Long post with many pictures. Get ready and hang on!
I spent last night opening up the STm and testing it. On the outside the machine is generally very clean and in good condition. I don't see any cracks, dings, or damage. Some of the keys are yellowed and some are not. Under proper light it almost looks like a checkerboard. I think someone may have swapped keys in and out.
The top of the motherboard is generally clean as well. I see a few jumper wires coming out of RF shield boxes. None of the chips popped out of their sockets but I did press down gently to re-seat them. Almost all of them "clicked" into the socket by a small distance. The bottom of the motherboard is a completely different story. Very messy with lots of dirty rework. I know Atari performed official rework on finished products to fix bugs and to complete circuits but this looks sloppy. I see flux on many pins and globs of glue. Is this normal? I can't imagine Atari authorized this work. The rework in my STe machines is very clean compared to this. Maybe I got a "Monday Morning" computer and the technician was recovering from a strong hangover? Underneath the RF shield is not better.
I powered up the motherboard without a keyboard and floppy using monochrome cable. It fired up almost instantly and I saw desktop minus icons. No funny pixel glitches or bombs. Hooray!!
I powered down and added mouse. Also good result! LIke before but now I have an arrow to move!
Next I try add my external floppy drive. This was surprising. I had a hard time inserting the cable. In order to do I had to support the connector from behind with left hand and push the cable in with my right hand. It went in but with difficulty. Maybe the connector is dirty? I powered on the motherboard and now I see icons. Floppies were unreadable. Power down, remove cable, reinsert cable, power up. Now I can read floppies! Hooray again! I may have been battling two issues : poorly seated cable and dirty connector. I can format and write to disks on my SF314 + STm and my STe can read them. Likewise, the STe can write to these disks and STm can read them back. So, maybe no major incompatibility issues exists here.
Finally I add keyboard. Now behavior becomes strange. Pressing certain keys would cause the computer to hang, usually keys in upper left corner. I think the keyboard may be shorting power lines and causing glitches. Perhaps the keyboard is flexing? I need to investigate more.
I ran SYSINFO and played a demo. Everything looks OK so far. I also ran YAART for more than one hour and no errors appeared.