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IBM PS/1 model 2121 monitor issue just in text mode

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First post, by abrunetto

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Hi Folks, how are you?

It has been a long time of my last post. I am rebuilding my retro cave and I started adding an IBM PS/1 model 2121. I am facing an issue with the monitor. It seems to have some kind of vertical sync issue (text blinking vertically up and down, but only when It is in text mode. In graphics mode, I mean Windows 3.0, IBM DOS 4.0 graphical dosshell, boot dashboard or even games, it works like a charm.

I tried another monitor (IBM 8513) and it works just fine in both graphics and text mode.

I know that this monitor has presets inside the case to adjust image geometry but I don't know if it is a misadjustment or another issue. I didn't try to adjust those presets yet, since in graphics mode everything seems to be working just fine.

I will highly appreciate any help on this.

Best Regards,
Ariel

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Reply 1 of 4, by mkarcher

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A standard VGA monitor has three presets for the vertical stuff (but some settings might be shared between them)

  1. One preset for 350 visible lines. This preset is used for the EGA-compatible graphics mode and 43-line text
  2. One preset for 400 visible lines. This preset is used in text modes and low-resolution 200-line graphics modes (CGA graphics, low-resolution EGA compatible modes)
  3. One preset for 480 visible lines. This preset is used for the high-resolution VGA graphics mode

Possibly there is an issue with the vertical hold potentiometer for the 400 line mode.

Reply 2 of 4, by abrunetto

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Thank you for your answer. The monitor has several potentiometers. The geometry ones are the seven in a row. You think that adjusting the right one could fix the issue?

Regards

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Reply 3 of 4, by abrunetto

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mkarcher wrote on 2023-07-15, 16:31:
A standard VGA monitor has three presets for the vertical stuff (but some settings might be shared between them) […]
Show full quote

A standard VGA monitor has three presets for the vertical stuff (but some settings might be shared between them)

  1. One preset for 350 visible lines. This preset is used for the EGA-compatible graphics mode and 43-line text
  2. One preset for 400 visible lines. This preset is used in text modes and low-resolution 200-line graphics modes (CGA graphics, low-resolution EGA compatible modes)
  3. One preset for 480 visible lines. This preset is used for the high-resolution VGA graphics mode

Possibly there is an issue with the vertical hold potentiometer for the 400 line mode.

How could I identified the preset on the mainboard?

Rrgards,
Ariel

Reply 4 of 4, by mkarcher

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abrunetto wrote on 2023-07-15, 18:50:
mkarcher wrote on 2023-07-15, 16:31:
A standard VGA monitor has three presets for the vertical stuff (but some settings might be shared between them) […]
Show full quote

A standard VGA monitor has three presets for the vertical stuff (but some settings might be shared between them)

  1. One preset for 350 visible lines. This preset is used for the EGA-compatible graphics mode and 43-line text
  2. One preset for 400 visible lines. This preset is used in text modes and low-resolution 200-line graphics modes (CGA graphics, low-resolution EGA compatible modes)
  3. One preset for 480 visible lines. This preset is used for the high-resolution VGA graphics mode

Possibly there is an issue with the vertical hold potentiometer for the 400 line mode.

How could I identified the preset on the mainboard?

If there is a label called "V-HOLD" or something like that next to a pot, that's a prime suspect for your problem. The first picture you attached shows pots labelled "MODE 3 V-SIZE" and likely "MODE 2 V-SIZE" next to it, supporting my post that there are different presets for different "modes". If you can get hold of a service manual for the 2121, you can look up which mode (1, 2 or 3) is the 400-line mode. V-SIZE is the setting that is usually separate for the three modes, so it makes a lot of sense to find mode-dependent V-SIZE pots. Instead of checking the manual, you can also experiment which of the likely three V-SIZE pots affects text mode. Then you know the mode number for the 400-line mode.

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