Samsung LE40B541 - volume goes up and down erratically - defect volume-up switch

I heard about Samsung sets changing the audio volume uncontrollably. I recently got a chance to look at one myself. I switched on the device and after some minutes it started pulling the volume up and down unpredictably. During normal phases, the buttons on the remote and also on the TV would work just fine. Thus, my first instinct was not to check for broken switches.

Nevertheless, it was pretty likely that either the switch board or the control board, which also contains the IR sensor, was to blame.

So I went ahead and unplugged the switch board first. The TV was stable. That was interesting. How could a switch break without getting used at all? I still did not believe it. The IR board contained nothing but the sensor and the connector to the switches. Others have a controller on it. Not here, the controller was integrated on the main board. I swapped a 47µ cap on the board, just to be sure. No effect.

Ok, so the culprit must be the switch board. I removed it an put it on the table. While pressing down on the connector solder joints with my meter probes, the TV would not show any symptoms. Aha. I took the board in my hands and twisted it slightly. Clockwise twisting reliably produced the problem.

The board has two output lines. Each button is connected via ground and a pull down resistor to one of the lines. This produces an individual voltage for each button. The line with the volume buttons was erratic with an unstable voltage. I unsoldered a resistor, a cap and a diode. Still the same. So I finally measured the volume-up button and what can I say, it changed its resistance wildly. Not quite down to zero, just in-between so that the controller sometimes decided that the other button was pressed. 

What the...? The button worked totally fine, yet it produced a leakage current. I have never seen anything like this.

Button removed and everything was back to normal. I had no replacement at hand, but the owner did not mind to leave it out, as it had never been used anyway(!)

Some wire-redressing with fresh tape and the TV was good to go.

Let's take the switch apart and see what we find:

The body shows some shiny residue, which might be conductive. Not easy to tell, but that is the only irregularity I could find.

My theory is that some material had been creeping into the switch during the re-flow phase of production. On top of that just a slight amount of oxide during 7 years and that could be just enough to connect the outer contact with the central pin. Who knows?

So, I guess, these switches are actually fixable with a proper cleanup.