Old MiniScribe 8438, 30 MB Hard Disk Repair - Index Pulse Not Detected

[n45048]

Hi guys,

This post comes off the back of a previous post I made here.

I'd like to try and get this drive working (as finding a suitable replacement for a reasonable price is proving difficult). The error that is being generated is "Code 4 - Index pulse not detected during spin up". The manual describes the following power-up procedure:

+12 VDC powers the spindle drive motor.  The microprocessor verifies that the disks are spinning at 3600 rpm and then activates the automatic Track Zero positioning.  -TRACK ZERO, -SEEK COMPLETE, and -READY will become true upon completion of the Track Zero positioning sequence.

Further on down the manual, it goes on to specify:

-INDEX     This 300 microsecond (typical) interface pulse is provided by the drive once each revolution (16.67 ms nominal) to indicate the beginning of the track. Normally, this signal is a high level and makes the transition to the low level to indicate -INDEX. Only the transition from high to low is valid.

In my case, the motor is spinning fine. I can physically see it make several full rotations upon power-up before shutting off with the error code. There are other error codes which relate to the actual speed:

Code 5 - Unable to reach 3600 rpm in 30 seconds
Code 6 - Unable to stabilize spin speed in 10 seconds
Code 7 - Unable to maintain spin speed to 0.5%

As I'm not getting these errors, this indicates to me there is absolutely no output from the sensor at all.

One of the forum members, Chris, posted this:

You do not have to remove the cover to check the index sensor, its located under the PC board, I remember the filecards and the 8438  RLL drives.

I've pulled the PCB off the drive and found what appears to be the index sensor. There is a small silver circle (A) on the edge of the motor which passes a small black piece (B) as it rotates. Now I'm stumped at where to start troubleshooting. What kind of sensor is this?

Unfortunately I don't have a scope to probe around with (Dave, interested in a repair video?)

[SeanB]

6 wires on the motor, so likely 3 are the phases, with a star point, and the other 3 are for the hall sensor that detects the index pulse. Look on the board where it plugs in, there will be 3 heavy traces going to the motor driver, and a trace connected to ground and another going to 5V, with the last thin one going to the motor control and the MCU. Solder a test wire to it, put the drive back together and power it up. There should be a pulse on that wire, if not either the pull up resistor is faulty or the sensor is bad. As you have to dismantle the drive to get to the bottom of the motor hope it is the resistor. If the sensor you can get a new one and break off the original one and epoxy one to replace it to the magnetic shield, and route the 3 wires to the connector.