I've been travelling overseas, but now I am back in the lab earlier this week I finally upgraded my 121GW firmware and installed the shim on the selection knob.
Today I went to measure a 12V DC source and the meter displayed 166V.
Then it decided to show OFL, even when measuring a 9V battery!
My initial thought was I had accidentally manually selected a low range, or it was in mV mode, or I stuffed up in alignment of the knob position during reassembly. But the display matched the various selected range positions. I took the meter apart and couldn't see any issue... until I removed the range selection switch... and the below image is what I found
I have not been measuring any mains or other high voltages on the meter. The only thing that comes to mind is that the 12V I was measuring was at the input to a couple of POL DC/DC converters which was at the end of a long cable, and the DC/DC converters only had ceramic caps. The DC/DC converter (my own design using TPS54622) is rated at 5.2V 6A out but was struggling to power a Raspberry Pi & pocket 3G router. Measured with another (Tenma or EEVblog/Brymen) meter I was seeing only about 3.8V instead of the expected 5.2V.
I added a 330uF electrolytic capacitor at the input to each of two the DC/DC converters and it solved the problem, so I guess with the long cable from PSU to the ceramic caps was creating a nasty LC circuit and maybe causing some voltage spikes. The cable was ~3m of 18AWG figure 8 speaker cable to simulate what was on-site, and it did help to clearly identify the equipment issue I was tracking down.
No (obvious) damage to the DC/DC converters, Raspberry Pi, or connected equipment, but apparently the meter didn't like it. Or the meter came to me this way as I've hardly used it apart from measuring low voltages mostly out of my sig gen. I doubt that as I'm sure I would have noticed this measurement issue in previous testing.
It was late when this happened and I haven't done further inspection or tried cleaning the PCB yet.
Any thoughts?