Weired pattern on 1501486 CGA adapter

[jens.hafner]

Dear all,

I recently received a CGA adapter of type 1501486. When I put it in the computer and boot it up, I see a pattern like the one in the photo attached. Every second character seems to have a red background, spaces are replaced by an apostrophe, colons) are replaced by a 'z', periods (.) by the letter n and so on.

Since these types of CGA cards are pretty rare here in Germany, I would like to rescue it. I assume that some RAM chip is defective. Has anyone any idea on how to diagnose this issue and find a solution on how to fix this?

Best Regards

Jens

 

[GiGaBiTe]

If it has socketed RAM chips, try swapping them between sockets to see if the display changes. If it does change, some or all of the chips are bad. If it doesn't change, something else on the card is bad.

 

[per]

Looks like bit 6 of video-RAM is stuck high. You can see it from the fact that all ASCII characters have been "OR 0x40"ed

 

[modem7]

On my CGA card, I simulated an 'output stuck high' failure mode of the RAM chip for bit 6, chip U51. The resulting screen display is shown at [here].

Compared to what the OP is seeing, the background is different (always red versus sometimes red). Maybe that is due to a different failure mode of U51.

 

[jens.hafner]

Thank you modem7, per and GiGaBiTe for your helpful responses. I attached a picture of the card below. The RAM chips are not socketed. I understand it is consensus that there is some issue with bit 6. It is high most of the time, but apparently not always, hence the checkboard pattern. I also understand that bit 6 is the responsibility of chip U51. So I will buy a new memory chip and replace the existing chips in U51. Is it save to solder in a socket and put the new chip in that socket? Or should I then expect any new issues?

 

[modem7]

Is it save to solder in a socket and put the new chip in that socket? Or should I then expect any new issues?

By putting in a socket, you are doing the soldering of the U51 position once. That is good, because if the replacement RAM chip you buy is faulty, then you will not need to do any soldering when you put in the second replacement RAM chip.

 

[jens.hafner]

That sounds good. I learned that every connection and every socket may cause signal reflection and signal degradation of signal strength and quality. But this is not relevant here, is it?

 

[modem7]

That sounds good. I learned that every connection and every socket may cause signal reflection and signal degradation of signal strength and quality. But this is not relevant here, is it?

In this case, not significant enough to cause any concern.