8050 which will write but not format

Gents,

I would not normally post 'please help me fix' type questions here but 
this one has me stumped. Some may already have seen the thread on 
vintage-computer forums here:

http://www.vintage-computer.com/vcforum/showthread.php?34572-8050-Floppy-Repair

In a nutshell, drive 1 works 100%, drive 0 will read and write but not 
format. It usually stops on track 1 with a #21 read error but 
occasionally will go as far as track 3 or 4. If I swap the mechanisms, 
the problem stays with drive1. This implies a problem in the analogue 
board (micropolis pcb #8050006) but I can't find it and I don't 
understand what kind of problem would allow it to write but not format. 
I know the mechanisms are good and well aligned and, as the drives read 
OK, they are properly speed adjusted.

Do any of you disk wizards know the algorithms by which these drives 
format a disk? I hoped this might give me a clue.

Rob

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