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MDA blown SRAM chip

R

Retro Canada

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I got this IBM MDA card to test with my converter but it has a blown SRAM chip :/

The lower 4-bits for the lower half of the screen are stuck on LOW. It seems a single SRAM 1Kx4 is bad. Would any of you with experience in repairing these boards could spot it ? I could try to find with a scope but the chips are difficult to reach.

I will have to buy SRAM chips now... If I had to replace many I will end up paying more than I paid for this board...

mda_blown.jpg
 
Is it a real IBM card or a clone?
The later clone cards that are switchable between CGA and Herc mode use just 2 RAM chips. Given that you have the four low order bits failing I'm guessing its a single faulty 41464 chip (64kx4)
Sometimes the RAM chips are surface mount and sometimes through hole, the card I have in my hand now is fitted with surface mount and has unpopulated provision for through hole chips.
 
Not an expert, just thinking out loud:

Errors start at around 800H, or 2048, so that's going to be the third bank (2K-3K) ?

But looking at the logic diagrams in the tech ref, I'm not entirely sure what chip is responsible. My guess would be U12, but it's an assumption based on the chips being numbered based on their position in memory space (e.g, 1K LO, 1K HI, 2K LO, 2K HI, 3K LO, 3K HI, 4K LO, 4K HI). Hopefully someone smarter can provide more useful information, just thinking out loud.

Sometimes I just enjoy the puzzle.

(Edit: but I doubt you'll use it for more than testing anyway would you? All of my IBM MDA cards are in static bags with Hercules cards in each machine)
 
Last edited:
SpidersWeb said:
Not an expert, just thinking out loud:

Errors start at around 800H, or 2048, so that's going to be the third bank (2K-3K) ?

But looking at the logic diagrams in the tech ref, I'm not entirely sure what chip is responsible. My guess would be U12, but it's an assumption based on the chips being numbered based on their position in memory space (e.g, 1K LO, 1K HI, 2K LO, 2K HI, 3K LO, 3K HI, 4K LO, 4K HI). Hopefully someone smarter can provide more useful information, just thinking out loud.

Sometimes I just enjoy the puzzle.

(Edit: but I doubt you'll use it for more than testing anyway would you? All of my IBM MDA cards are in static bags with Hercules cards in each machine)
Click to expand...

Sure, Hercules are way more fun, especially with RAMFONTS. But I'd like to play a little with assembly, see what can I get from the 6845 :)
 
Can you find the schematics for the board? I don't remember if the MDA board was fully documented.

2114 SRAMs shouldn't cost all that much. I could sell you some tested/working 2114s, but I suspect it'd be more to ship them to Canada than they're worth.
 
glitch said:
Can you find the schematics for the board? I don't remember if the MDA board was fully documented.

2114 SRAMs shouldn't cost all that much. I could sell you some tested/working 2114s, but I suspect it'd be more to ship them to Canada than they're worth.
Click to expand...

I paid 25 bucks for the board :p

I got some 2114 from Bulgaria, 15 dollars for 10 of them, let's see If they ever come. If I find the blown chip I can try adapting a 6116 I have here meanwhile... :)

The MDA manual on minuszerodegrees has the schematics but it's too confuse. There are many, many TTLs working together...
 
Sorry, the static RAM I have left are 2102-3.

The 2114s were apparently sold a couple of months ago in bulk.

Perhaps someone else (MikeS) has one laying around that he could send you.
 
The computer I just bought had an MDA with printer port adapter in it. I have no monitor to use it with. Unfortunately, I am in the US so shipping might be excessive.
 
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