What appeared to be an easy problem to solve eventually became a pretty hard to solve, after opening the oscilloscope and take a look at the contacts of CH1, I checked that was all right with them. The problem appear after several hours around components of CH1.
A google search revealed that this component is an attenuator, and for my frustration is not a common part to find. Already looked on ebay and did not find the exact part.
Anyone know where I can find this part.