Monday, June 8, 2015

1053 margin rack disassembled and old lube removed, diagnosed why Pertec drive won't load and go ready


I put out feelers for a spare part to replace the margin rack latch I broke yesterday, meanwhile I will think about how I might repair the one I have if that is my only option. I may be able to epoxy a tube over the remnant end where it broke off - assuming there is enough to affix to. Otherwise, I might have to drill a hole and attach a threaded rod in place of the missing vertical bar.

The 'good news' is that, since I have to remove the latch itself to fix or replace it, I might as well remove the entire margin rack assembly. That let me totally remove the stale old lubricants that might have been impeding its movement. With the parts disassembled, I have everything clean and ready to put back together with only new lubrication.

What a difference in crisp return of the margin rack when released. I also see enough of a stub that I should be able to recreate the missing part of the vertical bar which trips the latch release to end a carrier return.  No guarantee it will work, but worth a try. If I can buy a good replacement for the margin rack latch, that would be ideal.

Another option, since I have the end that broke off, is to see if it can be welded in place. The material is not very ductile, sort of a cast aluminum or pot metal, and the inside where it broke is granular in appearance. Not sure whether a repair can be made by welding but will look into it.

PERTEC DISK DRIVE (5440 style single 14" platter cartridge)

I began checking out the condition of the drive, using my multimeter to check various switches and the power levels. My cartridge presence switch is working, as is the switch that signals the platter cleaning brushes are in their parked position. The power supplies are delivering the four DC voltages needed, +10V, -10V, +5V and -5V, thus I am digging into the startup logic to see why I am not getting a start attempt.

The problem was quickly found - the start/stop signal from the interface is not pulled up by the terminators to a logic high level. None of the input signals were pulled up, they were only pulled down to ground. The + voltage side of the resistor dividers were only connected to the ribbon cables, not to the +5V from the Pertec drive.

Apparently the termination power is supplied by the host system - or in this case by unconnected cables. When I verify that this is what is happening, I will temporarily bridge the high side of the dividers to the +5V of the drive, which should pop the interface signals to their default state.

With the label on the drive, I can tell this D3422-E024-NSC drive is a 200 track per inch, 2400 RPM, top loading (5440 format cartridge) with 24 sectors, using the normal Pertec interface, but I only have the special Diablo compatible interface described. The ebay listing had the number mixed up, which is why my earlier posts listed a 4xxx model number.

All the docs on bitsavers describe the interface cables as 36 conductor flat ribbon cable, yet the cable and connectors are 50 pin. The interface from the terminator board onto the logic board is 100 conductor, which may mean a simple one to one assignment of the 100 lines to 50 lines on each of two cables. I will do continuity testing to figure this out.

When I know the interface and how power is applied to pull up signals, I can move forward to work the machine towards loading the heads. I will go cautiously, ensuring the drive spins, blowers work, and the disk brush sweeps the platters before I let it go far enough to put the heads onto the live surface. In fact, I need to clean the heads, the fixed platter and the platters of the three removable cartridges that I own before any heads are loaded.

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