This HP85 system had been in storage since about 1982. I had worked for a business that had sold these and if I recall correctly, this one had a problem with the display and was boxed for return to HP. The business went out of business shortly after and this system was among the pile of "trash" as the business was being liquidated. The system was complete with software and documentation and two 16K memory modules. I didn't have much use for it so I left it in the box in various storage locations over the years until about 2009 when I decided to restore this system.
I found it had several problems, most of which were caused over time, such as the tape drive capstan (the wheel on the tape drive motor that runs the tape) rotting away. And the tape oxide on several tapes had stuck to the band in the cartridge or had just flaked off of the tape causing them to become unreadable. Both of these are common issues with this system.
Repairing The HP 85 Display Board
First, I had to repair the display. The video was skewed, smashed, and jumping, completely unreadable. I was unable to find the schematics at that time (found the service manual later at www.hpmuseum.net ) so I spent a lot of time tracing and drawing out the video board and analyzing the signals with a scope. The horizontal and jumping issue was traced to a resistor in a DIP package. Quite odd, but it seemed this resistor was intermittently changing value. Most likely a manufacturing defect let in moisture into the package. I bypassed it with an external resistor and this restored the vertical. I found a similar problem on the vertical circuit, another bad resistor in a DIP package, repaired the same way.
Rebuilding the HP85 Tape Drive
Now I moved on to the tape drive with the disintegrated capstan. With some research I found that this was a common issue and several ideas on repairing it. What I have found that works best is to make a roller out of some Latex Tubing, 3/8" (1/4" inside) diameter, which is exactly the right diameter for this use. The trick it to find a glue that will hold it to the aluminum shaft. I used Permatex Contact Cement which so far has seemed to work.
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After removing the drive from the computer pull the motor and optical sensor
connectors.
Remove the motor, remove old rubber from the capstan roller. Cut a piece of latex tubing and apply glue as indicated by the glue manufacturer. Reassemble and be sure to let the glue dry completely before you attempt to use the drive.
Just to document the tape drive controller board...
Latex Tubing for use in repairing capstan drive roller...
Using the HP85 System
With the concept of wanting a "working computer museum" I wanted to be able to demonstrate these computers and peripherals in operation. I had also picked up a couple of HP plotters (9872A and 9872C) and a hard disk / floppy disk drive system (9133) over the years with the idea that these peripherals would help in presenting what these systems could do.
I was able to obtain the necessary ROM's and HP-IB interface to make the plotters work with the 85. As I was unable to read any of the damaged tapes I no longer had the Standard Pac or any other useable software. I do have the manuals for the system and the Standard Pac manual has listings for all the programs. I'm not fast at typing and after keying in a couple programs I decided to look for a better solution. I knew someone had to have a library preserved for this system and I found one at www.series80.org however, these are in binary and floppy images. Creating a series-80 format diskette on a PC is easy enough, however my 9133 drive is not compatible with the 85A storage firmware. A possible solution is to use the 85B firmware and I found the PRM85 project which has a Super ROM with the 85B firmware for I/O and storage. The PRM85 is a PROM board for the 85/87 series to replace HP's EPROM board and allow for the larger EPROM's with 8 ROM images in one chip. The PRM85 board was not available at the time, and when it was it sold for $100. Looking for another solution I found a reverse-engineered HP PROM board at http://retrobytes.ljcv.net/ and a revised schematic with his own design. Using this information I decided to build my own board that could support the SuperROM EPROM, the project can be found here {under construction}.
I also found another interesting project to emulate HP storage peripherals on a PC equipped with an HP-IB / GPIB Interface. {need links}