I’ve been trying to fix this problem for over a week, and I cannot find a solution. I've been told that you guys know these things inside and out so I was hoping you could help me. I absent-mindedly tried plugging an external drive to my IIc while it was on and running (more details here), and now the drives won’t read or write. Here are the facts:
- The computer runs entirely fine, except for disk things. I can use the monitor, BASIC, and bootstrap ADTpro and they all work fine.
- When trying to read, the head twitches back and forth very quickly (making the signature clicking noise), but it doesn’t move to the beginning as I believe it’s supposed to (this goes for both drives)
- I’ve tried both drives in both bays and vice versa, which leads me to believe that the problem is possibly on the main board and not with the drives
- I probed the IWM with a voltmeter (I won’t have access to an oscilloscope until after I go back to school), and at startup pins 2, 27, and 28 all toggle on and off while pin 1 does not (these pins are the stepper motor pins control pins). I don’t know if this is what is supposed to happen, but I found it odd that one pin didn’t toggle while the other three did.
- I’ve done the RAM test using closed-apple open-apple control reset and everything appears to be fine there.
- I do not recall mis-inserting the 20-pin bus to the internal drive so I do not think the drive's 74LS125 is busted, especially since both drives are acting the same (unless my initial error blew both of them simultaneously.
I think my only option now is to wait until I get back to school and then start probing components with the oscope until something looks wrong.