combining RGB to mono

[blackevilweredragon]

Ok, so in my old IBM Model 25, which has a mono monitor, I added a VGA card into it with 1MB VRAM, to ditch the onboard MCGA..

So, I wanted to test it out, and just for now I combined the RGB lines into one wire going to the tube..  (the video card don't support mono mode, and i need all colors)...  when i combined them with no resistors or capacitors, the monitor worked fine, though quite bright..  i was able to turn down the brightness and contrast, and made it look great..

so, do i need to make modifications?

What I did notice however, for a flaw, is that there is a slight ghosting next to high contrast objects on the screen...  is that a reason for the resistors and caps?

i used 22 gauge wiring for this mod..

[blackevilweredragon]

oh, and how do i prevent a BIG ghosting like this?  It does this even with no mod, but not as "much"..

http://blackevilweredragon.spymac.com/wavy.jpg

[viletim]

Ghosting is the effect you get on your TV's picture antenna isn't pointing in the right direction. What you're seeing on you monitor is noise from the horizontal output section getting into the video driver section. The video amplifier is usualy powered by a auxillary winding on the horizontal output transformer. Noise can be coupled into the video amp if the filter capacitors on this line go bad.

Set your VGA to monochrome mode with the mode select pins.

[ido8bit]

http://www.tkk.fi/Misc/Electronics/faq/vga...g.html#vga_mono

has the circuit you are looking for.  This should give you a better picture without having to mess with the constrast/brightness controls.  

You'll still need to change those caps to get rid of the noise in the image though.  After ~20 years it's hardly surprising that some will need to be replaced.  

[blackevilweredragon]

Ghosting is the effect you get on your TV's picture antenna isn't pointing in the right direction. What you're seeing on you monitor is noise from the horizontal output section getting into the video driver section. The video amplifier is usualy powered by a auxillary winding on the horizontal output transformer. Noise can be coupled into the video amp if the filter capacitors on this line go bad.

Set your VGA to monochrome mode with the mode select pins.

I said this in my original post:  "the video card don't support mono mode, and i need all colors"

the video card will NOT work in mono mode with the pins selected...  The color pallet gets all screwed up and black becomes white, grey becomes black, etc..

[blackevilweredragon]

http://www.tkk.fi/Misc/Electronics/faq/vga...g.html#vga_mono

has the circuit you are looking for.  This should give you a better picture without having to mess with the constrast/brightness controls.  

You'll still need to change those caps to get rid of the noise in the image though.  After ~20 years it's hardly surprising that some will need to be replaced.

cool thanks, i'll use that..  it looks like that is a weighted greyscale mod..  i decided weighted isn't what i want, but i see how i can mod that to be linear

as for the caps, is there another way?  getting to the video board means discharging the CRT, removing 300 screws (no kidding)...