The Huion Tablet Pen Repair

476 words | 2018-8-30

When using a graphic tablet, you need to be very careful with the pen. I was convinced of this on my own sad experience (as usual :smile:).

Here is this little cute tablet: HUION 420 tablet in the box

Once, when I was trying to portray myself as an artist, I dropped my pen from the tablet to the floor. As it turned out, it was a disaster: the pen did not draw any more, well, it drew — the line was always drawn, you could not even touch the surface of the tablet:disappointed:.

Disassembling the Pen

The pen is very easy to disassemble, the main thing is not to forget to first extract the rocking key

Assembled pen

Disassembled pen

No external damage, except a broken capacitor \(C_5\). But the impact of the fall could not have been so strong as to break the SMD capacitor, rather I damaged it during disassembly.

I contacted Huion’s support service and they were so responsive that they contacted the engineers who explained to me that this capacitor can not affect the pen so much. They advised us to send the pen to the nearest service center for repair.

Study the pen

After some time I got a new pen (:smiley:), a small oscilloscope and a few inductances (10 Henry) and proceeded to investigate the mechanism of the pen.


Measurement of the radiation frequency of a working pen

These measurements are summarized in the table:

Feather Mode Frequency
Serviceable idle 4 kHz
  press 8-13 kHz
  near button 7.86 kHz
  far button 16 kHz
Defective idle 8 kHz
  press 12-17 kHz

Now it became clear why my pen constantly draws: it just radiates at a frequency corresponding to pressing the tablet.

Modeling the Pen

To figure out how to fix a pen, we’ll draw a simple model:

Scheme of the pen

This is an oscillating circuit whose resonance frequency is determined by the formula:

\[2\pi f = \frac{1}{\sqrt{C\cdot L}}\]

To change the frequency \(f\) it is necessary to increase either the inductance \(L\) or the capacity \(C\). I tried to change the inductance by rotating the coil, but it ended up with a wire break:disappointed: (When you solder these wires, put paper or something to not melt the coil frame.)

So let’s move on to changing the capacity. I need to reduce the frequency from 8kHz to 4kHz, for this I’ll try to connect another capacitor \(C_2\) in parallel with the inductor:

Pen scheme with extra capacity

I soldered the first capacitor, which is suitable.

Soldering the capacitor

Look closer (100 pF):

Soldered capacitor close-up


I use the battery (temporarily soldered because I’m tired of pressing it) and try the pen on the tablet:

Pen Test

Here you need to look at the reaction to a different force of pressure:smiley: Pen test

Final Touches

It is necessary to isolate the capacitor by a piece of film in order to avoid unnecessary closures. Also it is necessary to throw out this lever for buttons since I never use them, and \(C_5\) is missing :smiley:.

Capacitor Isolation