Tuesday, November 7, 2017

Various HP 1000 restoration activities


HPdriver disk emulation facility setup

The post office did find my package this morning, one clean National Instruments PCI based HP-IB board ready to install in the Windows based PC that will arrive tomorrow. The remaining items before I can use this with the HP 1000 are a 12821A HP-IB disk controller card, the cable between that card and my PC card, and possibly a different boot loader ROM.

I am waiting for the seller to ship the disk controller and cable. Since he is in Southern California and I am up in the bay area, it won't be a long transit time.

2645 terminal tape drives

I received my rubber 'tires' to fit over the metal capstans in the DC100 mini tape drives for my terminal. Bought from someone who had plenty of good feedback for using these to repair HP85 tape drives. Those drives use the same DC 100 cartridges for media.

Unfortunately, the assumption that same cartridge would mean same diameter capstan is proven false. These are far to large, swimming over the capstan and therefore unusable for my restoration. Now I have to find a plan B to repair my drives.

The original manufacture of the capstans had them dipped in a 'rubber' compound of some type and then trimmed to diameter by a machine tool. The solid rubber turned to goop over the years. I have to find a suitable means of replacing the missing rubber.

Some have used soft plastic tubing, glued into place, while others have used the soft rubber that is stretched over tool handles. Finally, there is the rubber dip that coats the capstan (Plasti-dip) and can be trimmed to size by spinning the capstan, more like the way they were originally built. Dip is ordered and on the way.

7906 disc drive

I am concerned about how I can accomplish the head alignment for the top head that I removed to clean after the recent crash. I have about half of what is needed according to the HP service manual. Some I suspect I can do without, but there is a big uncertainty at the core.

Aligning the heads on the HP drive is done by attaching the disk service unit (DSU) and adjusting the head position until the value on a meter and digital display on the DSU is at the right point. The DSU indicates distances of 12.5 microinches, either plus or minus from the zero point on the special alignment track on a CE cartridge.

I did buy the cartridge, which has something recorded on cylinder 245 which is read by the DSU to indicate the deviation. In addition to the DSU unit itself, which I have, there are two printed circuit boards that must be installed in the disc drive.

One of them replaces the normal preamp board with a special board that is said to both eliminate the risk of damaging the alignment cartridge and to improve the signal from track 245 to increase accuracy. The first role is apparently by blocking write and erase currents but I don't know what is involved in the second role. Perhaps different bandwidth or response curves?

The other board is added to the card cage in the drive and is named the Head Alignment board. I have no idea at all how this works. It has a 20 signal connector on top onto which a ribbon cable attaches, the other end of that cable hooks to the DSU. 

Besides the 20 signal ribbon cable on the DSU, there is a 50 signal ribbon cable from the DSU to the main interface input of the disc drive. This allows the DSU to command seeks and other operations. 

I don't have either PCB. Without that board feeding the DSU, the meter will have no useful output and I can't tell whether the head is in the right position or not. There is no alternative method mentioned, such as putting an oscilloscope on the read amplifiers and evaluating the resulting pattern. The scope and pattern method is used with most disk drives, such as IBM, DEC and Diablo, but not HP.

Nothing in the manuals or online describes what is written on cylinder 245 of the alignment cartridge. I have no clue whether I could use a scope and determine positioning, since the recorded signal and processing in the Alignment Board is opaque. 

I also lack the tool to move the head forward and back during alignment, but believe I can fudge together something that will be adequate. My DSU can move the heads to cylinder 245, but I don't have a means of evaluating the correctness of the head position. 

Here is what I am thinking as a workaround, unless someone pops up with the two missing boards and can lend them to me. That is an unlikely scenario. Instead I will make the assumption that the second head, which I have not loosened at all, is correctly aligned. 

If I can watch what is read from that head while at cylinder 245, I may be able to discern a pattern that would be useful for setting the top head. If I can't tell from what I see, then I will just set up the head to be as close to the same distance as the second head, visually, and lock it down.

If I can't align and have to accept a rough approximation, it won't affect my operation on my own blank disk cartridges. Whatever I write there I will be able to read perfectly. The issue is that what I write may be off compared to any other 7906 drive, therefore I can't take my cartridge and read it properly on a different system. 

If I even had a theory of operation or description of what the DSU and its two PCBs did, or schematics of the boards, I could probably work out a suitable method of driving the DSU meter myself. There could be ROMs, custom logic chips or other impenetrable components that stop me from reverse engineering the schematics, but if they are all known chip types I have a chance. 

Both the top and bottom heads for the removable cartridge platter are now clean as a whistle. I still haven't satisfied myself with head and platter cleanliness for the lower fixed platter, nor have I finished cleaning those two heads. 

2262A terminal cable assembly

My Centronics 50 male connector is due to arrive tomorrow, after which it take very little time to solder on the six wires from the serial board cable and install the one jumper. The tape diagnostics will tell me if it works properly when I try to use the terminal in conversational mode. 

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