TDS7404B No DC coupling, Does not display DC portion of signal.

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?

Hi Doug,

The issue is that the TCA-BNC is 50 Ohms only and the P6139A requires 1 MOhm termination.
For this reason, you should use the TCA-1MEG adapter instead: https://www.tek.com/en/manual/tca-1meg-2

That would certainly 'splain it! Huge thanks!!