Help with Tektronix TDS-3012B Oscope

[HellasTechn]

Hello !

I Use a Tektronix TDS-3012B Oscope and i get this little strange behavior on both channel A and B. Even with no probe connected or with the probe connected and shorted to GND, i do get readings on the display that range from 0 to 100mv peak to peak. and from 0 to 400K frequency.
like shown in the pictures below. The same happens when i measure DC voltage. Also the resolution does appears to be far from perfect. I know that it takes samples of the signal but should it not look more like a solid line? also i do not know how to set it to use maximum sampling rate. Not to mention that i have an Oscope back from 1979 that its trace is sharp as a blade. I do not expect the display of an old digital scope like this to be as good as the display of an analogue one but... the readings ? This is a problem for me specially when i try to measure the ripple voltage of my buck converter.
I have done the signal path calibration but no change.

What am i doing wrong ?

THANK YOU!

 

[Externet]

I would first perform the same test in another room, house, environment, mains power, to compare, with as many things as possible around it off.

 

[HellasTechn]

I know... I have but no change. I have also tried two other scopes of the same model and all give me the same thing...