To see if I can isolate the cause of the XL1201 instability, I am going to physically inspect the two main cards with a loupe and flashlight. If I see anything suspect, I'll take a picture with a (cheap) USB microscope.

I am doing a detailed inspection of the solder side of the Merlin II CPU card first.

Overview picture:

Discrepancies

There are only two that are concerning to me, but several others were found. The two of most concern are:

• Seemingly bad solder joint or problematic solder joint
• Material in a empty socket hole - could it make shorts between the layers? Irrelevant?

Material in a hole of empty socket BB12

Bad-looking solder joint between CC25-26, DD-25-26

Black blemish near CD23

Poorly cut lead near EE25 but seems fine

FF11 - Slightly discolored joint

FF30 - Black stuff near solder joint, seems to be just a bit of fabric?