25 Jan
2005
25 Jan
'05
2:40 a.m.
without really trying:
Part amusing story, part warning. I have my OSI C4P all buttoned up with
the switcher supply tucked inside and I decide to fix another thing that
has been bothering me for some time. OSI used MPI B51 floppy drives with
most (all?) of their 5 1/4 inch systems. The B51 has an ejector spring
that operates via a two step latch (mechanical latch, that is). The first
step locks the spring when a diskette is inserted (so you can close the
drive door), the second step releases the spring once the drive door is
closed so that the diskette will be ejected when the door is opened. The
latch alignment is finicky and this drive had the spring locked and stuck.
So I open up the drive, straighten out the latch and then, since I have it
apart anyway, decide to clean the head. I get a Q-Tip and some alcohol,
lift the pressure pad arm and clean the head. (Anybody spot the problem
yet?) I put everything back together and fire it up. Now the drive is
flaky as heck. Read fails about 50% of the time. What did I break? I
check to see if the track zero sensor moved. Nope, original paint spots
are in place and uncracked. Did I, in fact, gunk up the head when I was
cleaning it? I look with a strong light and magnifying glass. The head
is smooth and clean. Did I pull some wire loose? Not that I can find.
After fooling with it for a while, I notice that the click of the head
load does not sound the same as before I took it apart. I look at the
solenoid, everything seems OK. I troubleshoot this for several evenings.
Finally, I go back to the head load investigation. I defeat the head unload
rail with a wedge of paper (so that the head will be loaded whenever the
drive door is closed). No help. Then, finally, I use a dental mirror to
look at the head and pressure pad with a disk inserted. The pad is not
in contact with the diskette surface! What the!?!?!? It turns out that
the pressure pad arm is "hinged" not with a proper hinge but with a thin,
somewhat springy, metal strip. There is also a proper coiled spring on
top of the arm to provide more pressure but, when I lifted the pad arm,
I flexed the "hinge" enough that the coil spring couldn't push the pad
into contact with diskette surface. If you just look at it with the
door open, everything looks fine. The pad arm seems to be firmly resting
on the unload rail, ready to drop down onto the diskette when loaded. So,
I disassembled the arm, gently flexed the "hinge" back to straight,
reassembled and back in business. One of the oddest things I think I've
ever had to work out. Maybe this is getting me ready to tackle my ASR33.
Bill