EEVblog #905 - REPAIR: HP85 Vintage Computer - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: July 22, 2016, 01:35:59 am
content: Dave screwed up and now he has to fix it.What went wrong with the vintage Hewlett Packard HP85 personal computer?

author: Silveruser
author_rank: Regular Contributor
timestamp: July 22, 2016, 03:59:42 am
content: As you say; back in the day, I used a few pieces of gear with those tape drives. In fact one was an HP logic analyzer. The tapes tended to come off the reels, they were not fastened to hubs. There were a couple holes near the end of tape and an optical sensor was used to stop it. Easy to take the tape cartridge apart and respool the tape. Great set of videos, I too lusted for one of these beasts.George

author: Twoflower
author_rank: Frequent Contributor
timestamp: July 22, 2016, 04:38:19 pm
content: I really wonder why it was so clean. Not even the high voltage showed any of the usual dust. Who buys such an expensive machine and store it nearly hermetically sealed away. As this would be the only explanation for the lack of dust.In the repair video you mentioned that the tape refuses to work. I think the tape chemically degraded. To my knowledge there shouldn't be any liquid stuff on the wheels. It could be the wheel in the tape itself degraded or the tape itself.

author: Fungus
author_rank: Super Contributor
timestamp: July 22, 2016, 05:02:39 pm
content: Quote from: Twoflower on July 22, 2016, 04:38:19 pmIt's from a University. They do stuff like that all the time just to use up their budgets.I think it was used a little bit - it had stuff printed on the printer paper in the machine and the tape cartridge had been written on.Quote from: Twoflower on July 22, 2016, 04:38:19 pmIt came in a huge zippered bag.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 22, 2016, 06:53:55 pm
content: It might have been cleaned, e.g. when doing a repair of memory upgrade.Sometimes places where the computer runs just is rather clean, e.g. an lab room or just a clean computer room.Form the program running at the end, it seem's to be a rather slow computer.

author: Fungus
author_rank: Super Contributor
timestamp: July 22, 2016, 07:11:36 pm
content: Quote from: Kleinstein on July 22, 2016, 06:53:55 pmUmmm....that's how computers were back then. It's a 1970s microprocessor running interpreted BASIC (no compiler).

author: Wytnucls
author_rank: Super Contributor
timestamp: July 22, 2016, 09:07:04 pm
content: Collector's notes:The HP-85 is a terrific little computer. It's small, easy to use and very reliable. Of the dozen or more units that the museum has processed, all have worked. The paper-advance and printhead drive (timing) belts decay, but they are easy to replace. The paper advance drive belt has 80 teeth and is 162.56mm long. A belt width of either 3.175mm or 6.35mm will work. The printhead drive belt has 175 teeth. It is 355.6mm long and 3.175mm wide. Be careful about spending too much time removing and replacing the internal assemblies. The ribbon connectors that connect the modules are very fragile; we are not aware of a replacement source.The only problem with the 85 is the tape drive. Very few of these drives are operational. Although the gooey capstan wheel can be replaced, other components are failure-prone. Regardless of whether the drive works, the old tapes are now very marginal. They are subject to breaking easily; it is also common for the magnetic substrate to flake off (even with brand new tapes). So, it's best to get any programs or data that you want to keep onto disc as soon as possible (this also makes the programs easy to archive). The 85 is easy to program and the small screen is surprisingly adequate.http://www.hpmuseum.net/display_item.php?hw=33

author: tatus1969
author_rank: Super Contributor
timestamp: July 24, 2016, 11:14:09 am
content: what surprised me most was that huge amount of bulk capacitance on the supply board. The small transformer doesn't indicate a large power consumption, and its all digital and should not require super flat DC rails. That looks entirely like a design where they did not yet have that cost-cutting managers flying around their heads all the time. Good ol' daysThat flex cable to chassis ground looks like they may have had EMC problems with the design, and they needed a low impedance connection to chassis. I think that this flex cable is way better than a regular wire of equal cross-section, because the skin-effect related impedance rise is much less.

author: Kjelt
author_rank: Super Contributor
timestamp: July 24, 2016, 11:30:21 am
content: Nice to see those dates stamped in the head of the electrolytes. Easy to know when to recap them.Are you going to recap this device Dave?Future eevblog: How Dave builds an USB to HP85 tape drive interface board with an STM32

author: Tinkerer
author_rank: Frequent Contributor
timestamp: July 30, 2016, 10:51:21 pm
content: Quote from: Twoflower on July 22, 2016, 04:38:19 pmYes, it was very clean. But like mentioned, if you store it away, it will stay clean.With that said, I have a toshiba laptop that was designed so its practically hermetically sealed. The airflow for the fans is sealed off(not like biohazard sealed mind you) from the rest of the computer as is the key board and any other thing where you might have an opening. Very few places for dust to creep through. Wish more computers had cases designed like that.