I have 4 Disk II drives, but only one of them is working. I am convinced that one of the 4 is a clone, as it has a different analog board with no Apple part number on it and has asian characters on a label on the bottom. I am attempting to get at least one other working drive as I intend to sell one of my Apple II systems and want it to come with at least one good drive.
I am focusing on one of the drives at the moment, and have done some various testing with my osciliscope and the Apple Disk Alignment Aid floppy. I am a noob when it comes to using a scope, but luckily the Disk II service manual has settings for the scope in it to make it easier. So far the only issue I have for sure identified is that the amplitude is slightly under the 150 mA the manual says is the minimum. It's so close though it could be that my scope is probably somewhere around 30 years old and hasn't been calibrated in who knows how long. I cannot get the Azimuth tests to show up correctly on my scope, but I think that is an issue with my trigger control being out of whack. Also, the drive speed test on the floppy never shows the drive's speed. I don't know if this is because it is not reading the disk or not. So, I used a strobe app on my phone to adjust the speed using the wheel on the bottom. Still no dice on reading disks.
Anyways, I had the genius idea of swapping the analog board from the working drive into the faulty drive. This gives me the same issue of the drive not reading disks. So then I took the unkown condition analog board and installed it in the working drive, and wouldn't ya know, it works.
So, what test or procedure should I be looking at next to see what the issue is with the mechanism on the non-working drive? Is it possible I just have a bad read/write head? I have cleaned it multiple times already.
Thanks for any comments or expertise you send my way!
You already narrowed it down quite a bit.
Double check that your method of adjusting the speed was correct. Use the working drive to check your approach and the app you are using (without messing with the trimmer of the working drive, of course). And if you played around with the trimmer too much, you might be off by more than 1/50th or a 1/60th, so the stripes look stationary again - even if the speed was completely off. An incorrect speed is the most common issue I observed. You can do the fine tuning using one of the Apple II disk utilities - but you first have to get the speed about right, before these even start working.
Otherwise, you already ruled out the analog board by swapping it. So use your oscilloscope to check the digitial read output signal of the drive. When you have inserted a known-good disk, you should see the data output signal becoming alive as soon as the disk spins. And the data signal should toggle, even if the speed wasn't correct. If the signal was still dead when the drive spins, then you have an issue with your read head (dead, completely misaligned, wires broken etc).
Do you know where I would attach the probes from my scope to see the digital read output signal? And what voltage and time settings will show it? Thanks for your help!
The alignment test on the APTEST disk gives me all zeros on that drive, which leads me to believe that the head isn't reading anything. The disk speed test on that disk also never shows a speed value.
Also, with the working drive plugged in and a bootable disk in it, the 2 drives I haven't really dived into yet prevent the machine from booting! It just sits there with the Apple II logo on the screen. If I try with the drive I am currently working on and the known good drive in port 1, the machine boots fine. Any idea what that might be?
It's always good to have schematics handy when working on such things:
https://mirrors.apple2.org.za/Apple%20II%20Documentation%20Project/Peripherals/Disk%20Drives/Apple%20Disk%20II/Schematics/
The schematics show the Apple engineers conveniently added testpoints: TP1 is the drive's digital output signal (RD). TP3 is ground. These were added for exactly that kind of analysis you're currently trying to do. Pretty neat, right? :)
Connect the signal and ground probe, and set your oscilloscope to something like 2V/DIV. The time setting doesn't matter, you just want to find out if the signal is completely dead or if it's alive. Start with something slow. If there is a signal, you can try faster settings.
Possibly, but not guaranteed. The test programs do not have access to the raw data bits returned by the drive. They can only get the output as decoded by the "Woz machine" on the Disk II interface card. This statemachine requires a valid signal (from a properly formatted disk) to work.
That's normal. If the disk II card is plugged, the Apple II will just sit and try to spin the disk forever, until it detect a valid disk - or you press CTRL-RESET to abort. There is no timeout.
This is with a known good disk in a known good drive on port 1 though. Wiith either of the 2 drives I haven't dived into yet plugged into port 2 the computer will not boot from the good disk in good drive 1. It boots fine however with the drive I am currently trying to fix plugged into port 2.
Thanks for the test point location. I will try it this evening and see what happens.