Olivetti M24 (AT&T 6300) color monitor

[3pcedev]

Hi everyone,

After a long break from working on old computers (built a house and took up rally driving) I finally have some free time to spend on this stuff again.

Out of nowhere on eBay an M24 complete with color monitor appeared for sale which never happens in Australia.... Of course I had to buy it. I love my green and amber monochrome screens but have always wanted a color one to complete the collection.

Despite the worlds worst shipping I have ever seen (the computer was placed into a cardboard box, the monitor was placed on top just like you would on a desk, and then the keyboard was placed on top of the monitor, it partially survived. There was no packing except for a broken piece of 1/2" polystyrene between the keyboard and top of monitor. The plastic on the monitor is scuffed in places from rubbing on the cardboard, and the top of the PC case is dented in due to the weight of the monitor (and probably being dropped) however the system however does work as advertised...

The model of monitor seems to be different to the US AT&T supplied units; I believe it may be same as the Italian delivered unit. The model is DSM 2412C.

The monitor has a few issues which I need to sort out....

First issue is it came with a non original DB25M-DB25F cable. The supplied cable looks like a serial cable, it's very short, thin and probably has very little shielding. This may be the cause of all my issues...

Second issue is when I connect the monitor with said cable and boot the computer up I have very little control over brightness and contrast. When at the bios screen (or in DOS) with just white text on a black background it works acceptably. It's not great in terms of focus, but it's ok. As soon as it loads into GEM 3 (was on the machine) the display is overly bright and I can see the raster zigzag. Adjusting the brightness and contrast doesn't do much at all, but if you max out the potentiometers the convergence and focus go out terribly (see photos). I don't think this is due to shipping, the eBay photos clearly look the same with the raster lines in GEM 3.

Before I tear into the monitor I need to confirm the cable between the PC and monitor is suitable. I found the pinout for the computer side of the display adapter on the web, but I cant find any information for the monitor side. I see that two pins are used for ID (i.e. color or monochrome connected). What I don't know is if those pins are bridged in the cable; or if those pins are bridged in the monitor itself. I also don't know if the pin mapping in the cable is 1:1 between the computer and monitor (i.e. pin 1 on video adapter actually goes to pin 1 on monitor).

Could someone with a color display check the pin mapping on the cable? I really want to make sure this is correct before continuing down a rabbit hole. I know this is a bit of a pain but it will really help me out!

Also if anyone has a service manual, circuit diagram or any sort of technical literature on this monitor it would be greatly appreciated!! I did a search last night but came up with nothing....

Linked below is some photos. Ignore the brown circular spot on the display; that's just a camera artefact. The color blur, raster and lack of focus is real.

Thanks!

 

[Chuck(G)]

You can find pretty much anything you need to know about the 6300 by calling it an Olivetti M24 and going to this site. Your connector pinout can be seen under "Technical Documentation".
The DSM2412 C is a Toshiba or Mitsubishi monitor (the -A and -B were made by Hantarex).

 

[3pcedev]

Hi Chuck,

Thanks for the reply. Took a look at the site and the pinout for the PC side is handy to have. It would also be handy to have the technical documentation for the monitor side too; I assume they mapped the pins 1:1 through the cable, but I just want to double check they didn't do something stupid....

 

[Chuck(G)]

I've never used the Olivetti-branded color monitor, so I can't say for certain. I've never been able to find a manual for the DSM2412C; maybe another forum member knows where to find it, if it exists.

 

[1ST1]

No idea, there are no manuals for these monitors. Once I have seen somebody using an alternative cable for his color monitor and it was looking like an industrial made standard cable, not hand made. So I think the cable has a standard layout like 1:1. As you have a working cable you can just measure the cable using a resistance meter. And then you can solder a new better and thicker cable, which has shielded rgb, h/vsync.

 

[MadMax2023]

Hi,
I also own since a while a M24 Olivetti computer which is in great working order with its green monochrome screen.

I got few days ago a color monitor for it, it is a DSM 2412c. And guess what, I have the same issue as you with the over brightness.
When I set-up the brightness to see correctly the DOS characters on screen, if I launch a game the brightness increases suddenly. So I set-up again to reduce the brightness, then on the next screen the brightness changes, the picture is too dark, so I increase it again, and so on... I am always setting up the brightness.

The switches on the mainboard of the M24 are setup for a color screen, no problem with that.

I opened the screen and checked one by one all the capacitors, they were all in good condition except 2 with a loss of 10-20% of charge, I replaced it, it changed nothing.
there is no IC controlling the display settings, so I don't have a clue what to check for further tests.

 

[1ST1]

Maybe you have to clean the potentiometers for brightness and contrast to get better picture.