HP 85 gummy wheel repair


i have a lovely hp85 microcomputer. however, inevitably the age has rotted the pinch wheel to gunge. is there an article on diy repair for this (i found ones for 67, 97 but not 85) or does anyone know the ideal sized part for replacement.

thanks for helping,
-- hugh.


I repaired mine using heat shrink. It takes a little trying to get to it and you have to be very careful because there are some small parts which can fall out while trying to access the part the needs heat shrink.

Also, don't use Radio Shack heat shrink because its too smooth (and isn't very good for anything). I used 3M. I has a nice grippy surface and works great. I even found some in red which somewhat matches the original gummy wheel.

Best of luck...



I took my 9915 tape drive apart recently -- AFIAK it's the same unit as in the HP85. Although there are some small rollers, etc that will fall out if you take the top section apart, it's easy to remove the capstan without doing that. All you do is remove the tape drive from the machine, disconnect the tachometer lamp leads from the PCB (some machines have these soldered in place -- my 9115 did, others have them plugged into little pin sockets on the PCB).
Unplug the motor wiring from the PCB. Remove the 4 screws round the motor on the underside of the unit. Remove the motor and capstan complete. Undo the 2 Allen screws (0.050" key) holding the capstan to the spindle, and pull it off. Take great care not to damage the tachometer disk.
I need to rebuild the capstan in this drive (and in some other similar HP drives I have here) -- can you tell me what the outside diameter of the capstan should be?


thanks for the help tom, i am following your instructions. i have removed the motor unit. the capstan is 3/8".

i am thinking of using some heat shrink like andy suggested, although i can only find the smooth stuff (maplin). also the capstan wont pull off, i've undone the two allen screws (i had to buy another set to get the small 1/20") should i just pull harder? although if i use skrink, i figure there's no real need to remove the capstan, now i have the motor out.

recommendations for shrink or another idea most welcome.


..oops, tony, why did i call you tom. this is what happens when you're fixing your hp's and cooking the dinner at the same time :-)


Don't worry about it -- I've been called many worse things than that :-)


The setscrews raise little burrs on the motor spindle which makes it hard to get the capstan off. What I had to do was remove the setscrews completely (they're not hard to handle on the end of the Allen key), then support the capstan under the capstan roller part (NOT the tachometer disk) with a pices of hard plastic (nylon?) clamped in the vice. I then used a soft metal (brass) drift and hammer to tap the motor spindle out. Oh yes, have some way of catching the motor when it finally does come free.
For reassembly, I smoothed out the burrs on the spindle with 1000 grit wet-n-dry paper.
When you say the capstan diameter is 3/8", I assume this is the diameter of the rubber part, not the metal hub (which is still intact, of course, in my drive).


But why go to all the trouble of removing the spindle, when you can replace the plastic coating of the capstan roller part while the spindle is still attached to the rest of the motor assembly.

Of course the first time you go through such an operation you may go a bit too far (I took the whole tape drive assembly apart and then had to put it together while all the little plastic bits tried to escape). Now I leave this alone and only remove the motor.



thanks. yes, i noticed looking down the holes that the
spindle has been dented by the screws. i'll try a bit more force as you've suggested otherwise i'll give up and try to repair it whilst attached to the motor, which should be ok, unless i need to do something scary.

i measured 3/8" after i've cleaned off all the old rubber residue. my motor looks *exactly* like the picture here from vassilis (even the stray wires on the left).


i have fixed it with some heat shrink and it works. i can load programs and they run. what fun!

it rumbles somewhat whilst spinning the tape. was it originally real silent? i didnt know the original rubber thickness, so i put on two layers of shrink and hoped.

anyone interested can see by bodges here:

many thanks to people on this thread for their help!


Tony Duell (UK) wrote:
> can you tell me what the outside diameter of the capstan should be?

Here is a picture of the capstan. This is the original material which has turned into goo, but it is still retaining its original shape. The scale of the items in the picture can be extrapolated from the diameter of the metal spindle (which I assume is the same on all drives).

> Take great care not to damage the tachometer disk.

This is the semi transparent disc at the far end of the spindle.



thats *exactly* what it looks like. except i've cleaned off all the goo. the capstan head is 3/8".


Here's how I fixed the tape drive on my 9815:

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!


thanks. that sounds like a possible alternative idea. i dont have a home depot (i'm in london uk) but there might be something similar to magic wrap in my local diy store.

its sunny today and just for fun, i attached the motor to my 12v solar panel. it really spins! my alternative energy plan is on hold currently until i figure out how to make inverters happy with inductive loads :-)


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



it worked with heat shrink in the end, although it rumbles a bit. maybe i needed one more layer or its not as smooth as the original rubber.

i'd like to thank people here for their help with the details. here's my bodgery anyhow;


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