Thursday, November 2, 2017

7970 tape drive run through all diagnostics, plus a switch issue fixed


7970 tape drive diagnostics and repair

I ran the diagnostics for the tape drives and controller, validating timing, movements and correct read/write operation of my 7970E drive. To do this, you have to swap over to a scratch tape at the right time, otherwise you might write over and ruin the diagnostic loader tape.

While everything ran fine during the session, the tests do attempt to address the drives as various unit numbers, from 0 to 3, not just the default I use of 0. Switching the tape drive to respond as the other unit numbers highlighted a defect in my drive. When the buttons for 1, 2 or 3 are pressed, they don't latch into place. Only unit button 0 does this.

Unit select pushbuttons that were failing to latch (1, 2 or 3)
I suspect a mechanical problem in the unit selection switch assembly. Prior to beginning actual disassembly, I reviewed the diagrams and other documentation to understand how it should work. Those weren't of much assistance for the mechanical aspects of latching one button down.

In fact, these can't be disassembled without desoldering them entirely from the PCB. Further, it doesn't appear even then that they could be opened. However, I did see a rod that seems to be the locking bar, which seemed a bit sticky with accumulated dust/sludge.

A shot of alcohol spray freed it up nicely, and it began working properly immediately. I reassembled everything into the drive - the PCB holding the buttons and lamps, a larger PCB that attaches at right angles, three molex power type connectors, plus two ribbon connectors to the control board inside the drive enclosure.

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