Sunday, November 5, 2017

Work on 7906 disc crashed head, preparing to wire 2262A terminal cable


7906 disc drive

I removed the top head from the drive, which had suffered the head crash that dug the groove into my disc cartridge. Initially the head looked quite bad, with a sort of metallic smudge and raised area across the head poles. However, I rubbed it a couple of times with kimwipes soaked in isopropyl alcohol (IPA) and it looks quite a bit better already. 

Head with small remaining crash material to clean off

Looking at the head under the stereo microscope, I could see that there is one raised spot right on the read/write pole in the short groove above the main channel in the picture above. There is some dark residue n the short groove on either side of the pole iron. Also, along the bottom edge there are two remaining oxide/aluminum smudges to be removed. 

I will give this a bath in the ultrasonic cleaner at Marc's home, after which I believe it will be clean enough to reinstall in the drive. The cartridge itself is ruined, requiring me to find a new cartridge before I can load heads. 

Alignment is going to be a challenge. The procedure involves a specially written alignment cartridge, much like the ones used with the Alto Diablo drives and others, but one made specifically for the 7906 drive. It also uses a special tool installed on the drive.

I found an HP alignment cartridge on eBay, but can't tell from the listing whether this is for the 7906 drive or the incompatible 7900A that uses thinner platters. Since it was listed for a reasonable amount even if it turns out to be the wrong type, I bought it. Once it arrives I will inspect it to check it for usability with my drive. 

The tool for doing testing and alignments consists of a main panel that hangs on the back of the drive, connected to the main disk interface with a 50 pin ribbon cable. It also provides a special pre-amp board to replace the normal PCB in the drive. That special board is connected to the tool panel with a 20 pin ribbon cable. 
Disk tool on drive, 50 and 20 pin cables hook on top
I have the main tool panel, but do NOT have the special preamp board or the two cables. The cables can be made easily, but the board is pretty hard to find. Since it provides the signals to drive the meter on the main tool panel which is the reference for setting the head alignment properly, I am going to perform some serious improvising. 

2262A terminal restoration

The main task remaining before I can use my 2262 terminal is to make up an adapter for the serial connection. The terminal has a connector block that is quite different from the 30 pin cable that hooks to my 12966A serial board. It is a Centronics 50 female.

Connector on back of terminal
It will require me to first determine what pins are used on the terminal's Centronic connector, then locate the corresponding signals from the regular 30 pin connector. I can see that only a subset of the 50 possible connection points have any metal fingers attached, instead only allowing for 34 signals.

30 pin edge connector on serial cable
Finally I may have to jumper some wires as they enter the 30 pin connector for signals just to set up the board and terminal to communicate. 

Serial cable hood, room inside to jumper connections
After a long time hopping between a number of manuals - for the serial card, 2262A terminal and a general cabling book - I have the desired hookup for the centronics connector. Using the existing cable from the serial card, I can open the hood on the 30 pin connector and get access to the six signal/ground wires I need. 

These six wires will be placed on specific Centronics pins, plus I must jumper pins 36 and 46 to each other but not to any signal wire. The six wires are color coded, making the connections to Centronics easy. 

  • Green wire to pin 48, common signal ground
  • Red wire to pin 42, transmit data
  • Blue wire to pin 50, receive data
  • Brown wire to pin 12, secondary line signal detect
  • Yellow wire to pin 13, ring indicator
  • Orange wire to pin 44, secondary channel data
  • pin 36 provides +5V (logic 1), jumpered to pin 46 for receive line signal detect 
Existing 12966-60008 connector, six wires, to build Centronics 12966-60010 cable

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