HP97 The journey begins

[teenix]

Hi all,

I sprayed some circuit board lacquer onto the back of the display PCB to protect it. This is the stuff that you can solder through if necessary.

It was one of those nights where something is on your mind and you cant sleep from trying to solve it. The problem was the rotted dampening shaft material. I though of all sorts of solutions and ended up considering the silicon approach, but I didn't like the idea of the mess with small parts and trying the centralise everything, then hold it there somehow while it set. Some non liquid form of repair seemed to me to be the best approach.

My thought was, this coupling is supposed to dampen vibration so if the assembly is off centre then it will vibrate anyway. The coupling (I think) is also there to provide a release mechanism if the card seizes in the slot and therefore save the motor from burning out or stripping gears. HP's solution was elegant but difficult to mimic for a home constructor.

After some searching around and some guess work I went down the local electronics hobby shop this morning and bought some cable that I thought might do the job.

As it turned out, and I am still gob smacked, the first cable I tried was almost perfect. It took three goes to get a working coupling, but the last try only took under 10 minutes.

All you need is the following...

A piece of 75 ohm RG58 coax (75c per meter)
Stanley blade
A 5/64 drill bit
Preferably a bench drill.

Method...

Cut about 2 inches of the outer sheath off and strip the underlying wire braid
Use the Stanley blade to cut the inner translucent wire sheath at the same point and pull the sheath off.

Unless your motor has a different shaft size, the inner hole fits perfectly onto it and provides lots of drag and can be easily removed.

Next, place the drill bit into the bench drill and have it at a "medium" speed.
Taking care here, hold the translucent sheath vertical and gently push it up against the rotating drill bit.
You only need to remove about 5mm of material from the inner hole, any more and the drill may drift off centre. The inner hole will serve as a drill guide.

Once done, if the hole is out of centre, discard it and start again. Look through the translucency in bright light and you will see the drill path. It should be even all around.

Place the worm drive against the sheath, then with the Stanley blade cut off what you think is enough for it to sit on the shaft end.

On the other end, cut the sheath to the length that will let the motor sit in position without binding it.

Finally, push the motor shaft onto the sheath and you are done.

As this assembly is so short, it has good mechanical strength, so I did not replace the aluminium sleeve as I don't think it is needed.

I tried it with some cards just by powering up the motor from my bench supply and the cards went through quite nicely.

I haven't tested it by reading/writing but will do so later today. Hopefully all ok.

cheers

Tony