The Museum of HP Calculators
HP-65 HP-67 HP-97 "Clutch" Fix
This article is specifically aimed at users of HP-65/67/97
calculators who have had the defective gummy wheel replaced, either by the
Silicon Tubing or O-Ring fix, and are now experiencing card jamming and
misreads. Those who have a calculator with a gummy wheel fix that was performed
by others are highly encouraged to read through the documentation describing
both solution procedures before attempting any "clutch" fix.
A brief history. Faced with that gummy stuff sticking to
program cards, and with a small amount of research into the problem, I was,
thanks in most part to hpmuseum.org, presented with two disparate yet equally
elegant solutions. One is Mike Davis' Silicon Tubing solution (https://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/articles.cgi?read=179),
the other Mark Hoskins' O-Ring solution (https://www.hpmuseum.org/guest/hoskins/67crfix.pdf).
At first I opted for the O-Ring solution, as Mark (email@example.com) offered not only a
detailed step-by-step procedure, but the actual O-Ring material as well, and
for an entirely reasonable 4 or 5 bucks. His documentation also included some
excellent diagnostic material. When my HP-67 read its first card after all of
those years of sitting dormant, Mark became my instant hero.
His instructions also included a reference to the slippery
"clutch" problem, which I promptly ignored, even as the cards became more and
more twitchy to get to run smoothly through the gauntlet.
Well, not content with simply accepting a single solution, I
opted to attempt the Silicon Tubing Solution. Everything was perfect,
everything fit, and yet, the thing stalled time after time. I'd get a motor
buzz, followed by an "Error" display. During reassembly, I was also greeted
with detached power wires. Not the ones that plug into the power supply board,
but the ones that solder to the underlying main board. Oh, joy.
So I naturally went back to the O-Ring solution, which
worked great for a while, before cards eventually began to stick every time.
This is when I revisited Mark Hoskins' clutch slippage solution advice, which
he graciously supplied with his O-Ring fix documentation.
The advice was to use a little super glue to tie things
together. This left me feeling a tad apprehensive.
So, lets look at what we have. Though we can only get at the
drive wheel with complete disassembly of the calculator, access to the motor
and "clutch" can be attained with removal of the back panel. I removed the two
slotted screws holding the motor to the frame, holding the calculator body on
edge to allow gravity to keep the tiny slotted screws in their case holes (as
suggested in Bernd
Schmeling's wire insulation clutch solution https://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/articles.cgi?read=262),
and, remembering where they connect, unplugged the red and black motor wires
from the power supply board. I was faced with the following...
A motor with a thin 5/64 spindle extending into a 3/32 I.D
aluminum sleeve, and, from the other side of the sleeve, a brass screw shaft
that meshes with the grooves of the card reader's plastic drive wheel. I used
rubbing alcohol to remove EVERY trace of gummy "clutch" material from the motor
shaft, the screw shaft, and from inside the aluminum sleeve.
Well, here's the thing. The motor shaft engages loosely with
an indentation in the screw shaft, and, originally, some kind of gummy material
adhered the two pieces within the oversized 3/32 I.D aluminum sleeve.
I tried everything but the super glue: the wire insulation
fix, minute silicon 3/32" O-Rings I purchased from smallparts.com, candles and
incantations, nothing worked. Things jammed completely, or either the motor
shaft or the screw shaft would spin freely.
Until I came up with this. It was like an epiphany.
The thing isn't really a "clutch" at all. It's a start-up
torque inhibitor and vibration suppressor, all meant to make screw gear and
wheel engagement not only be kind to each other, but to be mutually forgiving
as well. If the two little nylon ball switches that let the calculator know
that a card is in place but not moving ever fail, and the assembly does indeed
act as a last resort clutch, it would probably have to be replaced after a
short time anyhow, and the main board electronics would have to be deemed
Assuming that the electronics and nylon ball switching are
operational, the following fix, though a tad tedious, works perfectly.
Please read the entire thing through before commencing. Once
all of your parts are cleaned of all residues, there's no going back.
A small tube of DAP 100% SILICONE Household Adhesive,
hopefully fresh. Don't scrimp on this, its only, like, two or three bucks a
Some scraps of 24 lb glossy paper, or equivalent.
3/32 metal bit (won't be used for any drilling).
