IBM XT bad memory diagnostic cheat

[Dagwood]

I few weeks ago I was tinkering on a 5160 that would only count to 64K even though it had 256K onboard + 256K on a card. I have a cheap thermal camera so I took a peek and... viola, it stood out like a sore thumb. I swapped the chip and it went to 256k like champ. (see attached pic, center'ish)

The same trick didn't work on the memory expansion card though. I was able to see different blocks of memory were at different temperatures though and upon closer inspection they were different brands. I was able to determine which was the first bank on the card and based on the on-screen error determined it was the parity bit. I swapped the first and last chips and sure enough the error moved, so I replaced the offending chip and bingo... 512K.

I later tested the two chips (outside of a computer, so maybe not a great test) and the one from the mainboard failed, while the one from the expansion card passed... then they both went into the trash bin.

 

[SpidersWeb]

Nice work.

I just run my finger on top of each after running for a while and see if any are hotter than the others.
However it only works sometimes, completely depends on how the DRAM has failed. But is a nice check to see if you get lucky

 

[Flamin Joe]

I few weeks ago I was tinkering on a 5160 that would only count to 64K even though it had 256K onboard + 256K on a card. I have a cheap thermal camera so I took a peek and... viola, it stood out like a sore thumb. I swapped the chip and it went to 256k like champ. (see attached pic, center'ish)

The same trick didn't work on the memory expansion card though. I was able to see different blocks of memory were at different temperatures though and upon closer inspection they were different brands. I was able to determine which was the first bank on the card and based on the on-screen error determined it was the parity bit. I swapped the first and last chips and sure enough the error moved, so I replaced the offending chip and bingo... 512K.

I later tested the two chips (outside of a computer, so maybe not a great test) and the one from the mainboard failed, while the one from the expansion card passed... then they both went into the trash bin.
View attachment 57121

Great work.

Out of interest, which thermal camera are you using? I've been interested in a while in getting one myself for the same purpose but wasn't sure which to buy.

 

[Dagwood]

It's the 'Seek Thermal Reveal' which is the self contained unit vs. the one that attaches to a smartphone. It was $250 about 3 years ago but evidently the price has gone up a bit. I bought it to find sources of energy loss at my home and office and it's more than paid for itself in just that task. Since then I have found it super useful in problem solving and/or detecting many things (including an ear infection in the cat when half it's head was glowing hotter than the rest).

It's low resolution but perfectly adequate for home and hobby use. I got the standalone version because I figure it should always work as long as it can be powered whereas the model that plugs into a smartphone is fragile both physically (tiny USB connection) and 'mentally' (has dependencies on apps which have dependencies on operating system versions which means someday it won't work anymore when the manufacturer quits caring).

 

[modem7]

... (including an ear infection in the cat when half it's head was glowing hotter than the rest).

I panicked when I thermally observed the same with my cat, Jenny, but the cause turned out to be that she is very analytical/logical, i.e. very left brained.

 

[Dagwood]

I panicked when I thermally observed the same with my cat, Jenny, but the cause turned out to be that she is very analytical/logical, i.e. very left brained.

In the words of Jerry Reed, "When you're hot you're hot!"