Nice! I have a set of springs on order so hopefully one of them will work and let me start looking into this again.
Best of luck!
I eventually reached a situation that I was happy with. Several further learnings;
1 - I recommend arranging a longer cable, for example an external drive cable, to avoid the EMI interference problem (from the Mac’s screen) while making adjustments with the drive’s top cover removed. I used a 20-pin ribbon cable from the LEGO Interface A!
2 - This 400K drive I’m working on is an early one - March 1984 on the side sticker - and seems to have one annoying characteristic compared to later drives: it does not re-seek Track 0 by checking the sensor when inserting or formatting a disk. So, after adjusting the Track 0 sensor, the power has to be turned off and on for the adjustment to take effect. This really slows you down! Check that the drive will format a disk and adjust the track 0 sensor until it does. Also, adjust the sensor to obtain a Happy Mac icon when attempting to start from a startup disk. Sometimes, a working startup disk will be damaged by a wrongly-set track 0 sensor, so it is necessary to keep testing the startup disk in another drive.
3 - Turning the head stepper motor *does* have an effect while the drive is on (no power cycle required). This makes small changes to the head position, and also helps it move around track 0 in the startup process. If the heads don’t move at all before the disk is ejected, the stepper motor position is probably wrong.
4 - I then found the 400K drive could format a disk, copy a startup disk from the second drive (800K external), start up from that disk, and - here’s the punchline - the 800K external drive could start up from the disk made in the 400K drive. Yet the reverse was not true; the 400K drive would not start up with a disk made in any other drive.
5 - Eventually, after further adjustment of the head stepper motor, I managed to format a 400K disk in a 1.4MB drive and copy a disk image to it (in my Powerbook 540c), and this disk starts up the 400K drive, and also works in the 800K drive. A disk made in the 400K drive starts up the 800K drive (and can be read in the 1.4MB drive). Yet, a disk made in the 800K drive still won’t start up the 400K drive.
I give up at this point
At least I can make disks for the 400K drive in several 1.4MB drives, such as my Powerbook 100 external drive.
Adjusting the head gain potentiometer and attempting to measure the results with an oscilloscope did not produce any meaningful result, incidentally. And overall, the Sony Test program was no help at all in adjustment steps 2 and 5
Here is another interesting screenshot: the ‘failed disk’ icon that appears with System 7.0 (1983!) when an attempt is made to start the 400K drive with the Sony Test disk. I’d never seen this before and hope never to see it again