Saturday, November 11, 2017

Repaired minicartridge tape drives in 2645A terminal, Marc repaired an HP85 tape drive


tape drives in 2645A terminal

The capstans on the minicartridge tape drives in the terminal had their rubber liquefy years ago, but no spare parts exist by which to get new capstans. After some research I determined I would try to build up a rubber on the capstan using a product called Plasti-Dip. 

It is a rubbery material that coats and object then dries in place. It was originally used to coat the handles of tools but some one discovered that it makes a good material as a substitute capstan for these HP drives.

I disassembled the two tape transports, taking off the motor with its attached capstan at the top. The original goo was scraped off the metal of the capstan and I began dipping it in the solution. Each layer dries in about five minutes, after which another layer can be added on top. 

To make it even and circular, we applied power to the motor and rotated it at a slow speed, enough to counter any sagging and circularize the shape. After about 6 or 8 layers, it was thick enough and looked ready to be used.

I put them back together and brought them home where I will let them finish setting, a process that should take four hours according to the instructions on the Plasti-dip can. In the afternoon I reinstalled them into the terminal, so that I could do some testing of the drives and my supply of tape media.

I ran the self test in the terminal that checks out the function of the tape drives. With tapes inserted in both drives, the test will rewind, write a 'worst case' pattern of data, rewind again, and read what it had written. It does this for each drive in turn, reporting any errors. None were found.

I wanted to boot up the tape diagnostic system and run a more detailed diagnostic program on the terminal, but discovered that no such test exists. If the terminal's self-test routines aren't sufficient, HP required a special tool to be installed in the terminal, a Cartridge Tape Unit (CTU) which would control the drives and report status via LEDs. 


I had bought a rubber 'tire' designed to slip over the capstan of an HP 85 computer, which also used the mini cartridge format and also suffers from rubber decay. They were too big for my terminal's tape drives, but I figured Marc could use them.

He popped the tire on his HP85, completed other adjustments and cleaning to the system and then conducted tests. Besides the new capstan rubber, Marc increased the erase/write current by modifying a resistor on the PCB, allowing him to use DC2000 cartridges instead of the native DC100 media. 

A bit of filing to fit the larger DC2000 cartridge, and he tested by writing, reading back and listing files. The HP85 works great now. Unfortunately for owners of the tape drives in the 2645A terminal, some mechanisms that HP built were too cramped to allow modification to take DC2000 media, unlike the HP 85. 


I attended a wonderful event at the CHM, dedicated to the Xerox Alto and its creators. Twin Altos sat on the stage as various programs were demonstrated and discussed by their inventors, along with often humorous memories of the happenings back at the Palo Alto Research Center. Among the people talking and attending a reception earlier were Bob Sproull, Charles Simonyi, Larry Tesler, Avi Tevanian, Tom Malloy, Doug Brotz, Dan Ingalls, Chuck Geschke, John Warnock, and David Boggs. It was a privilege to meet and hear from them.

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