Just picked up yet another 3400A. Coincidentally, I was just last week trying to fix a problem in two other recently acquired units, and this latest one acts the same. They seem to work fine with definite input signals, or with no input while a low-Z terminator is present. Without it, they tend to have some reading around 10 to 100 mV with nothing applied. In contrast, the ones that work right are clean all the way down to the 1 mV range, reading residual noise and offsets around 5% of FS. IOW, say you've got some signal reading nicely in one of the more sensitive ranges, then completely disconnect it, and it pegs the meter.
I'm assuming the problem is due to line ripple in a PS due to bad caps, causing high residual signal, or local leakage somehow getting into the front when open-input. Another possibility is maybe something is oscillating up front. The main thing is, that they all seem fine when the input is from low-Z. My test setup for these is a 50R terminator in front, and HP3325A feeding it. With 1 mV up to the most the 3325A can do (2-3 Vrms?) for input, they all show about right, in the appropriate ranges. So, I think the basic amplifier and RMS engine work fine - it's just this extra junk that's always present, but is swamped out when the input is at low Z.
Anyway, I haven't checked anything much yet, but before digging into the details, I figured on getting some background info. Does this symptom seem common or familiar to anyone? I don't recall any discussions of this behavior.
Scoping them will be the first thing, but I won't be getting around to these again for a couple months. I want to avoid total re-capping, since there are a helluva lot of caps, times a helluva lot of units (nine now) involved, if I wanted to really do a proper, thorough job on the whole stable. Over the years I've managed to patch them up as needed with minimal changes. I assume (and hope) these last three have identical problems. This last unit also means I get to do yet another IEC power connector mod - I've already done all the others (I think) that needed it over the years.
Ed
