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Retro Canada
October 11th, 2017, 08:17 AM
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...

41273

David_M
October 11th, 2017, 12:25 PM
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.

Retro Canada
October 11th, 2017, 12:49 PM
Its a long green IBM 1501985 0 Black and White Parallel Card. It has 8 NEC uPD2114LC Static RAM chips 1Kx4.

SpidersWeb
October 11th, 2017, 02:17 PM
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)

Retro Canada
October 11th, 2017, 02:28 PM
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)

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 :)

glitch
October 11th, 2017, 04:48 PM
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.

Retro Canada
October 11th, 2017, 06:56 PM
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.

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...

Druid6900
October 11th, 2017, 07:28 PM
Living in Canada, like I do, I could mail you a 2114 for about a buck.....

David_M
October 11th, 2017, 09:07 PM
It shouldn't be too hard to work out which one is the problem. I don't have a schematic for it or I'd work it out from that for you.

modem7
October 11th, 2017, 11:20 PM
I don't have a schematic for it or I'd work it out from that for you.
The 'IBM Monochrome Display and Printer Adapter' link at [here (http://minuszerodegrees.net/oa/oa.htm)].

David_M
October 12th, 2017, 01:16 AM
The 'IBM Monochrome Display and Printer Adapter' link at [here (http://minuszerodegrees.net/oa/oa.htm)].

Based on the errors, its the low nibble of the 2nd character code chip, that is U14

Retro Canada
October 12th, 2017, 08:38 AM
Living in Canada, like I do, I could mail you a 2114 for about a buck.....

I couldn't find it in your website. If you have it available I will want. The ones I bough will take a month to arrive...

Druid6900
October 12th, 2017, 07:33 PM
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.

MikeS
October 12th, 2017, 08:19 PM
Perhaps someone else (MikeS) has one laying around that he could send you.

I ignore requests from people who can't be bothered to put their location on their profile; gets tedious after a while to always have to ask.

Retro Canada
October 12th, 2017, 10:43 PM
Updated. I live in Vancouver

Retro Canada
October 14th, 2017, 01:12 AM
It does not look pretty but it worked... Yes it was exactly the U14, thanks for the tip :)
41311
41312
41313

MikeS
October 14th, 2017, 10:14 AM
It does not look pretty but it worked... Yes it was exactly the U14, thanks for the tip :)

Nice! And thanks for adding your location.

I assume that'll keep you going until the Bulgarian chips arrive?

m

KenEG
October 14th, 2017, 11:15 AM
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.

Retro Canada
October 14th, 2017, 12:24 PM
Nice! And thanks for adding your location.

I assume that'll keep you going until the Bulgarian chips arrive?

m

Yes, I had this just for testing. I placed a socket in the extracted chip so just unplug the brain implant.

Retro Canada
October 14th, 2017, 12:25 PM
Thxs, don't worry I have fixed mine when my chips arrive It will look better :)

Trixter
October 14th, 2017, 01:07 PM
But I'd like to play a little with assembly, see what can I get from the 6845 :)

Be careful -- while it is difficult to blow a CGA monitor with goofy timings, it is easier to blow an MDA monitor with goofy timings. Stick to the BIOS-initialized modes until you're extremely confident, then when you start playing with registers, avoid H/V sync total and sync width.

Retro Canada
October 14th, 2017, 05:55 PM
Be careful -- while it is difficult to blow a CGA monitor with goofy timings, it is easier to blow an MDA monitor with goofy timings. Stick to the BIOS-initialized modes until you're extremely confident, then when you start playing with registers, avoid H/V sync total and sync width.

I don't have CRTs monitors, it wil be with my converter :D and i can't blow it :)