Memorex 102 20MB Hard disk

Steven Hirsch snhirsch at
Sun Apr 11 13:25:09 CDT 2010

Has anyone on the list seen or work with one of these?  I received a 
Morrow M-20 8-inch hard disk along with my N* Horizon and found the 
Memorex 102 disk mechanism inside (what a beast!).

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).

The platters move freely, so the motor certainly wasn't bound up. When I 
checked power supply voltages (it's an analog supply), I discovered that 
the +24V rail was delivering +44V (!) and the -12V rail was -15V.  The 5V 
rail was right on spec.

For a quick reality check, I plugged the supply into my bench variac and 
adjusted input voltage to yield +24V at the motor supply.  Both other 
rails were within spec at this point.

When I tried again to spin the drive up at the correct voltage, the series 
resistor became extremely hot although, again, the drive was on its way to 
full rotational speed at the time.  I measured 20V across the 10-ohm 
resistor, which calculates to 2A and a dissipation of 10W.  The latter 
number explains the heat.  Not wanting to tempt fate, I shut things down 

So, opinions and suggestions please?  A two-amp draw does not seem 
excessive to me, although clearly it's outside the design parameters given 
the rating of the dropping resistor.  The power supply is certainly 
reparable, and before anyone asks it is delivering clean DC - ripple is on 
the order of 10mv.  Why it's doubling its output is yet to be determined, 
but I did find a diagram at bitsavers.

One thought that occured to me is that the +24V rail is probably powering 
the voice-coil positioner in addition to the motor.  I felt around 
extensively, and none of the power devices involved with head motion or 
servo is excessively warm.  None of them appear shorted or open.

Not sure where to go from here.



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