troubleshooting an SA-400 disk drive

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Jun 11 18:34:22 2003

> I have been monitoring this new problem for a few days. Actually,
> it first surfaced weeks ago, but I had zero time to look into it.
>
> Anyway, I have a three drive Smoke Signal Broadcasting disk system,
> in which one drive, 2 (last one on 0,1,2 chain), has this nasty whine
> when
> powered on. The other two drives work and sound fine.
>
> The faulty one's driver motor never stops when powered on whereas the

OK...

[...]

> as being bad; either the small circuit board or the drive motor.

The small circuit board is the spindle motor speed controller. It
normally contains an LM2917 Tachometer IC and a few other components. The
spindle motor has 4 wires -- 2 are for the DC (brush-type) motor, the
other 2 are the AC tachometer generator. The frequency of the latter is
measured by the LM2917 and used to control the spindle motor.

On this PCB there's a power transistor, often a darlington type. If this
shorts (and it sometimes does) you get exactly the problem you're seeing.
The spindle motor runs at full speed all the time (hence the whine)

>
> Also, when 2 is selected the red LED does light up, but no diskette can
> be read.

Not suprising. The motor is running far too fast.

>
> Something is forcing the drive motor to stay on or the drive motor
> itself is defective.

I can't think of a fault in the spindle motor that would cause this. If
the tacho winding goes open circuit (_very_ uncommon), the motor will run
at maximum speed when turned on by the controller, but the controller
will be able to stop the motor.The fact that the motor runs all the time
implies a fault on the control PCB (the 'small circuit board').

-tony
Received on Wed Jun 11 2003 - 18:34:22 BST