bad contact in the small video board connection with the CRT, but the initial problem does not change, only some color fails.

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Then, I have two others to repair:

A Samsung SyncMaster 793s that I must examine in more detail. It powers up (the LED turns green), but immediately goes into standby (the LED turns orange), and powers up again and repeats the cycle.

A Samsung SyncMaster 997MB that seems to have a problem in the power supply, which I cannot find what it is. A very slight intermittent clicking sound is heard (every second or so) in the power supply side, and it does not power up, the LED does not turn on. The part that worried me the most, a power supply control chip, I replaced with the one from the 793s, which is equivalent, but the problem remained the same, so "luckily" it wasn't this part.

Any advice on this?
what should I check first?