12 Apr
2010
12 Apr
'10
1:08 p.m.
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).
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.
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.
Ouch. It's possible there's some kind of overvoltage protection
downstream of that resistor (like a fat zneer diode) which has shorted.
Or maybe somthign took a dislike to the high votlage.
Or perhaps there is a real fault. Since it spins up, I guess the spindle
motor is OK. Can you diconnect the positioner actuator and see if the
current drops then?
Also, find some of the analouge chips in the read amplifier circuit and
measure their supply voltages. They should almost certainly be
symmetrical about ground. If the +ve voltage is low or missing, trace it
back to where it comes from (maybe a 3 terminal regulator like a 7812)
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
Am I missing something? 2A and 20V is a power dissipation of 40W.
number explains the heat. Not wanting to tempt fate,
I shut things down
quickly.
So, opinions and suggestions please? A two-amp draw does not seem
Firstly, fix the PSU. I would be much happier knowing that that was
nothign to do with the problem.
Measure the current drawn from the 24V line (I susepct it's rather more
than the 2A measured through that resistor, and that some parts fo the
drive are not suplied via it)
Remove the reissotr and power up. Does the spindle run? Can you find
which parts of the drive are now lacking a +ve supply line?
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.
If you know it's a voice coil, try measuring the voltage across the voice
coil. If ti's being driven hard, it will draw a high current from the
supply (after all the coil has a pretty low DC resistance). Maybe the
heads have stuck so the thing is not managing to move the heads to find
the servo data (but it's still trying). Maybe there's something wrong
with the driver circuitry, but not the output stage.
-tony