hmmmmm........ what do you mean by "dirty"? a dodgy solder joint or something? And by "Earth" I think you mean the -ve common "bus" I've got on the board? (black wire).??Hi,
That waveform looks very "suspicious" and my inclination is that "Earth" is not what you think it is, BUT.....
If the "Earth" (local to the point that Channel 1 is measuring) is "dirty", then connecting it to the PCBScope might cause damage, so: Connect the Channel 2 probe to the Earth (0 volts) point close to the Channel 1 probe, and "tick" CH2 . The (horizontal) blue line should be just that, of course; if it isn't then you've learnt something.
ummmmmm.... my eyes have glazed over with that one.That's another feature that "big" scopes have; a "differential" mode, either a "dedicated" facility (or the early Tektronix 'scopes actually had an optional "plug in" Differential Pre-amplifier module), or simply an "invert signal" for one of the channels and an "Add" mode for the two channels (or of course a "Subtract channels" mode).