I opened up my scope to do some tests and try to confirm my theory from above.
I did confirm that the relay part is A150-20-12:
Also, I traced out the 50ohm path. It goes through the first relay, through the Teledyne attenuator, then terminates into the 1NB7 IC:
I also put it on the TDR and took some measurements. The blue trace is with the scope set to <200mV/div (good), yellow is with it set to 200mV/div (bad), the solid marker is lined up (in time) with the pad exiting the first relay, and the dashed marker is on the pad of the 1NB7 IC. So it does look like the Teledyne attenuator is bad, I think.
I have a DSO7054A and the internal 50ohm termination appears bad on channel 1. I've not opened it yet but reviewed some of the internal pics posted and dont see an obvious 50ohm resistor. Is the termination built in to the Teledyne relay as suggested here?
On mine at least, the 50ohm termination is built into the Agilent 1NB7 IC. The Teledyne relay is a switchable attenuator sitting in front of that termination. However, since yours is a 500MHz model, it shouldn't have the Teledyne relay installed at all. I'm not sure where the termination is for those models.
How often do you use all 4 channels on 50 ohms.. ? Maybe swap it with Ch4 ?
Yeah, that's a good idea. I might just do that.