6128's floppy disk head: Slight movement upon every power up.

[Ace]

Hi to all! I 've got a strange little problem with my 6128's floppy head. Upon every CPC's power up, I was hearing a brief noise coming out from the disk drive like the motor's buzz and when I opened the case to check, I observed that upon every power up the floppy's head is moving slightly to the front, because the motor is spinning about an 1/4 of the circle. If I force about 10 on-offs in the row , the head eventually goes to the end point where it cannot move any longer and upon every further power up, the motor tries to move the head further where it resists because it has already reached the end point. But when I give a disk drive command like cat or run the head reallocates itself to the normal position.

Besides that I have not any other problem, the disk drive is perfectly capable of work. No read errors, it can read, write, copy, format, with no problems, it even reads the sector 42, that I think it's a a bit difficult and not all the drives can do that.

With further observation I discovered that something is not right with the disk drive's power cable:
[attach=2]
The two black cables from the power cable are wired into one which ends up at a different place at the motherboard that it should. The job was done not by the factory off course but obviously by a amateur. I don't know if this is the cause of the head's movement but when I completely disconnected the two black cables the head was still slightly moved upon every power up. Also with the black cables disconnected, the drive continued to read fine, so what is the purpose of this cables?

So anyway, two questions: Why the head is moving upon every power up and why the two little black cables of the disk drive's power cable are ending up in one common different motherboard's place?

[Bryce]

Hi Ace,
         First off, the two black wires are ground wires for the 5V and 12V supply to the drive. They are fine where they are.

The stepping isn't good though. It sounds like the pull resistor inside IC208 (74LS38 ) has failed. The IC and drive would still work properly in this case, but on power-up the signal would be low and cause the head to take one step forward.

Bryce.

[Ace]

...first of all, thanks for the reply!
I replaced a few hours ago the disk drive, with a newer EME-156 according to the serial number, (plus it has a few physical differences located both at the pcb -which is bigger- and the mechanism) and it doesn't has this behavior with the same motherboard and cables. So it seems that it isn't the motherboard to blame but the disk drive?

[Bryce]

Interesting, the controller IC must be dodgy then.

Bryce.

[Ace]

I connected the first drive with the odd head movement behavior, to the other motherboard that I pulled the "good" drive off and guess what, it has exactly the same problem: The head moves forward, towards the last track, upon every power on, until it reaches to the end (Track 42?). So two motherboards, one drive, exactly the same problem: head movement.
So the old drive, according to SN, with the smaller PCB and the head movement is a no go? I should keep using the newer drive?

[TFM]

As long as the old works I would use it. You can replace it whenever it really fails. That way the new one will live longer.