I once had a situation where the signal (in the 1 MHz region, square wave) looked like it was being AC coupled when I didn't want it to be - the strange thing was that the time constant of the coupling was very high, so that the signal looked like the derivative of the actual expected signal. After fiddling around with lots of stuff I finally found that the little "hat" on the scope probe tip wasn't pushed in all the way so there was no DC contact - only a few pF of capacitance between the probe tip and the "hat" contact - which was enough to high-pass-filter the signal and pass it with almost the correct peak amplitude :) Sean On Tue, Mar 26, 2013 at 3:43 PM, James Holland wrote: > >> Subject: [OT]Oscilloscope puzzle or how to feel like a fool. >> >>Snip >> >> Colin >> -- >> cdb, on 26/03/2013 >> >> > > Yes I know the feeling. Today I spent some time unplugging and rebooting = a DSO because I couldn't get the input to change from DC coupled to AC. Afe= tr determining that the scope was faulty and fetching another I started goi= ng through all the set up process again, when I got to the trigger menu I r= ealised that I had been switching the trigger from AC to DC and not the inp= ut signal! Fortunately I didn't get as far as sending the original back for= repair. > regards > James > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .