19 Feb
2010
19 Feb
'10
1:45 p.m.
Chris Elmquist wrote:
On the Siemens drives also used in the H89 and H17,
there is an inductor
in series with the +12 to the spindle motor. It's a hash choke that keeps
the motor noise from going back into the +12 rail. When the grease in
the spindle motor gets old and sticky, the motor draws way too much current
at startup (if it ever does start spinning) and toasts this inductor.
I don't have "real" schematics for the drive and I have not
had time yet to work out the whole circuit but what I do know
is that one side of the inductor is connected to +12. The
simple diagram would be:
+12----<inductor>---+---<tantalum cap>----ground
|
|
somewhere else
not yet traced
When the tantalum cap shorted, burned and popped, the inductor
was carrying +12 to ground. That's how it got "toasty".
The spindle motor seems to be nice and free. Judging by the
strobe sticker on the bottom of the flywheel, the disk RPMs
are spot on.
Interestingly, I could not reproduce the failure last night.
I went as far as putting a small piece of carpet on top of the
H17, blocking the vents. It got nice and warm, but no failure.
The last thing I did Wednesday night, two nights ago, before I
posted the first message, was to unsolder one of the leads on
the inductor so I could put an Ohm meter on it. It was not open
and, as others have suggested it should be, was almost a dead
short. I wonder if the desolder/solder heat has "fixed" it.
Bill