Tuesday, October 3, 2017

Continuing working on HP 1000 system restoration, starting on disk and tape


I opened and looked over my 7906M disk drive. This drive has a single platter removable cartridge plus a fixed platter underneath. The multiple heads share one access arm. The drive itself looks very clean except for the crumbling foam of the pre-filter. 

Front of 7906 drive, cartridge inserted and control panel visible below

Underside of 7906, nice condition
After I remove and replace the pre-filter, I have to verify the cleanliness of the air seals for connections that are between the HEPA filter and the plenum and cartridge. On similar drives, there is a foam filter as a seal which is known to deteriorate. 

The heads look fine, albeit with a small amount of oxide smudging that is typical for a drive after some hours of use. The heads need some cleaning. I will carefully inspect and clean the platter in the removable cartridge and the fixed platter if I can reach it. This will take a few days.

The HP drive does not use a seal at the cartridge air flap so this problem is not an issue for the 7906. Once I finish the inspection and cleaning of platters and heads, I am ready to fire this up and test out its loading behavior.

Air door on bottom of disk cartridge

Mechanism that pushes up the air door but does not use a foam seal
I looked into the main rack power switch that was sticking in the off position. It appears that a grease was used which hardened a bit with age. I cleaned it out and applied some alcohol to help with the main barrel sliding in and out of the switch. It now works great, just waiting to be reinstalled tomorrow.
Disassembled switch before cleaning up solidified grease
Toggling in instructions helps me learn the machine architecture and prove out the functioning of the system at the same time. I have spotted an anomaly where something is overwriting memory with an alternating pattern of all-one words and all-zero words - this doesn't happen constantly but often enough to wipe out my hand entered code. 

The direct memory access functionality provided by the DCPC card, which allows selected peripherals to read and write to memory autonomously, might be the agent doing this writing. I decided to pull the disk and tape controller cards and see if that resolved the issue. No change.

Definitely flaky  behavior of some kind. I will be checking the memory and other card configuration settings, then yanking cards or disabling functions until the bad behavior goes away. The one definite problem with the system.

I worked a bit on the 7970B tape drive, which is a 9 track 800 BPI 45 inch per second unit. My controller cards are for the 1600 BPI model I don't own, so I can't do reads or writes until I sort that out, but I can get the mechanism and offline behavior working correctly.

Mag tape has a metallized reflective spot glued on that identifies the 'load point'. The drive will forward through the tape slowly until it sees the reflective spot. However, my drive didn't see it at all. I hit rewind to see the tape move the tape back to the supply reel at higher speed. That too will only stop if the reflective spot is detected.

Tape threaded on 7970B tape drive

Mechanism of the drive
Time to do some basic troubleshooting on the drive, first repairing the reflective spot detection and then moving on to its other behaviors. Tomorrow I will dig into this.

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