Loss of Colors/Color Contrast on CGA monitor...

[T-Squared]

One thing I've noticed with my Sanyo MBC-775 is that the internal monitor does not show the proper CGA palette. I know I've been saying this in multiple threads, but I found it has gotten worse.

The colors affected are Cyan, Magenta, and White. (I want to say there's one more, but I can't be sure.) The intense versions of those seem to not even show up on the monitor properly, as there is no tinting difference.

On that note, is there any way to adjust the electron gun intensities through their resistors so that they output the proper color? (I know about the circuitry for Color 6, but that's a different story)

 

[Chuck(G)]

Are there any "hidden" potentiometers on the video board that might affect contrast?

 

[T-Squared]

I did consider that, but I did adjust some the other day.

Also, as I've said in the other forum posts, there seems to be a problem with the intensity in-general, because the colors seem to not be contrasting enough.
I've noticed that there is indeed dark gray, but it's way too dark for an RGB screen. (again, almost as dark as its unintensified counterpart)

I noticed the problem as I was using the CGA_COMP video testing program.

At the moment, I'm still trying to get 640k to work on this machine, so I'll check back later after I've gotten it. I am so close. It reads and writes the memory, but not at the correct size. Everything to try getting this memory to be addressed properly in the MBC-775 seems centered around the logic equivalent of the 24S10 PROM used in the 5160.

 

[T-Squared]

All right, I have made good progress on the computer. At VCF Southwest a few weeks ago, I found a prize that I think really deserves a permanent spot in my Sanyo:



Yep! An AST SixPak Plus! I gasped as soon as I saw it, and it only cost me $15! I now have not only the full 640k capability that I was looking for, but I also get serial mouse capability too!

On my second attempt to add memory, after my first where I attempted to expand the on-motherboard RAM, I tried a Monotech MicroRAM module card, TWO actually, but neither of them worked. The BIOS threw up an error code. I actually feel a bit bad, because JD actually sent me the second one free of charge. I even changed out the RAM chips on my first module just to make sure there was no problem.

I hope to make an extension daughterboard that gives me one more ISA slot, to add an XTIDE-CF-Mini. (as there is a pin header footprint that matches the pin count of the two original ISA slots)

But enough about those. Now to turn my attention back to the colors on the Sanyo Monitor:



I did more research on what I'm missing. All of them are not quite correct, but the colors that seem to have little-to-no-contrast are:

• Magenta and Light Magenta
  • I started Commander Keen 4 CGA to test the capabilities of the memory card (It needs 640k), but the pre-game purple frames and option text completely disappear into each other.
• Cyan and Light Cyan
  • These disappear into each other too.
• Blue and Light Blue
  • A bit of contrast, but not enough. This one is most noticable on Space Quest I.
• Light Grey and White
  • Again, these disappear into each other too. I keep seeing normal DOS text as white, not light grey.
• Black and Dark Grey
  • Dark Grey is actually missing. That is expected, given that the circuitry in the monitor is not the same as IBM's, so brown is also just dark yellow.

Would there be resistors in the monitor to change out to get the color to the proper shading mix?

 

[daver2]

First things first...

Have you found the schematics or service/maintenance manual for the monitor?

If so, can you post a link to it.

This will make our lives much easier if you can find this...

Dave

 

[T-Squared]

I tried multiple times, especially in the past, to find anything having to do with the monitor.

I just tried again, but I can't find anything.

The monitor board is labeled "CM100A 4-2269 25800" (I assume "CM" stands for "Color Monitor"), and the CRT itself is an NEC "220DB22-TC06". (The latter can be found on eBay, but it tends to be expensive.)

 

[daver2]

OK, so if we don't have the schematic, what about some photographs of the monitor PCBs?

Especially of any preset potentiometers that may be present (along with any text that may be present on the PCB related to the preset potentiometers that may give us a clue as to what they do).

Without details, we are all guessing...

Dave

 

[andy]

There appears to be a slight difference between the light and dark colors, but not much. That means the intensity signal isn't completely missing. Do the contrast and brightness controls do anything? Usually the brightness affects all colors, and the contrast only affects the light colors.