No replies after 24 hours indicates an "I don't know" scenario, so I decided to search the forum for an hour and found this 2016 comment by Uniserver. It was his personal opinion that all the Apple logo RAM chips were just waiting to die and he didn't feel that selective replacement was a good idea, opting instead to replace them all. Yet later in that thread I see one person who placed one chip successfully. Whether that Mac is still running now in 2020 is unknown though.
It also isn't clear what the speed of the original 128K chips was. Some people say 200ns and others 150ns. The JAMECO RAM chips are mentioned as being 150ns, which indicates they must be the Siemens HYB 4164-2 chips mentioned in the datasheet they provide. But if we compare the A.C. Characteristics on page 6-16 of that Siemens Datasheet to the same A.C Characteristics on page 1-4 of the Micron MT4264 datasheet, we see speed differences even between the 150ns chips...
(The Micron MT4264 (-15 or -20) were the stock chips used by Apple in the 128K motherboards.)
I am going to guess that the speed differences between the HYB 4164-2 & MT4264-15 are "within tolerance" of whatever the Apple 128K motherboard requires, but if anyone knows for a fact, please chime in.
(While it would seem prudent to just swap out all the stock RAM chips for JAMECO's 256x1 DRAM chips so as to get a 512K motherboard, such would also require the addition of a multiplexer. Such is a non-trivial mod.)
My biggest problem at this point is that I do NOT have a Hot Air rework station. I merely have a standard Desoldering Station. Chip removal for multilayered motherboards like these are likely problematic at best for a regular desoldering station, and I would assume that a hot air station is really best to avoid problems on these RAM chips. Have any of you been able to accomplish the job with a regular desoldering station? (Soldering in new chips is easy. I am talking about the desoldering and removal of the bad chips.)
I haven't followed this thread as much as I should/would have wanted to so pardon my ignorance, but wouldn't it be possible to design a small daughterboard with a SOIC attiny85 and a 32.768kHz crystal?
A bit like this:
If all we need is to remove the 10/33pF cap, it's not too much of a problem? Again, I haven't taken a good look at the schematics yet so I may be talking nonsense.
It's been a very long time, but ISTR my TrueVision card w/Xilinx on board came up from DeclROM as a startup screen without a driver, but that's NuBus? Dunno, given built-in monitors, maybe some SE/30 PDS VidCards weren't designed to Apple's spec?
Thank you for taking the time to summarize these findings, the knowledge is invaluable for anyone wanting to tinker and explore an old classic. I look forward to the new topic that you plan on posting sometime in the future. Thank you, much obliged.
I don't know about converting video formats but will mention that some of the video cards use Xilinx FPGAs which are configured by their driver. In other words the FPGA doesn't know what it's doing logically, until the driver loads and downloads a bit stream to the FPGA on the video card.