Fluke 8100A DMM Nixie Tubes Display All Zeroes - Page 1
Forum section: electronics / repair

author: doombot1
author_rank: Contributor
timestamp: January 05, 2019, 09:21:22 pm
content: Hey guys!First post, but I've heard a lot of good things about EEVBlog before. I'm fairly new to actually fixing electronics, but I know quite a bit about test equipment & soldering, as well as a very large amount about PCBs.So, to start off. Last year, I picked up a Fluke 8100A DMM from my university that was being thrown out (can you believe it?). Plug it in, and to my surprise, the 50+ year old machine turns on! All five of the Nixie tubes do in fact work. However, the DMM shows all zeroes! When a voltage is inputted, the display shows all zeroes, but very briefly flashes every handful of seconds. I found out that if I take a slow-motion video with my phone and look at the video, when the DMM flashes, it *very* briefly shows the correct reading, before returning to zero. Upon a very basic inspection of the internals, nothing appears to be blown, but I'm going to give it a closer look later tonight.Does anyone have any idea what could be causing this issue? Please advise - I would really love to get this thing working again! If it helps at all, I do have some equipment to test it with. I have a Fluke 10 DMM as well as a bench-top Keithley DMM (an older one, but it does what I need it to). My good OScope is back at college (I am home over break), but I do have a refurbished Tektronix scope from the early 70's, so although it won't give me precise readings, I can use it to visualize basic waveforms.Thanks for the help!~DTP.S. I can try to get some pictures of the thing up later tonight as well; right now I am away from home.EDIT: Images of PCB:TOP:BOTTOM:EDIT 2: Video of Tubeshttps://imgur.com/a/F7CSswZAlso, I accidentally had the model as 8100B. It is not. It is a Fluke 8100A.

author: bitseeker
author_rank: Super Contributor
timestamp: January 05, 2019, 10:56:48 pm
content: Welcome, boombot1!I have one of these meters, too, and it's still quite accurate. Just off the top of my head, some things to check:1. Cycle the range and function buttons. Dirty switch contacts could result in loss of input.2. Does tapping on the meter or flexing different areas of the PCB cause a change in behavior? If so, there might be a bad solder joint.3. The usual starting point when diagnosing faulty equipment is to verify that all the required voltages from the device's power supply are in fact correct and not excessively noisy. A simple DMM and themanualcan be used to verify.4. If all that looks good, then trace the input signal from the binding posts and see where it disappears.

author: PaulAm
author_rank: Frequent Contributor
timestamp: January 06, 2019, 12:46:18 am
content: You need to get a service manual so you can trace through the schematics.Googling 8100a manual turns up a bunch of links, here's one picked at randomhttp://www.nixiebunny.com/fluke8100b/fluke8100b.htmlDo you have a scope? You'll probably need one to track down what's happening. You don't need anything special, anything as good or better than a 50MHz analog scope will work.I haven't looked at the schematic, but generally the count on these kinds of displays is strobed into a register or latch which is then decoded to the display itself. Maybe the latch is being enabled longer than required so it sees the correct value from the counter and then the 0 when the counter is reset for the next acquisition. So look around that type of circuitry; maybe there's a bad cap that's stretching the enable signal (or a marginal or bad IC).OK, I looked at the schematic and it appears that the display is continuously scanned since there is only 1 decoder/driver chip that is multiplexed across the displays. That will make it more fun. Look around the counter chip A4.