As readers may remember, we have been battling this drive which reads just fine but has problems writing correctly. If we take a cartridge written on other drives, this one will read it perfectly. If this drive writes anything, it is very likely to have errors reading back on this or any other drive.
The problems occur with both top and bottom heads. The oscilloscope showed that the previous contents of the sector were not fully erased. We swapped PCBs with other drives but the problem remained.
Today we decided to swap out the heads on the drive with known good heads from another drive, just to eliminate the heads as a cause of these symptoms. Even with all PCBs and the heads from a good drive swapped into this unit, the problems continued.
There was not much left that could be wrong - a cable from a PCB to the disk head connectors, the backplane carrying signals between PCBs, and the electromechanical drive and arm mechanisms. That is, if this is not a case of possession by malevolent spirits.
We ruled out the backplane after checking that all the signals required for writing were reaching the PCB that connects to the heads. We ruled out the cable and connectors to the heads by using a Time Domain Reflectometer and also measuring the resistance of each line. The TDR would show us shorts, open wires or other anomalies that cause reflected signals which may impact the quality of recording.
At this point, Marc began to closely examine the mechanism that loads the heads down onto the rotating disk surface. This is a pair of brass prongs that pivot to force the two heads together, pinching them against the disk surface as it rotates. The back of the disk head has a raised metal ball that sits under the end of the brass prong, through which the prong exerts the diskward pressure.
Marc could see the ball because the prong did not extend over the ball. The prong was about 3/32" shy of the ball. Looking at our other drives, we saw that the prong sits right over the ball on correctly operating drives.
|Prong does not cover small metal ball on disk head|
|Adjusting bracket holding prongs|
The failure mode was failure of the prongs to press the heads down against the disk surface as fully as on a correctly working drive. Just as seriously, the point where pressure was applied is now offset compared to the aerodynamic center of the head.
This would cause the head to fly at an angle, not parallel to the disk surface, and have reduced pressure to force them down. The spacing from head magnetic poles to the disk surface was larger and uneven.
|Prong properly positioned over metal ball|
We swapped back all the PCBs to the ones from this drive, moved the drive to the Alto and booted up to ensure it was working well. Problem solved! All that remains is to swap the heads back between our drive and this one, run an alignment, and do a final set of tests.
We believe this problem is related to the bad disk crash we experienced early on in the repair of this drive. It was shipped to us by the owner, who followed our recommendation to secure the disk arm from movement by using a cable tie. It is likely that the cable tie was tightened much too much, bending the bracket outward and shifting the prongs out of position.
|Very slight gap of bracket against metal of disk head holder|
The misalignment was very small and subtle. Worse, it is not a position measurement that is called out for checking in the maintenance manual, so we didn't spot it in all these months. At least it is corrected now and the drive can go home to support its Alto system.