Sn63Pb39 - something is wrong with this marking ;) It is a mass percentage composition of an alloy, but this one sums up to 102% :lol: I always used solder wires by Cynel Unipress: leaded Sn60Pb40 (60% tin, 40% lead), lead-free Sn99Cu0,7Ag0,3 (99% tin, 0,7% copper, 0,3% silver). I also have some sol...

I believe 1st and 3rd are the same, but differently broken. Only the 2nd one is different, possibly some early unit. For sure all STf and STe had the same keycaps (but there were at least 2 slightly different keyboards there).

It rather is an RPD-60A... no need for 3.3V in the ST.

He wrote 68Ohm, so 68R - all fine.

Make sure you haven't damaged any traces between chips. Which exactly upgrade were you trying to install?

BTW, I still say that it is enough to cut one resistor to safely disable the on-board RAM on the ST.

AT-Once has only DIP socket, while STe has PLCC CPU. On that auction link you gave in the 1st post, there is a MegaBus adapter, but if you look closely at it, you will notice, that the PCB is universal and can be either MegaBus or PLCC adapter: https://obrazki.elektroda.pl/9253080700_1708177526_thum...

In STf there is no option to keep the shielding in place... at least not without cutting a big hole in it.

My card has a Harris-branded 80C286 CPU in it. It is a CMOS/low-power version.

There were adapters for MegaBus and PLCC socket.

That's strange as 286s or even most 386s used in PCs never required heatsinks. I have AT-Once in my 1040 STf machine and it works without any heatsink. To be more precise I have this AT-Once+ replica: http://atari.myftp.org/atari16bit/atonce/atonce.html I know the Author had serious difficulties wit...

I'm guessing you're aiming at something like this, but with an automated solder feed via the modified extruder:

ICs from different manufacturers may have slightly different pin profiles, thus a new CPU may not have good contact with a socked in which another CPU has been sitting for the last 30+ years. When swapping a CPU for another, you must use Deoxid or a similar product on the socket, especially because ...