Just a quick post: today I've succeeded in repairing one of my SFD-1001s. Now I have two working units. :)
Last time I wrote that I found out that it has a defective 6502. MMSZoli just gave me a 1541C with probably a power supply fault. It responds (responded) to serial bus but disk operations failed. Even the power up cycle failed to spin the disk. So I have my first IC desoldering from this unit. I took off the 6502 and plugged it into the socket of the SFD. It just works fine. :)