Doug Bonham

That would certainly 'splain it! Huge thanks!!

I have two used TDS7404B scopes with exactly the same problem - they only display AC coupling, the DC portion is not displayed. Surely this is a known problem. All four channels on both scopes.

Neither the 'Default Setup' nor "Autoset" fix it.

I'm trying to display the probe calibration signal (simple 1KHz square wave), but it only displays pos and neg spikes where the transitions should be. Vertical setup:  DC Coupling, Full bandwidth, Term 50 ohm, Display ON, 100mV/div, Offset 0v, 

Probe: Tek P6139A with a TCA-BNC adaptor. 

Is this a probe compatibility issue? Setup or config? Did the front end blow out on both scopes?