John, as i wrote all R/W tests with a formatted disk are successful.
Josh, after testing around the last two days, i have tried almost everything (also switching off/on).
The drives and the cable are known good.
And, as i write before, mid yesterday the format function works perfect.
Today morning i poking around with the scope, to find something.
I notice, that all Z80 data has little peaks (about 1V) when they should be low.
Also i changed the clock for the Z80 to 4MHz. No different.
I changed the PIOs, the SRAM, the EEPROM and the WD2793. No different.
Then i change all TTLs on the board. No different.
The only unchanged active part is now the Z80 (ST Z84C00886 / Z80BCPU, same as on the Z80 CPU board).
I only found a crusty Mostek MK3880N-4IRL in my boxes. i give him a try. And the formatting works at once.
The peaks on the data lines are now gone.
To secure, that is is the Z80, that is not working, i changed it back and formatting again is not working.
I changed the two ST CPU i have between the Z80 CPU board and the ZFDC board.
The Z80 CPU board works fine, the ZFDC is now completely dead.
Changing back the Mostek Z80 back to the ZFDC board, all is working properly.
Has anyone an for explanation for this?