Hello again!
I checked today the voltages. A proper +5V and +12V voltages is coming from the PSU. On the FDD power plug is a voltage of 4.7V without FDD connected. When I connect the FDD it's 4.6V.
I also recapped the FDD today and at the first test I got the same result as already mentioned. The LED on the FDD and the Keyboard lights up for approx 2-3 seconds and only the platter is permanently spinning. So I disconnected the FDD and traced a bit of contact spray in the floppy cable connector. After that the FDD is now recognized again. But now the same issue: It doesn't read floppys any more and I have no clue why.
Here a video of it (stored on my private cloud): https://cloud.jotoma.de/f/8269f19c37ab4ec2bebc/
I know the best thing is to try another FDD but I have only a 1.4MB FDD here. Exxos sells a Diagnostic Cart but I don't know if I should buy this first or a new used FDD. And I also know that there are some alternatives but I want to keep the floppy feeling. What do you recommend me?
Ah btw. I have schematics of my 1040 STFM.