800XLF rogue Samsung RAM

[TZJB]

I have been going through my Atari collection to see what still works. This 800XLF was known to be faulty, but not known to be a 800XLF, and was sitting in my garage until recently. I was therefore not surprised when it still refused to boot at all. I tried Shoestring's ROM/RAM tester blown into a 27C128 EPROM after socketing the Atari OS ROM and at least that gave me a frozen screen proving some main chips were working, but I was still doubting it could be the DRAM as it was Samsung manufactured and not Micron Technology as is usually the case with failed DRAM.

 

To that end I swapped Freddie, Sally and PIA before I measured the voltage at the DRAM which was lower than I would have expected at 4.81 Volts with a new Meanwell 5V 3A supply. Not having another tester to hand I tried the drop of isopropanol on each DRAM chip trick, to see which one might be faulty by observing the rate of evaporation. It was 6 of the 8 DRAM chips that turned out to be faulty! I replaced the DRAM and reworked the extra RAS pull-up resistor so that it's underneath the board now, together with the TFHH mod of an additional 100nF capacitor across the Freddie chip supply.

 

I attach pictures:-

 

 

Edited July 10, 2022 by TZJB
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