I also have a Corcomp fdc story, I have 3 corcomp fdc's(2 are mine and 1 is a friends) with the 2793 chips. I purchaced both about 5-6 yrs ago
and both controllers didn't work correctly, they had inconsistant/flaky operation, error 16, 'not initialized' messages. Both would not
read/write/format DD disks. They would sometimes read/write SD disks but were inconsistant. I tried to repair one of them about 3 yrs ago by cleaning terminals,
testing voltages, replacing the ele. capacitors, swapping the socketed chips...with no improvement. Then just put them aside and moved on to other projects.
Reading this topic got me thinking about these cards again, so I broke one out. Started testing again and adjusting the 50k and 10k var. resistors
again with no improvement, so I reset them back. I then went to the var. capacitor and adjusted it with no improvement again. I looked very closely
and saw slight green funk in\under the capacitor, on the schematic it shows a fixed 10pf & var. 4-60pf capacitors on that circuit for a total of 14~74pf.
Not having any variable capacitors, I just clipped on a 22pf cap(across either capacitor) for a total of 32pf and the card came to life. It formatted/reads/writes DD and SD
disks with '0' failures. I let it run for about a week cycling on and off, tested with diff. drives and disks again with '0' failures. So I broke out
my 2nd controller, removed the cover and simply clipped on the same 22pf cap. and turned it on...0(zero) failures! I tested it like the other one for about 3 days
reading, writing, formatting with no failures.
So I got a hold of the 3rd card and we did the same thing, clipped on the 22pf cap. On first test it was a little lazy and said 'not initialized' once,
then after about 30 seconds it began to read/write correctly. I swapped it with a 27pf cap. and it began to work flawlessly, I kept the card for about
a week testing cold startup and long runs, '0' failures ever since. I ordered some variable caps(5-50pf) and replaced them now all 3 cards work.
Maybe this will help someone else.