The HP-85 is part of the 80 series of early personal computers from HP. It was announced in 1979, and sales started in 1980. It was not exactly meant as a home computer with a price tag of over 3.000 USD (which would be about 11.500 USD in 2024). Rather, it was meant as a device for use by an individual engineer or scientist in a lab setting. Due to the relatively lightweight, all-in-one form factor, it was possible to take the machine home or on field trips.
The 85 was the first offering in the lineup and probably the most attractive since it integrates a CRT, printer and cassette drive in a tabletop case. It is expandable via four cartridge slots at the back and a multitude of expansions were offered for it, including RAM, Software and even a Z80 CPM cartridge. For use in the lab, the HP-IB option was available allowing to connect the machine with measurement equipment, plotters, disk drives and harddisks.
The ecosystem around it made the HP-85 a success and widely used in engineering. Its formfactor is similar to the earlier IBM 5100 released in 1975, which however, was not a huge commercial success.
Restoration
The machine came off ebay at a very reasonable price but it was dirty and required some maintenance to get it into an operating condition.
Keyboard
The keyboard was filthy; however, I did not follow my usual procedure of pulling out the caps and washing them. The reason is that the plungers of the Hi-Tek switches used in the 85 keyboards get brittle over time and split when pulling / re-inserting the caps, which leads to sticky keys.
The approach thus was to clean the keys without extraction, using a toothbrush, cotton swaps and lots of cleaning alcohol. After quite a bit of time and elbow grease, the keyboard turned out quite nice and it felt safe to touch.
Tape Drive
As usual with these drives, the tape drive capstan wheel had disintegrated. My approach to repairing this was to lasercut a 10.4mm rim around the cleaned capstan and a disc to attach to the top of it. Then used shrinking tube to add bit of grip to move the tape.
I did the QIC mod to the drive which should allow to use DC2000 cartridges. It involves modding the capstan to be approx 2mm higher so it reaches the drive wheel in the taller QIC cassette and adding a parallel resistor on the PCB to increase the write current.
The tape does move with this contraption, however with the only DC2000 cassette I have, I keep getting READ and SEARCH errors, even after running CTAPE and several ERASETAPE commands. The tape is a used one, so probably needs to be demagnetized in order for the HP-85 being able to format it.
I tried a wide variety of things, including demagnetising the read/write head, running a strong magnet over the tape, cutting away a length of tape that could have been in contact with the drive belt etc. Nothing works, so either the tape or drive is toast.
Printer
The printer was absolutely working and had its belts replaced by the previous owner. So the only difficulty was to find paper with the proper width as the transport wheels in the printer are at the sides, so narrower paper than 108:ish mm won’t work.
I addressed that issue by buying a thermal fax paper roll and cutting it about in half so that I got one piece with the needed width. While the paper is still rolled up, the whole thing can be cut with a hacksaw or other wood working tools. I added a round of blue tape to prevent the roll from unspooling while sawing it and that worked perfectly.
I aimed for 108mm (which I read somewhere), and that works, but it is a little bit too wide to fit into the lower part of the paper compartment, so 107mm should be right. Propping it up with a pencil works, though.
Monitor
This one was a bit of a let-down since the eBay seller had presented the item as with a working monitor. However, it had an issue with the first two character columns, which were warped around. Also, otherwise, the monitor was out of adjustment since it did not show all 16 lines.
Looking at it, it was a similar issue to what “curiousmarc” – who is an HP restoration genius – had shown on his youbtube channel and website some years back. He had the same warping happen at the right side of the screen, which points to the horizontal retrace interfering with the video signal.
Hooking up an oscilloscope confirmed the suspicion – the video signal (blue) starts while the retrace (yellow) is still going on. The nice thing is that the HP service manual is freely available and goes into quite a bit of detail including sample images of how signals should look like.
The suspects here are the 555 (U22) timer chip and an 843pf (C34) capacitor, which generate the retrace pulse. As in Curiousmarc’s case, the cap was in the typical tolerance range. However, the 555 turned out to be defective. I replaced it with a new part and added a socket, solving the problem. Then I adjusted h/v position, height and focus, and everything was fine.
Expansion Bay and IO Cable
I got an extra 16k RAM module with the machine, which however does not appear in the LIST output – it stays at 14576 with or without the module. This is related to the IO cable that connects the motherboard to the expansion bay. Beeping out the lines, I found that the /RC signal was not coming through, which means that the CPU cannot see the expansion module since it can’t switch the bus to “read”.
I found FFC cables of the right pitch and width on ebay, however I have to fabricate a 50cm cable out of 20cm and 30cm pieces. The solution was to solder pin headers to the ends of both cables to join them together.
Replacing the FFC cables to the IO board did not improve things concerning the RAM expansion. It is seen now by the CPU, but it acts up in a weird way, showing random lines of characters and advancing the printer paper once in a while. The machine is unresponsive then, reset does not work nor is any keyboard input possible.
Another avenue is the work that Bas van Reeuwijk posted on the HP Series 80 group, proving that the FFC cable can be replaced by a regular flat cable and IDC connectors. He has designed a small PCB to adapt the connector footprints and shared it online: https://oshwlab.com/wbvreeuwijk/hp-85-flatcable-replacement.
Working with the HP-85
The 85’s strength is clearly its built-in, easy-to-use but powerful BASIC geared toward scientific calculations and visualisation. It features graphics commands to produce plots easily, with concepts very close to HPGL, so plotter output should require no program changes.
Outstanding work
- Diagnosing the RAM expansion to find out if it is faulty or if there’s an issue with the computer still
- Fixing or replacing the tape drive – I’m thinking of adding a EBTKS board so I can use virtual tape images and exchange data with a modern computer
- Adding a serial interface card so I can use my 7574A plotter with this
- Maybe adding an HP-IB card to connect to other old-school HP devices
HP-85 Technical Details
| Model | HP 85(A) |
| CPU | HP Capricorn @650kHz |
| FPU | built in |
| Memory | 16kB |
| Harddisk | N/A |
| Removable Media | 210kB DC100 minicartridge |
| Video | Monochrome Text (32×16) and graphics |
| Other | Built in thermal printer |
| Manufactured | 1980 |
| Release Price | USD 3,250 (USD 12,450 in 2024 inflation adjusted) |
Sources
- HP Series 80 on Wikipedia
- CuriousMarc’s HP 85 Website
- HP Series 80 Group, IO Flatcable replacement
- Bas van Reeuwijk’s PCB design to replace FFC cables with regular flatcables and IDC connectors
- HP-85 Schematic Diagrams
- HP-85 Service Manual
- HP-85 Owners Manual and Programming Guide
- HP-85 Pocket Guide
- HP-85 IO Programming Guide
- HP-85 ROM Mass Storage Manual
- HP-85 Assembly IO Manual
- HP-85 Repair Course
- QIC Drive Modfication Drawing
Links to other HP items in my collection
An HP 7475A plotter, showcased in the IBM P75 restoration
A HP 9000 / 712 workstation running Nextstep
