Creating the Head Adapter Tool for aligning the Diablo drive

PROCESS IS DIFFERENT FOR STANDARD DENSITY (1130) CARTRIDGES

The method of aligning the heads takes advantage of the wider spacing of the poles of the erase coil in the head compared to the read/write coil. To do this, a special tool is connected between the J10 printed circuit board and the connector from the heads. It rewires the connections to switch the read preamplifiers from the ends of the read/write coil to the erase coil. 

This provides a very sharp null position over the track centerline so that only signals that are offset to the sides will be detected. The CE cartridge that is used to align packs had track 100 recorded with a special drive that offset the center of the spindle. 

This caused the written track to shift during a rotation, putting the center of the recorded track close to one edge and then the other of the .01" track width when it is read back on a drive with a normal spindle position. Since the center of the erase coil does not see a signal, the only signal picked up is from the portions of the special track that wander off center to be under one or the other pole of the erase coil. Correct alignment gives equal signal strength on both poles, otherwise we see more signal on one or the other so that the scope trace is unsymmetric. 

image for properly aligned head

High density machines, including the Diablo 31 we used to archive the Xerox PARC cartridges from the Alto computers, cannot use this method because the track width is too narrow. It must use the read/write coil for alignment. 

HEAD ADAPTER TOOL REWIRES THE CONNECTIONS TO USE THE ERASE COIL

No schematic or board view exists of this tool, but its purpose is described enough to work out a substitute. I will create a mini PCB that sits between the socket and the plug of the connector from the heads. It will also provide scoping connections.

Google AI gave me a start on the wiring, but it has several problems. As usual, it creates plausible text but can be way off on important details. The pin numbering from the AI output does NOT match any schematics of the Diablo drive. 

It refers at one point to the heads as having two erase and one read/write coil, which is NOT correct. It claims there are two erase coils (yes, but one per head) and a common return line (not true, separate wires from the heads only joined on the J10 PCB to form a common erase return path. 

Further, it doesn't seem to work with the method of selecting the head to align as described in the alignment procedure. The select line (a common line to all three coils in a head) has to be grounded. Grounding pin 5 of the J10 input pads selects the upper head; without that, the lower head is being selected. The wiring of the erase connections must conform with this. 

The AI discussion has both erase head ends connected across the differential amplifier, which is one for the top head and one for the bottom head. This could work if we have a floating select line hooked to one side of the erase coil in the head we aren't watching and an activated select line hooked to the other side of the coil on the head we care about. The two select lines go where the two erase return lines are connected on the original schematic. Here is what I think the wiring should be - from the head wires on the connector through to the socket on J10 PCB.

The normal wiring from the heads into J10 connects the two ends of both head's read/write coils together and they both feed the differential preamplifier, also called head bus A and head bus B. The erase coils are hooked on one end to the select line, as is the center tap of the read/write coils. The other end of the erase coil on each head comes onto J10 and are tied together there into a common erase return. 

All I need to do is disconnect the read/write coil ends and reroute the erase coil lines. Select lines remain unaffected. The erase coil line from one head goes to head bus A and the erase coil line of the other head is attached to head bus B. Very simple rerouting of the signals. 

HEAD SELECTION ON THE DIABLO DRIVE

The wiring of the heads uses a Select wire that is connected to the center tap of the read/write coils plus to one side of the erase coil. The two ends of the read/write coil are hooked to head bus A and B, while the other end of the erase coil is connected to a common return line that is grounded when erase current should flow through the erase coil. 

The select line is set to +14V when the write gate is turned on, thus the erase gate also drives the erase coil in this case. However, when the head is not selected, the select line is floating thus we don't have any erase or write current flow. For reading, the select line is set to 0.1V on the desired head and floats on the unselected head. 

As a result, even though the erase coils of both heads are connected to the differential preamplifier channels, only one select line is connected thus a signal voltage is only developed on one of the head bus channels at any time. 

WILL HAVE TO DETERMINE PIN NUMBERING OF ACTUAL CONNECTOR ON DRIVE

I have to sort out which pins lead to the read/write and erase coils of the two heads. The Diablo schematics should four pins per head, eight total. I know that it uses shielded cables to each head thus there are two more pins used for those shields. There is also mention of a chassis ground connection in the AI, but no documented in the Diablo manuals. That means I have 10 or 11 pins used in a 2x6 connector. If that matches what I find on the actual board and drive, then I can start developing the logic for the adapter.

I must work out how to connect the two erase coils (each with a return line) to the preamplifiers and how to cause only one to drive signals (selection of the head). I have to be convinced I have a rational wiring scheme before I design and manufacture my adapter board. 

Also to be determined is the pin spacing and size of the connector. I will have to put pins on my adapter board that slide into the J10 PCB with good contact, as well as mounting a compatible socket on my adapter where the connector from the disk heads will plug. Any elements needed to select which head I am aligning will also have to be placed on that board. A few turrets may be added to attach scope probes for raw measurements.