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AST Six Pak Plus - 5150 won't post

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First post, by polishvito

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Wondering if anyone can help me with this one:

Working on my IBM 5150 (Rev B). I got this AST Six Pak Plus in a box of random parts and I wanted to try using it for this machine. Unfortunately when this card is placed in the machine, it won't post. The power supply, hard drive still power on/spin up so the power supply is not going into protection mode. I've tried in multiple slots (and can verify other cards work in those slots). I removed all the ram, ran them through a tester (only one bad chip surprisingly), deoxit in all sockets, cleaned connector etc. I've verified that the switch settings are correct on the card and on my 5150 (256k on the motherboard), but even if it was a matter of bad switch settings or bad ram, I don't understand why the machine would not even post.

I have a different ram card that adds 256k and when that is in the 5150 it works fine. I want my 640k though!!

I also disabled the serial/parallel ports by removing the jumpers on the card, and even with no other cards in the machine, it still won't post

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Reply 1 of 5, by BitWrangler

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Did you try disabling the top two banks to see if it works at 256/512 like the other board does... or tried it with only one bank?

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

Reply 2 of 5, by the3dfxdude

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Given the fans and hard disk spin up, but you have no beeps (correct?), doesn't rule out a problem with the card where a particular rail may be shorted. I'd use your ohm meter and check for shorts on all the rails through the card edge connector.

https://www.minuszerodegrees.net/5150_5160/MD … diag_config.htm
Minus zero degrees speaks of how to check the motherboard without any cards to make sure it is fine. I believe if you go through the reading based on your symptoms with the card installed, you would be getting to the point of checking for shorts on the rails. So this is why I am suggesting to go to checking the card itself, as you say the motherboard is working without the card.

Reply 3 of 5, by polishvito

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BitWrangler wrote on 2024-03-12, 15:48:

Did you try disabling the top two banks to see if it works at 256/512 like the other board does... or tried it with only one bank?

Yes tried with no difference. Even disabled all banks on the Six pak card with same results

Reply 4 of 5, by polishvito

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the3dfxdude wrote on 2024-03-12, 15:48:

Given the fans and hard disk spin up, but you have no beeps (correct?), doesn't rule out a problem with the card where a particular rail may be shorted. I'd use your ohm meter and check for shorts on all the rails through the card edge connector.

https://www.minuszerodegrees.net/5150_5160/MD … diag_config.htm
Minus zero degrees speaks of how to check the motherboard without any cards to make sure it is fine. I believe if you go through the reading based on your symptoms with the card installed, you would be getting to the point of checking for shorts on the rails. So this is why I am suggesting to go to checking the card itself, as you say the motherboard is working without the card.

Checked all of the voltage rails on the ISA slots with the card installed - all within range.

Reply 5 of 5, by polishvito

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Okay so here is an interesting twist: While I was playing around with the card, checking voltage rails and such, I felt the card after it had been running for a couple minutes - One chip stood out as being a little warmer than the others - A "SN74S280N" which in the picture is the chip directly next to the first bank of ram on the bottom of the card.

The chip is not hot by any means. But it was warm - most of the other chips remained cold, including all the ram.

Looking this IC up - https://www.alldatasheet.com/datasheet-pdf/pd … /SN74S280N.html

It is an "9-BIT ODD/EVEN PARITY GENERATORS/CHECKERS". Now I won't pretend to have any idea what that actually means, but it got me thinking. On the SixPakPlus switch block, position 8 according to the manual is for "Parity Check Enable". I had it set to "on". When I switch it to "off" - The computer posts. Now it does not work, I almost immediately get memory and Parity Check errors, but the system does actually boot my XTIDE - it gives stack overflow and crashes when I try to do anything, but its still a sign of life.

So I guess I'm wondering, and I know I'm grasping at straws - if that SN74S280N is bad, could that be enough to take down the entire system?

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