I had originally purchased a single Disk II during the initial preorder
when a discount of $100.00 was offered. I still have this drive with
a serial number of around 10,000. However, it wouldn't read some new
(old stock) Atari game disks. I learned that the drives can be adjusted
with a test program called aptest. I made the adjustments and cleaned
the heads and the problems have disappeared. This drive also apparently
had it's ribbon cable replaced somewhere along the way, as originally the
first production drives came with multicolored ribbon cables. By watching
ebay carefully I was able to get 2 drives with these older ribbon cables
for only $27, including shipping. It was easy to swap the cables.
I did notice that the original black connectors fit pretty tightly, which
is a possible reason that the original cable was replaced. Too much
tugging when pulling the ribbon cable off the controller card could have
damaged it. It also turns out that one of the two drives I bought on
ebay also had an early Shuggart drive, so I have added that drive to my
system, as well.
I also have PROMs and a number of disks formatted for DOS 3.1. The
PROMs were mounted on a clone Disk II controller card, that had a
slightly different wiring for the PROM sockets than Apple's production controller
cards. For this reason, one chip select pin on each PROM was lifted
and hard wired to ground. Fortunately I was able for find a genuine
Apple II controller card on ebay for just a couple of dollars. The
PROM have been transferred to that card where no extra wiring is needed.
However, because I feel the need to back up stuff on some of my more
modern computers, I needed to run ADT(apple disk transfer), which only
works with DOS 3.3 disks. I have purchased a second genuine Apple
II controller card with DOS 3.3 PROMs which I use on a daily basis.
In order to maintain the old inventory of DOS 3.1 of software I have,
I have copied them to a backup set of DOS 3.1 disks and also to DOS 3.3 disks
and have also copied the 3.3 disk images to a more modern computer with
ADT. I will maintain the 3.1 disks and controller in a safe place,
in case I want to demonstrate Apple's original operating system.
The picture at the top of this page shows an analog board from my
original Disk II. The yellow arrow points to resistor R-21 that
controls a delay between peak detection and read pulse output, as
described in Jim Sather's book, "Understanding the Apple II".
This resistor is fixed at 9.1K ohms which results in a delay of
approximately 2 microseconds. Later model drives, had this delay
made adjustable by adding a 10k pot and reducing the value of the
resistor to 7.2K. According to Jim's book, this delay was
increased to 3 microseconds to help read DOS 3.3 disks more reliably.
This can be seen in the image below of a analog board that has an
approximate serial number of 800,000. Note that the copyright
date hasn't changed on this much later board. I'm unsure if this
change is related to the problem I had reading some Atari disks with
this early drive or not.
Update 6/12/2009 -
I have put a completely reversable mod on my original controller to
allow the adjustable 10k pot to tweak peak detection. Sure
enough, read performance of those Atari disks have improved to close to
that of my later controller.
