HP-85 Tape Drive Capstan Repair

I would like to thank katie@wass.net for the following suggestion.

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 fuses 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!

I have looked at Home Depot and Magic Wrap was not to be found, but they suggested Nashua's Stretch and Seal Silicone Sealing Tape, which turned out to be ideal for the repair.

The silicone tape adheres to itself and most other surfaces, so there is no need to use glue. Just make sure that the old coating has been thoroughly removed before you begin the operation.

For a detailed discussion on how to take apart an HP-85 to get to the capstan (and a novel way to attach heat-shrink tubing to the capstan), check http://www.voidware.com/calcs/hp85rep.htm.

No matter what technique you use to repair the capstan, be sure to 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.

This picture shows the original rubber coating of the capstan. By comparing the thickness with that of the other elements of the picture, you can determine the correct thickness of the rubber coating.
Click on the picture to enlarge.

After the repair, if you still get "END OF TAPE" errors, then you may have a damaged tape. These are usually original HP tapes that are shedding their metal oxide coating. Once the metal oxide is gone, you are left with the transparent tape backing. The BOT/EOT (beginning/end of tape) sensor interprets this stretch of transparent tape as the EOT hole and complains.

Here are also a number of other suggestions on ways to fix the capstan.

rhahm@nycap.rr.com wrote:

I use 3/8" latex tubing from Lowes Hardware and LockTite glue. I also leave the capstan on the motor during the repair.

Ethan.Dicks@amanda.spole.gov 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 analyzers, VAXen, and a VT103, I have nearly a dozen drives to overhaul).

Will Kranz in his DEC TU58 page (the TU58 is a DEC tape drive that is mechanically similar to the HP-85 tape drive) mentions that jtcomputer.com sells replacement capstans (but at $75 they are a bit expensive).

Back to the home page.

	This picture shows the original rubber coating of the capstan. By comparing the thickness with that of the other elements of the picture, you can determine the correct thickness of the rubber coating. Click on the picture to enlarge.

	After the repair, if you still get "END OF TAPE" errors, then you may have a damaged tape. These are usually original HP tapes that are shedding their metal oxide coating. Once the metal oxide is gone, you are left with the transparent tape backing. The BOT/EOT (beginning/end of tape) sensor interprets this stretch of transparent tape as the EOT hole and complains.