HP-85 Tape Drive Capstan Repair
I would like to thank firstname.lastname@example.org 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
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
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
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
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
[...] 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
sells replacement capstans (but at $75 they are a bit expensive).
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