RCA COSMAC MS2000 MicroDisk Development System

[Joshua Rice]

On with the saga of the COSMAC MS2000…

I powered it up a few days ago. The ROM boots, nothing smoked, the drives whirr.

However, one of the RAM boards (the lower 32k) seems to have failed with a stuck bit. it outputs the rather humourous message of BAD RAM P00 (where P00 is memory page 00h). poking at various memory locations seem to reveal that they are populated with 40h instead of 00h. I tried reseating th card, and swapping backplane slots, to no avail.

Now my first assumption was that the tc40h245p bus buffer might have an open short driving one of the lines high, but probing it with a multimeter hasn’t revealed anything enlightening, with all resistance values being practically identical. Sadly, this seems to be the only thing between the RAM chip’s data bus and the card edge connector. Continuity seems fine on all pins.

Is it entirely possible that one of the RAM chips has an open short and is pulling the line high? How would i go about testing this without desoldering every chip off the board? I fear my soldering skills (and tools) aren’t up to the skill level required for such an endeavor, and i’m really paranoid about killing something that’s practically irreplaceable. One chip is probably fine, but a whole board is just asking for trouble.

I hope someone here can shine some light on the matter. I’m really flying from the seat of my pants on this one, and thoroughly understand i’m way out of my depth! It’s entirely possible there’s something i’ve overlooked.

For reference, this is the offending board: https://i.imgur.com/G2zMw7t.jpeg https://i.imgur.com/rrHvl4C.jpeg

The data lines on the card edge connecter start from the 3rd finger from the top, and carry on down 8 fingers. It’s a dual sided board, no layers here. 

Thanks, 

Josh Rice