I know that there are a few other threads on 1050 disk drive repair, so I hope I am forgiven for starting a new one. Hopefully some of my techniques might help others.
Back in November a bunch of Atari 8 bit stuff was put on Ebay UK by a seller - "ginger_colin". It was all "buy it now" and most items were "for parts or not working", even though for the computer systems he showed the computer as operational in the pictures. Someone grabbed a bargain Atari 800 for less than £100. A number of items sounded like they needed a clean. I bought a 1050 for £30, then a day or so later bought a second for £35. I bought the first to use the case for an Atari related project which I started back in 2000, but then the second just seemed too good a bargain to pass up. The second drive was badly protected (hardly any bubble wrap) and when I took the drive apart it was clear that it had taken quite a hit during delivery. I powered up drive #1, but it was completely dead. I powered up drive #2 (the damaged one) and switching on and off several times, it operated the drive once only. So I knew that I had a repair project on my hands.
I took both drives apart and cleaned the cases (using JIF and soap) and used a brush to get rid of fine black "dust". I suspect that the original owner was a smoker. Drive #2 had broken and cracked support pillars. The switch was bent up - quite a feat if you know the size of the 3 pins of this switch, as was the LED. The LED bent back, but the switch didn't. The tab to open the drive was completely stuck, as it was clear that the drive had been hit hard on the front. The brown front plastic surround was cracked and pieces broken off in the bottom left corner. Much later I also noticed that the front of the mech was also cracked. To his credit the seller refunded the money for drive #2 without me even asking. He was also selling a bare mechanism, and having communicated with him it sounded like this was due to him having to replace another drive (for a different buyer) which must also have been broken in the post (I assume that he must have had to replace the plastic case leaving him with a spare mech).
I removed both mechs and used a multimeter and scope to check the power supplies. Both drives checked out OK and the 5V looked very clean. The 6507 clocks for both drives were good and RSTB was working correctly. What surprised me was that for both drives the address lines were not toggling. This puzzled me as if the firmware was not seeing the mech I would have thought that the code would end up in a tight "error" loop so giving some 6507 address line toggling. I surmised that the CPUs have executed some bad opcodes which crashed the CPUs, possibly due to back RAM used for the stack. So I then started a more thorough examination.
I will post more later, so those that are not interested do not have to wade through a long initial posting.