intro#

Not much to say here; I occasionally use a Wacom CTH-680 drawing tablet as a mouse substitute and for sketches. I keep dropping the pen/stylus on the ground because I’m a clumsy fool. As it turns out, its pressure sensors are quite fragile, and (repeatedly) landing hard directly on the nib/eraser is just about the worst thing possible.

In my case, the eraser was working well but only part of the time; the pen was also flaky but only at low <40% pressures. I was hoping to find a simple mechanical fix and was only partially disappointed.

disassembly#

As seems to be common for a many Wacom pens, one needs to first remove the plastic button rocker - it pries out with a small flat screwdriver. Then, pulling apart (without unduly twisting) the two halves of the body. This needs a fair amount of controlled force to not throw small parts everywhere when the halves eventually separate.

The eraser end consists of the rounded hard-plastic “head” plus a plastic plunger similar to plain nibs.

To further take apart the pressure sensor assemblies :

• pull eraser and/or nib out. This frees the ferrite coil
• (extra risk for no benefit, not required: remove rubber bumper carefully)
• de-solder 2 pins of the sensor element
• sensor assembly + black plastic holder can now come off.
• White sensor assembly has a few parts too, I haven’t gone farther than this. One would need to release two metal tabs from the plastic body

failure 1 : simple mechanical breakage#

At the eraser end, I was lucky : the larger pin (actually part of the stamped sheetmetal) of the sensor assembly was broken off. Soldered a wire right beside. Broken pin was attached to near the top corner:

Luckily this worked; eraser now operates normally.

failure 2 : loose sensor + unknown trauma#

At the writing end, the sensor seemed to be un-clipped to the plastic body - I just pushed it back in place. Less fortunate here however, the sensor is still intermittent. Probably some internal fault .

To my knowledge nobody sells replacement sensors, and the stylus is discontinued since long ago, so this is now mostly a deadweight. Next time I’ll buy a spare stylus and keep it in a vault, safe from gravity and me…

pressure response test#

I wondered about swapping pen and eraser sensors, but it’s not clear if their response differ at the hardware or software level :

internals#

Mostly a Wacom ASIC that does the heavy lifting. Battery-less, a.k.a ‘EMR’ technology. Ferrite coil; power and signal modulated on 667kHz according to FCC filing docs. A few people have analyzed all this in more depth.

The pressure sensor is supposedly a capacitive element ? I have not verified this.