The Museum of HP Calculators

HP Forum Archive 14

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Message #1 Posted by Jeff on 7 Aug 2004, 7:51 p.m.

Are there any articles or information regarding replacement of the printer drive belts for these units. I see them advertised on e-bay frequently but was curious if there there was a standard part number for each belt that I could order myself. These stepper motor belts cannot be that oddball! Also, has anyone had to replace the rubber drive wheel in the tape compartment? Mine seems to be a little "funky" ie.. slightly melted/deformed but only slightly.

Re: HP-85B
Message #2 Posted by Vassilis Prevelakis on 8 Aug 2004, 9:50 p.m.,
in response to message #1 by Jeff

Jeff <> wrote:
> I am currently in the process of re-building an HP-85 and an HP-85B.

Excellent idea, these machines were very well made so, apart for one or two items, that have worn off with time, you don't need to carry out any particularly difficualt repairs.

Two things to be careful though:

1) take GREAT care while you have the HP-85's cover removed because there is high voltage around the display area. This is so EVEN WHEN THE COMPUTER IS SWITCHED OFF. The circuitry acts as a huge capacitor retaining lots of charge. Any short can cause large amounts of current to flow causing injury to both personell and (almost certainly) the equipment.

2) The strips connecting the various boards together (they look like ribbon cable used to connect PC hard drives to the motherboard) are made up of 3-4 layers. In most cases the glue has lost its adhesiveness so the strips are delaminating. Be careful when you handle them in case they fall to pieces. Also note that if you remove them, you cannot just push them back in place. In mose cases you need to use a thin plastic strip as a guide. The best thing to have before starting the repair is the service manual for the 85. This is available on the HP museum collection of manuals (check

> Are there any articles related to any of the following:

> * Ordering replacement belts for the printer. Part numbers to order by?

You can get these on eBay. I bet you can get them from some place like, but you have to know the part number.

> *       Tape drive rebuild. Fixing what looks to be a "gummy" capstan?
I have attached to the end of this email the current wisdom.

> *       Understanding why I get the stall error 74?

Most original HP tapes are shedding their oxide coating, leaving the transparent tape backing behind. The tape drive interprets this as a tape stall (since it assumes that if it advances the tape, it will move past the Beginning-of-tape hole. If you have old tapes don't bother trying to read them, you will simply destroy their contents forever. Keep them in case somebody ever figures out how to read them.

> *       Cleaning scratches on the plastic cover for the screen?

I have tried small amounts of tooth paste and they can get rid of small scratche s.

> I have been taking pictures and documenting everything that I have done
> and would be glad to send this to you for posting when finished.  

That would be great, as I am hosting a series of tips and advice.

Best Regards



How to fix your "gummy" wheel problem: wrote:
> In the plumbing department at Home Depot I found some stuff called
> Magic Wrap, it's a roll of very thin rubber 1" x 16'. I cut a 2" x
> 1/4" piece of this stuff stretched it out to about the width of the
> roller then wrapped it around (several turns) the cleaned off roller
> as tight as I could. Then I smoothed it out by spinning the drive
> wheel at high speed and grinding down the rubber with an emery
> board. This sort of melts the rubber and fusses it to itself getting
> rid of the overlap and making the whole thing perfectly symmetric.
> To spin the drive wheel I disconnected one of the motor leads and
> put it on a variable power supply. You can use up to 12 volts but
> I found 5 or 6 volts ran the wheel plenty fast with tons of torque.
> Basically, you want to use it like a lathe and I found that the
> tape head makes a good tool rest! wrote: > I use 3/8" latex tubing from Lowes Hardware and LockTite glue. I > also leave the capstan on the motor during the repair. posted the following in the vintage-computer mailing list: > [...] I have used 1/2" tygon tubing from the local hardware store > (think "large aquarium hose") to refurb DEC TU58 drives (same tapes, > same capstan, same melting problem). > > I got a foot of it a few years ago and have yet to run out (even though > between HP line analyers, VAXen, and a VT103, I have nearly a dozen > drives to overhaul).

I do not recommend heat shrink tubing as it tends to come off. Test the repair by retentioning a sacrificial tape (use the CTAPE command on the HP-85) two or three times in a row. If the tubing does not come off, then it'll probably stay put for a couple of years.

If you are wondering what is the thickness of the rubber on the roller, the following picture is of an original HP roller with decayed coating still in place.

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