Well, just a checklist:
*Ground coupling: is the effect the same?
I am almost certain that ground "coupling" in this scope is done in software, or at least it disconnects the input and then
additionally zeroes out the trace. I have never in my life seen such a clean 0V trace - I wouldn't even expect that from a properly functioning scope. It is literally perfectly blanked. (Out of curiosity, to find out whether there is a proper hardware disconnect, sometime I will get around to pumping RF into one channel, setting it 'ground', and seeing if the cross-coupling changes. That's not going to help this, though.)
*Physical inspection, inside and out.
Done, except under the shielding can. (I'll dig into that next week. I have exams to study for now.)
*Bring it back to 1052E see if the problem persists. If it persists, the problem is after the input section.
Good idea. Pain in the ass, but I'm sure this will be, whatever the problem is...
*Turn on bandwidth limiting, see if the problem persists. If it does, and the BW limit is in hardware, you know where the problem is physically located.
Damn. That didn't even occur to me.
Thanks! That narrows it down significantly. The BW limit has absolutely no effect on the noise, so it must be a hardware limit and the noise must be coming in after the limiter. The separate software filter
does reduce the noise, as expected, so I haven't hosed the digital section or anything.
*Self-calibration, after 30 minutes of warm-up (it really needs the warm-up)
Done multiple times.
*Broken shielding causing it to capture FM radio broadcasts internally? (almost out of question)
I can't get this much radio noise on the scope with a foot-long antenna sticking out of the BNC! Also, there's no effect at all when I take off the large, main shielding can around the whole scope, so I doubt it.
That's all what comes into my mind.
Thank you very much!