Magnifying glass (usually handled, used for reading small
how drastic this fix seems. When done successfully, your card reader
"clutch" will work like new. But please be cautioned. After reading
through these instructions, if you are at all unsure, please seek out other
the back cover, hold the case to the side, release the power supply
connector to the main board and flip it up in order to access the two tiny
slotted screws holding the motor in place.
the motor and shaft assembly, remember where the red and black wires are
connected to the power supply board, and gently pull on them to
disconnect. With the magnifying glass, note the tiny brass clips that
careful not to exert enough pressure to warp the aluminum sleeve, separate
the screw shaft from the motor...it will pull off. The aluminum sleeve will
usually remain attached to the screw shaft rather than the motor shaft.
sure that a tiny nylon washer is still surrounding the motor shaft, and
push it up against the motor body.
the aluminum sleeve from the screw shaft. After separation, use a small
knife, if necessary, to carefully remove the hardened gummy stuff
surrounding the end of the screw shaft.
rubbing alcohol and Q-Tips, clean every bit of residue from the motor
shaft, the now separated screw shaft, and the inside of the 3/32" ID
aluminum sleeve. Poise these three items on your desk and wonder, as I
did, how you will ever get them to work again.
- If the
aluminum sleeve is warped, due to handling during removal, use the blunt
end of a 3/32 bit to reform it, by setting the sleeve on end, and,
gripping the bit with needle nose pliers, force the blunt end into the
sleeve; then work the sleeve back and forth with your fingers until it can
easily be removed.
that the end of the screw shaft has an indentation that accommodates the
motor shaft. The motor shaft and screw shaft will meet here, while
entombed within the oversized aluminum sleeve. Practice inserting the
motor shaft into the screw shaft a couple of times so that you can get
used to the feel.
practice propping the motor against something stable so that the shaft is
pointing straight up...a small soft-edged bench vise would work well for
this...I just kind of prop the motor up against the inside edge of the
calculator case from where the thing had been removed.
make sure that the tiny nylon washer is on the motor shaft and is snug
against the motor body.
DAP 100% SILICONE Household Adhesive will be acting as a filler and
torque/vibration inhibitor between the inside of the oversized 3/32 I.D.
aluminum sleeve and the motor shaft/screw shaft connection.
- Put a
big old glob of your DAP 100% SILICONE Household Adhesive on a scrap of
glossy paper. Have a tissue handy. Have the drive motor standing by within
that little aluminum 3/32" ID aluminum sleeve in hand, plop it onto the
silicone blob from both ends, packing it up. Don't try to be precise. Just
glom both sides.
the tissue and, without trying to be too exact, wipe around the outside of
the miniscule sleeve to remove access silicon material. Wipe across the
open ends as well, the silicon will retreat into the sleeve, which is just
what you want, as it will be displaced by the motor shaft/screw shaft
Here we go. Place the silicon filled sleeve over the motor shaft, and,
while working to keep things centered using the thumb and forefinger of
the hand that's holding the motor, use the thumb and forefinger of your
other hand to insert the screw shaft into the other end and, by feel, mate
the screw shaft and motor spindle, within your silicon filled aluminum
step 16 finds you with silicon everywhere and nothing meshing, or even
with the tiny sleeve sticking to your fingers or falling on the floor,
take a breath. Work through the steps. Clean everything thoroughly with
rubbing alcohol and begin again at step 10. Clean things quickly...silicon
cures over 24 hours, but begins to set in minutes.
meshed the motor shaft and the screw shaft, all within the silicon filled
aluminum sleeve. Prop the motor up so that the screw shaft is pointing up,
and over the next several minutes adjust the outer sleeve by using your thumb
and forefinger to gently turn the shaft, and keep readjusting the sleeve
so that it looks centered.
while keeping pressure on the screw shaft to keep it engaged to the motor
shaft within the silicon-filled aluminum sleeve, pull back up on the sleeve
a bit to keep it just from touching that nylon washer. This is easier than
it sounds, and will make the sleeve seat up against the thicker part of
the screw shaft. You know you're right when you can see a slight
separation between the sleeve and the nylon washer that is sitting against
the motor body.
adjustment is difficult, due to the silicon within the sleeve beginning to
set, its time to throw caution to the wind.
the silicon set for about three hours.
the case on edge, reassemble the motor into the frame, over the drive
wheel, with the two tiny slotted screws. Reinsert the red and black wires
into the power supply board, and reset the power supply connector into the
all wire connections, and, holding a battery pack against the battery
compartment contacts, turn on the calculator and make sure everything
- If no
display, switch things off, remove the battery, and carefully check all
power supply connections to the motor, R/W head, battery leads, etc.
Although all wires attach with tiny brass clips to the power supply board,
everything is soldered to the main board underneath. I've had to re-solder
power leads a couple of times while replacing the gummy wheel. Have a
solder-sucker handy, and use a low-wattage gun.
- If all
is well, and the display is lit, and a couple of simple calculations
succeed, keep pressure on the battery pack against the contacts, take a
breath and test a card. Mine went through like it was on fire. Motor noise
was minimal, and the card took no coaxing at all. Smooth as silk.
the calculator off, remove the battery pack, and reassemble the back of
God knows how I came up with this last ditch, here-we-go,
let's get Western solution. But oh, man, does it ever work! Satisfies all of
the original criteria. Cards don't stall, and motor torque is buffered against
the inherent frailties of the neoprene drive wheel.
back to the main exhibit hall