Fluke 199 - Scope noise at faster time bases and Meter noise (With video!) - Page 1
Forum section: electronics / repair

author: michaelrcolton@gmail.com
author_rank: Newbie
timestamp: March 27, 2025, 08:13:32 pm
content: Hi guys,Thanks for taking a minute to look. I have a Fluke 199B that I picked up a few months ago. It's a nice tool, but I've noticed two issues that are bothersome and was hoping I might be able to ask for the insight of those of you that know lots more than me!The first issue is that in scope mode, both channels show a distinctive loss in stability whenever the scope is set faster than 5uS/Div. Like a sine wave is leaking in somewhere?The other (perhaps related to the first?) is that in meter mode, in (at least) DCV and Ohms measurement modes, the readings are pretty noisy, while the readings from my fluke 117 are rock solid.Maybe this is just a difference in the way these work or are built, but I find it hard to believe the far more expensive 199 would perform so much worse than the 117.I appreciate any insight you can offer. Thanks!https://youtu.be/EvI6ILdU-0w<iframe width="560" height="315" src="https://www.youtube.com/embed/EvI6ILdU-0w?si=B2uRuGXdPmfE5EQy" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>

author: Swake
author_rank: Frequent Contributor
timestamp: April 02, 2025, 05:00:31 pm
content: For the scope part: Environmental noise? Like SMPS for lights/computer/whatever picked-up by the test leads.For the DMM part: Have had something similar on other fluke's. It was either dirty contacts or the test leads being crap (mind you.... original Fluke leads).

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: April 02, 2025, 07:06:30 pm
content: Hello michaelrcoltongmailcom,Might be worth to check theinternal power railsfor ripple.

author: luke10050
author_rank: Newbie
timestamp: April 06, 2025, 07:37:41 am
content: does this only happen when the scope is powered by the mains adapter? I had the mains filtering capacitor come loose off the PCB in the power supply and it caused all kinds of issues.I'd recommend pulling the power supply apart (its servicable) and checking to make sure the transformer and mains filtering capacitor are still soldered to the board.

author: michaelrcolton@gmail.com
author_rank: Newbie
timestamp: April 28, 2025, 04:01:54 am
content: Everyone, thank you so much for taking the time to reply!@swake: Since I am using the same leads between the two meters, I think it rules out a lot of these possibilities (unless the scope meter is fundamentally more sensitive in some way (like, higher impedance inputs?))@timeandfrequency: Yes, I'll look into that. Looking at the block diagram (still haven't gone through the whole thing), I see a voltage reference (for the DMM part, at least) that I could check. Also, if I'm interpreting things right, it uses a linear opto-isolator with feedback to send the reading over the isolation gap, so I guess that isolator, or the feedback circuit might be suspect. For the scope part, there are separate paths for Low Frequency vs High Frequency (though I haven't yet found where the cutoff is) so that's a potential clue, maybe something in the high frequency path isn't right?@like10050: I'll have to check this. Since it can run on batteries, this should be really quick to verify.

author: luke10050
author_rank: Newbie
timestamp: April 28, 2025, 05:19:28 am
content: Pulled mine out today to have a look and I get +/- 1 count on battery. Eg. 119.7 ohm to 119.8 ohm for a 120 ohm resistor or 9.63-9.64 for a 10kNot great, but my 34401A does the same thing with the least significant digit in 6.5 digit mode

author: michaelrcolton@gmail.com
author_rank: Newbie
timestamp: June 03, 2025, 05:02:05 pm
content: Well friends, I may have figured it out. At least for the DMM part (which is the more annoying of the two issues)I suspected maybe the opto-isolator or the feedback circuit that goes with it were the problem, so I probed those with my (other) scope and found that the signal there was indeed jumping around, but as I traced it further back, it was clear that the jumping around was happing earlier. There is an adjustable gain stage that seems stable, followed by a mux to choose which path would be passed through the isolator to the rest of the instrument. The output of this was where the jumping around starts. I checked the control lines and it looked like it was being told to select different inputs (which could be by design) but the pattern had some irregularity to it (a lot of more or less regularly spaced transitions, with some less frequent very short ones) so I started following those signals back, eventually reaching all the way to the isolator for the control lines, the output from that looked really ugly. The digital signal was visible, but there was a very large ripple there (almost as big as the digital signal itself). Probing the +5 and -5 supplying that isolator I found that the -5 supply had that same ripple.Having that ripple seems bad in a few ways, as that is part of what feeds the various op-amps, but that could also cause some glitchiness in the control signals.I tried carefully injecting a little bit of -5v into that node with an external supply and the measurement from the meter cleaned up beautifully. I think this might be it. I'm ordering a replacement for the -5v regulator now and we'll see how we do!https://youtu.be/wRaiF2j67IQ@timeandfrequency, you were right on about ripple on the power rails. Nice!The ripple in the higher speed ranges of the oscilloscope still needs some attention (I can't imagine they are related, at this point) but that issue is much less problematic for me so that can wait for later

author: michaelrcolton@gmail.com
author_rank: Newbie
timestamp: June 04, 2025, 06:26:27 pm
content: Well, I replaced the -5 regulator and now the DMM is quiet. I hadn't realized how much anxiety those numbers jumping around was giving me! What a relief! (Also nice to get that thing put back together, off my shelf, and back into use.)I'll revisit the scope ripple issue later. This will do for now.Thanks everyone for your help!

author: michaelrcolton@gmail.com
author_rank: Newbie
timestamp: June 09, 2025, 10:15:51 pm
content: I have no idea when it was last calibrated, but after sufficient warm up, it's only off from the voltage reference by 0-1mV. Not bad!PS. They market it as having "2.5 GS/s real-time sampling", but elsewhere it says only 20-25MS/s for single captures. I was under the impression that "Real-time" in this context meant that was the literal capture speed of the ADC, and that the correct term for what they must be doing here is "Equivalent-time sampling" am I wrong? Seems like pretty shifty marketing to me. Oh well. I still like it.