ST506/412 failure modes? (in this case, an IBM 0665)

[Jules Richardson]

Afternoon all,

This may be forgotten knowledge - or perhaps more likely, something that 
was never known in the first place - but are there any typical failure 
modes of ST506/412-type drives (beyond the obvious mechanical damage 
between heads and platters)?

I've seen quite a few dead drives over the years, but of course haven't 
invested much time into thinking about what might actually be wrong with 
them, because they were once quite common. Now that working ones are 
getting scarce, it got me wondering if there were any obvious things to check.

In this particular instance, I've got an IBM 0665 30MB drive in a Compaq 
which spins up, bounces the heads around a little, then causes the machine 
to issue a fixed disk failure at boot time. This is an embedded servo drive 
with a voice coil, not a stepper type.  Oddly enough, it passes Compaq 
diag's spare cylinder read/write tests, but fails the seek test. I've not 
tried a LLF yet because I was interested in trying to salvage whatever data 
might be on it first; there are several large ASICs in the logic board, but 
also a lot of more common stuff, so it's possible that the fault is 
something fixable - but checking component by component is probably more 
trouble than it's worth.

Yeah yeah, MFM emulators and whatnot... but I do want to keep noisy old 
boat anchors alive in my machines for as long as I can - while a modern 
replacement is the only long-term solution, I always think of modern tech 
as detracting from the experience of using old hardware.

cheers

Jules