Don't use cleaning disks on 400k or other single-sided drives or you'll damage/dislodge the pressure pad and render the drive unusable. The pressure pad is a little felt pad that, on the 400k Sony drives, is on the arm on the top of the drive directly above the head. This pad presses down on the disk surface to ensure full contact with the head. If this pad is missing, it will not generate enough pressure to properly read the disk and may scrape up the top of the disk causing a lot of debris to be generated. Cleaning these drives requires disassembly and manual cleaning of the head with a lint-free swab and special cleaning fluid, but you can get away with carefully using a cotton swab and 90% iso alcohol. 70% iso also works in a pinch. You'll also want to check that the pressure pad is still properly attached. If it isn't it may be in the drive somewhere.
Even though your disk media may not look bad, it may be heavily shedding particles and/or binder material all over the heads of your drives. I probably wouldn't try using it again.
I took the keyboard apart and found an interesting hack. The '*' key's stem was broken off, the keycap sitting in the broken switch with cleverly bent pins glued to the keycap keeping it there, a spring making the key feel like it worked but of course it did not! I did not have a spare salmon Alps switch but I did have a spare white one so easily fixed.
I designed and printed a plastic mount for the 5G SCSI HD I had in a drawer. The original drive works, the big drive will be faster. I am not sure if what came with the machine is original or not but the two mounting holes do not line up properly with the holes in the HD at all. The steel has a different look to it than all the other steel parts inside. A number stamped on the bottom is "805-5066-08". Right number, but ugly.
Also 3D printed a programmer's switch. They are not that expensive but shipping anything from USA to Australia right now, unless it is by expensive courier, takes about 10 weeks.
Then the Mouser order arrived and I replaced every electrolytic cap on the logic board, analog board and in the Astec power supply. Mac is happy, clean and the CRT display is rock solid.
Next I went to plug in my monitor to see just what the Radius card can do on a 21" CRT with my new HD15-15 adapter... and found that I bought the wrong adapter. I got HD15M to 15F, not HF15F to 15M. So if I ever need to plug an old Mac monitor into an old PC I'm sorted, but now I have to wait for my new, new HD15-15 adapter.