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Xolod Offline OP
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Please who can help with advice on repairing MFM HDD. I have a NEC mfm hdd which does not pass the self diagnostics. And it has a single copy of Unix for Labtam.

Last edited by Xolod; 05/14/21 12:29 PM.
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Hi,
what do you mean by "doesn't pass self diagnostics"? Does it spin up but not unpark its head? Or does it spin up normally, rat-tat-tat-tat and all, but is not recognised by the controller?

It might be worth to try and image the disk with David Gesswein's MFM emulator board: https://www.pdp8online.com/mfm/mfm.shtml


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Xolod Offline OP
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Originally Posted by rfka01
Does it spin up but not unpark its head?
Yes, the relay will enable the stepper motor. There is a very small step of the head motor and it no longer moves.
I connected hdd to mfm emulator and it gives this error.[Linked Image from i.ibb.co]

Last edited by Xolod; 05/14/21 09:46 PM.
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write fault is a very odd failure.
does the drive have a terminator in it?

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https://forum.classic-computing.de/index.php?attachment/64459-20200511-211202-jpg/&thumbnail=1

On one of my NEC D3142 one of the black parts between the red resistor blocks and the solder pads of the power connector (bottom left) has let out its magic smoke. I know the picture is small, it's not mine and just for reference. It looks doable, but I haven't made an attempt yet.


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Xolod Offline OP
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My HDD Nec D5124. as I understand it is a problem in the head (step) motor. it cannot move and therefore the disc does not enable the ready signal?

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Originally Posted by rfka01
https://forum.classic-computing.de/index.php?attachment/64459-20200511-211202-jpg/&thumbnail=1

On one of my NEC D3142 one of the black parts between the red resistor blocks and the solder pads of the power connector (bottom left) has let out its magic smoke. I know the picture is small, it's not mine and just for reference. It looks doable, but I haven't made an attempt yet.
If you mean the small surface mount part(s) on the left of the photo they look like transient voltage suppressors (TVS Diodes). A straightforward repair if it has been blown by a voltage surge - in my case it was a lightning strike and after replacement the drive was fine.

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Maybe this can be of help:

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Xolod Offline OP
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Find out that the problem is not with the stepper motor. I checked two of its windings, the resistance of both is 22 ohms. oiled the spindle but did not give any results. the motors are controlled by two Darlington assemblies of the STA434A transistor. they are very hot over 50-60 degrees Celsius. I watched the signals with an oscilloscope on these assemblies of transformers, at one output of a constant voltage of 12 volts, and at the other output a high-frequency signal of more than 1 MHz and an amplitude of 12 volts is generated. It looks like self-excitation, due to the breakdown of the trasistor. if I move the head from the initial position with my hands (it is mandatory with rotating discs), then when I turn it on again, the head returns to its initial position. assuming Darlington transistors are punctured. I'm currently looking for where to buy them.


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