Memorex 102 20MB Hard disk

Tony Duell ard at
Tue Apr 13 14:11:17 CDT 2010

> On Mon, 12 Apr 2010, Tony Duell wrote:
> >> At initial power-up, a 10-ohm 5W resistor near the power connector let out
> >> a puff of smoke (the drive was in the process of spinning up).  I killed
> >> power and started checking components carefully.  There were no ominous
> >> readings across the 5V logic power rails and I traced things out enough to
> >> determine that this resistor was between the +24V power input and whatever
> >> lies downstream (probably motor servo).
> >
> > It would be worth knowing what that resistor feeds. The obvious things to
> > need the 24V rail are the spindle motor and positioner (which could be a
> > stepper nmotor in an old/small drive like this, or it might be a voice
> > coil). I would also suspect that the 24V line is regualted down to 12V
> > for some of the analogue circuitry.
> I traced things a bit further.  The 24V line from the power connector runs 
> directly to the NO contact of a relay.  The leg that runs through the 
> scorched resistor ends up at a TO220 transister.  I'll try to get more 
> detail on its downstream side.
> I'm wondering if the resistor might be there to limit inrush current? 
> Maybe the relay closes at servo lock and feeds power directly to the 
> driver transistors at that point?

I've had a look at the Memorex 112 manual on Bitsavers. If your drive is 
similar, you might bave big problems.

The relay contact is part of a 4-pole relay used to turn on the spindle 
motor (the other 3 poles connect braking resistors ot the windings) The 
10 Ohm (5W) resistor is the fred to the collector of a power transistor, 
whcih is the pass transistor for a simple 12V regulator. The emitter of 
that transistor is the output of the 12V regualtor and feeds a lot of 
analuge circuitry.

My guess is that the transsitor is OK (but it would be worth checking) and 
that you have something downstream of it that is drawing exceessive 
current. It might be a shorted decoupling capacitor, it might be an IC. 
And to  make things worse there seem to be some custom analogue ICs in 
this drive. 

First test that I would do is to measure the voltages (wrt ground) on the 
3 leads of that transsitor.
> The heads are not stuck.  If I tip the drive, I can watch them slide 
> across the platter from their own weight.  Should have mentioned before 
> that this thing has a translucent plastic top :-).

RIght..The Model 112 is a stapepr motor positioned, BTW. That is what I'd 
expect in a low capacity, but large platter drive like this. The trackls 
would be suufficiently wide that a stepper motor would be fine. You need 
a voice coil positioner taking a servo signal from the disk if you have a 
high tranck density, of course.


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