I am sure this has been answered somewhere, but a preliminary search didn't turn up anything definitive.
I have a situation where I cannot write disks. Disk reads are just fine.
For example, it goes through the motions to format (DOS 3.3 INIT HELLO) but nothing is written.
I have narrowed it down to the disk controller card. Switching that out has solved the problem but what about the Disk II controller might be bad?
Scope the disk port output pins 10 (/WRREQ) and 18 (WRDATA). A failed 74LS05 B2 might hold WRREQ' high or a an invalid level, and a failed 74LS174 A3 might hold WRDATA at a stuck or invalid level.
You say that the disk isn't affected by an attempted INIT, so it's probably not a bad sequencer ROM B3, but that could also cause writes to fail if its pin 14 output was stuck or the ROM contents corrupt.
Just to confirm the card you swapped, when you say "Disk II controller card" you're referring to the card which goes in the II and not the card inside the drive which is aka the disk II analog card?
Correct. Not the analog board
Got it. It was a bad 74LS05 at B2. Works fine now. Thanks, robespierre
I recently repaired a disk II controller with the same symtoms. In my case it was the 74LS05.
Good to hear you got it with an easy fix! I don't think that's a common failure point, although stivo says otherwise, may just be age but that would also be unusual.I did a quick check for any other wiring cause (connecting ribbon cable wrong) and appears the off by one errror could potentially back-feed +/-12V through the 74125 (analog board) depending on how that part fails. Usually it's just 125 which is destroyed. And since all pins of the 05 inverter are internal to the board with only pins 2 and 4 externally exposed, that could make sense. Do you know if it was just that pin 4 which was damaged? Or wanna send it to me to check? (Note: I'm sometimes too curious)
Pretty sure that's all it was - the card was fully functional otherwise.
I have been troubleshooting other issues with this machine's video output (another thread pending), and as part of the process was disk controller card swapping because I know that it commands various voltages from the motherboard invlusing +12V)
I got this card from my storage bin of peripheral cards. I have no idea what that card's history is or where it came from. I don't think I misconnected it - it may have been in the storage bin so long that it got to me that way when I bought it, probably as part of a lot, or maybe you're right, just part old-age.