Bad memory on a 64K 400

[ACML]

I'm not an electrical engineer, but have a question. I have a 48K 400 that uses eight 4164 DRAM chips and shows memory failures at the hex 6000 block. I assume this means only one part of one chip has gone bad. Am I wrong in assuming if one whole chip was bad, all 64K would fail since its constitutes one bit of every byte?

 

 

[Joey Z]

I'm not an electrical engineer, but have a question. I have a 48K 400 that uses eight 4164 DRAM chips and shows memory failures at the hex 6000 block. I assume this means only one part of one chip has gone bad. Am I wrong in assuming if one whole chip was bad, all 64K would fail since its constitutes one bit of every byte?

 

 

You are correct. If a whole chip failed, you'd have all RAM appear as bad.

[Rybags]

Failure in just one area of memory could also mean an address or selection failure.

 

A decent memory test program though would tell you if failures were at specific bits or entire failures at certain address ranges.

[tjlazer]

How does an Atari 400 have 64k of RAM memory??!?

[Joey Z]

How does an Atari 400 have 64k of RAM memory??!?

Well, you have 64K chips in it, and only use 48K of them. That's how all the Atari official '48K' cards were done. With a 2-chip mod, you can actually make the memory scheme XL compatible somewhat (you don't get self test, and as such have to use the 1200XL OSROM). I have done this, but it's a real mess to do. In theory I developed circuitry to also implement self-test with 3 chips, and also additional circuitry to do XE style banking with an additional SRAM. But given how much of a mess it was just doing 64K, I never actually did it.

[Kyle22]

Did you remove and replace the RAM card to clean the contacts? While you're at it, push down on all the chips that are in sockets. Sometimes they work loose over time.

[ACML]

Did you remove and replace the RAM card to clean the contacts? While you're at it, push down on all the chips that are in sockets. Sometimes they work loose over time.

Have not disassembled to try that yet.

[Kyle22]

Have not disassembled to try that yet.

Sometimes it helps.

[ClausB]

It's not that they "work loose," it's that they corrode over time. So pry the chips part way out of the sockets and then push them in, to scrape off corrosion.

[Kyle22]

Yeah, I know. It is just easier to say "work loose" than to explain temperature cycles causing them to lift slightly, and oxidation on the pins and sockets. Usually, 30 year old chips in sockets have lifted enough to be noticeable when you push down on them. This also wipes the pin and improves contact.

 

I agree it is better to lift them first, but that increases the chance of pin damage, and for the average non-technical user, I felt it safer to tell him to "push down".