IBM 5150 64-256k Motherboard Ram Test Failure.

[dabone]

So I'm working on a 5150 this morning, and when I started it had a short on cap c24 (+12v), So I've removed this and was able to power up the unit.

With the original bios installed, I only get a black screen, so I put a Supersoft Diag rom in u33.

The rom reported "16K Critical Memory Region" Failed, and indicated bit 7. So I removed and replaced bit 7 dram in bank 0.

I've also removed all the other ram chips and setup the dip switches for 64k only.

Replacing bit 7 ic still showed the same bit 7 error. Ram is know good, but I tried 3 others just to make sure. Soldering is fine and triple checked.

I then decided to remove bit 0 from the bank to see what happened... And the test still shows bit 7 bad. No other bits show failing..
Ok, then I removed the parity bit... Still bit 7 only marked as bad.

Memory starts showing failed tests at 00000.

Any suggestions?

 

[dabone]

Finally found the correct schematics, and it ended up being a broken D7 trace between U12 and U13.

Doh... Well, at least I didn't hurt anything adding the 3 sockets..

 

[modem7]

Good work.

I added the failure to the list at [here].

 

[dabone]

Thank you sir, your site made reviving it possible.