You sure the monitor adjustments for that video mode aren't just set up that way? Since it's one of those text+graphics modes it's not the same DOS text mode that it goes into when it clears the screen, displays the system configuration table and boots, so perhaps your monitor treats it as a distinct mode with its own size/position settings, and someone has tweaked with them on that monitor before.

jakethompson1 wrote on 2023-04-01, 20:23:

You sure the monitor adjustments for that video mode aren't just set up that way? Since it's one of those text+graphics modes it's not the same DOS text mode that it goes into when it clears the screen, displays the system configuration table and boots, so perhaps your monitor treats it as a distinct mode with its own size/position settings, and someone has tweaked with them on that monitor before.

Big thank you for the answer! Yes, it turns out that if I set the monitor position and size while on the post screen, it saves the settings just for it, not for the whole OS loading. Now the post screen looks better, a little bit off ratio, but still better than before, by filling more of the screen.
Thank you once again!

Don't be surprised when after monitor warming up the picture is off a bit again. That is normal for CRTs.

Thank you all for the replies. It seems that my monitor is going to die soon, so unrelated or not, I'm posting this here, not to make another topic:

Now my monitor started to look like this, and if I hit it from the top it fixes, but then, short after that it returns to this pincushion ugly picture. Is it fixable? Should I be getting a new one, or I can fix it? I have a little experience with soldering electronics, but I never dealt with CRT monitors.

nickolov wrote on 2023-04-09, 14:11:

Thank you all for the replies. It seems that my monitor is going to die soon, so unrelated or not, I'm posting this here, not to make another topic:

Now my monitor started to look like this, and if I hit it from the top it fixes, but then, short after that it returns to this pincushion ugly picture. Is it fixable? Should I be getting a new one, or I can fix it? I have a little experience with soldering electronics, but I never dealt with CRT monitors.

If you hit it and it fixes it, I is likely a bad contact somewhere. hopefully it's a cracked solder joint which is easily fixable with a soldering iron.

weedeewee wrote on 2023-04-09, 14:33:

If you hit it and it fixes it, I is likely a bad contact somewhere. hopefully it's a cracked solder joint which is easily fixable with a soldering iron.

Excatly. Any TV technician that wants to fix this monitor will be able to. The fault is obviously in the horizontal deflection, which gets quite warm in operation (and warming up/cooling down weakens solder joints), and has some heavy components like transistors on heatsinks, e.g. the horizontal output transistor, and the horizontal transformer itself, which are prone to pick up vibration, finally cracking the weakened joints. The fault is clearly not located in the picture tube, that's what makes me so confident that this is fixable. The deflection is generated by the yoke, which is "just a set of coils" (TV technicians: I know this is oversimplifying it) which is mounted outside the tube on its neck, and the electronics used to generate the drive current for the coils is mostly 80's technology made of discrete components. The geometry preprocessing unit is using modern difficult-to-fix proprietary ASICs, but is extremely unlikely to break in away like you are observing. A broken horizontal transformer also is not completely impossible to cause this symptom, but broken joints are so much more common that it is unlikely.

Thanks for the comments, people!
So, if it is a broken solder, is it bad that I have used it for like a day? I mean, have I shortened its life more, by using it this way? Is it worth repairing it?
Because at work I found this Samsung 15' from 2000. Its plastic is really yellow-ish, but at least I saw that it works (or who knows - I just tested it for like 5 minutes and the picture was good). I can swap this LG for the Samsung.

I had one of these like 20years ago, but the 17" model. It failed with this very issue. Vertical deflection problems... More specific, my case was due failing caps (LG StudioWorks line used chinese trash caps... At least the SKU which was being sold in my area/country).
At time I just trashed it away and bought a new one... But now I would say, recap it if you can. Aside trash caps, these were Ok monitors for windows and DOS usage.