I bought a RTO2014 the other day. Beautiful scope, but it has RAM artifacts. The seller made it right, but now I have to go and actually fix it
My first step was to identify the RAM chip with the artifact, and I did this by cutting data lines and observing where the errors showed up (and then repairing the cuts, of course). I didn't have especially good luck in guessing which chip was responsible, but the silver lining is that I now have a mostly complete picture of how data is deposited in the RAM. Even better, I have a procedure to share with the world so that the next person to have this problem can simply drop some cursors on the waveform artifacts and immediately know which chip is faulty.
The architecture is that 4 10GS/s channels feed two ASICs and each ASIC farms the samples from its channel pair out to 14 W9751G6KB SDRAM chips. This will be different if you have option B110 extended memory, because the math doesn't work out for B110 to use the same chips. Each chip has 16 data lines and every 100ps one of the DRAM chips will receive one 8 bit sample from ch1 and one 8bit sample from ch2. The chips will be used in sequence until, 14 samples later, the first DRAM chip receives its second pair of samples. The especially good news is that this pattern is stable with respect to the timebase: if you hit PRESET and then activate Channel 2 (Firmware 5.35.1.0), you can simply drop a cursor on the first artifact past (or on) the origin, measure the picoseconds, and look up the chip on the image I attached. Be sure to use the BLUE numbers; the yellow numbers are my own labeling scheme relative to a chip I arbitrarily chose as 0 and are offset 200ps from the BLUE numbers, which are the ones that correspond to "picoseconds past the origin".
Replacement chips are on order and while they slowly make their way to me I have begun to practice BGA rework. I've done a few one-offs on smaller chips, but this will be my first chip with this many balls on this thick of a board. My first practice attempts were on the cheapest DDR4 SODIMMs I could find, because the LED testers were available on Amazon. Those went well! I think I have the hang of reballing them now, and my success rate is 100% on 4 chips out of 4. My second set of practice attempts were on a chunky but obsolete graphics card and those didn't go nearly so well. My 450W preheat plate was not up to the task. I had to crank the temperature setting very high just to get one chip off, let alone remounted. The thermocouple I had on the back side was always much lower than the setpoint, even if I insulated with aluminum foil. I have an 850W station on order, hopefully that works better. In the meantime, here are some pictures and that timing diagram I was talking about!
EDIT: compression did a number on the blue numbers, here is a higher res version. Note that the dropbox preview is also compressed, download the PDF for full resolution.
https://www.dropbox.com/scl/fi/1roh0w3hl585cxuaygs77/DRAM.pdf?rlkey=7vpw6q7kcfpk3d3kpr7usy2qq&st=clrch9hl&dl=0EDIT2: diagnostic procedure in video form. If you want the repair story, look for that video a few posts down.
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