Keithley 2000 problem - Page 1
Forum section: electronics / testgear

author: McBryce
author_rank: Super Contributor
timestamp: August 16, 2015, 01:08:57 pm
content: Hi all Keithley experts,I've just observed a strange problem with my Keithley 2000 and I'm not sure if it is a setting or whether something is broken, because I'm quite new to the meter and to confuse things further, my Son (4 years old) was busy pressing buttons on it just before I noticed the issue.The problem occurs in the VDC measurement. If I connect the probes to a 10VDC source, it correctly reads 9.999998V or whatever, but when I disconnect the probes it's almost as if a capacitor is strapped across the inputs. It takes several minutes for the displayed voltage to return to (almost) 0V as you'd expect. As if a capacitor is discharging across the inputs.Is this some setting that I wasn't aware of, or has something broken in the Keithley?McBryce.Edit: Additional info - While the displayed voltage is ramping down, if I remove the positive lead, the voltage displayed jumps back up to around 8V and then continues to ramp down again??

author: sync
author_rank: Frequent Contributor
timestamp: August 16, 2015, 01:33:12 pm
content: It's normal. At 10V and lower ranges it has high input impedance (>>10GOhm). It's more like a capacitor (dis)charged by the input bias current of the input amplifier and external charges.My K2000 has about -20pA bias current. It's ramping down until -12V then changes to the 100V range which has 10MOhm input impedance. This discharge it and it starts again.

author: TiN
author_rank: Super Contributor
timestamp: August 16, 2015, 01:33:44 pm
content: I'm pretty suresame questionwas asked already few days ago. Likely you meter is completely fine, it's just high-impedance input picking up statics and charge with floating inputs. If you switch to 100/1000V your inputs would be 10Meg and essentially zero. If your meter pass self-test, it's with high probability totally fine.

author: McBryce
author_rank: Super Contributor
timestamp: August 16, 2015, 02:10:16 pm
content: Yes, it passes self-test. I've also noticed that it ramps down a lot quicker depending on the leads I use.After a few minutes of use, I'm starting to think that it has always done this, just I tend to only look at the display after I have attached the probes to something. As it passes all tests and is reading correctly I assume this is normal. I haven't had the Keithley very long and I'm only getting to use it seriously in the last two weeks. I couldn't find any online videos of anyone taking voltage measurements to compare so I thought I'd ask here first.The thing that confuses me is: If I take a measurement of 10VDC somewhere, then lift the probes and connect them to two disconnected points with no potential difference, the meter will still tell me it's around 8VDC and ramping down, which is not true! Shouldn't the meter always read correctly even if the probes are essentially floating?McBryce.

author: TiN
author_rank: Super Contributor
timestamp: August 16, 2015, 02:24:35 pm
content: But what is supposed to read, if nothing connected?

author: sync
author_rank: Frequent Contributor
timestamp: August 16, 2015, 02:29:46 pm
content: Quote from: McBryce on August 16, 2015, 02:10:16 pmIt is true! Your K2000 simply charged the two points to 8V potential difference. The meter input has capacitance. It's like putting a charged capacitor to the points.

author: McBryce
author_rank: Super Contributor
timestamp: August 16, 2015, 07:35:04 pm
content: Maybe it's just that I've got so used to using my Fluke meters for so many years. With them, as soon as you lift the probes it instantly drops back to 0V.McBryce.

author: sync
author_rank: Frequent Contributor
timestamp: August 16, 2015, 08:00:48 pm
content: Your Fluke has probably 10MOhm input impedance. This will discharge the input quickly when the probe are lifted. There are also bench DMMs which have switchable input impedance (10Mohm and >10GOhm) for the lower voltage ranges. On the 10MOhm setting they behave like your Fluke.

author: McBryce
author_rank: Super Contributor
timestamp: August 16, 2015, 08:21:27 pm
content: Yup, they are a Fluke 88 and a Fluke 79, both 10M internal resistance handheld devices. The Keithley is my first bench meter, I was never a fan of bench meters, used handhelds my whole life, but I decided to give the Keithley a go and I'm liking it up to now. But it takes getting used to.McBryce.

author: sync
author_rank: Frequent Contributor
timestamp: August 16, 2015, 08:29:08 pm
content: btw: when was your Keithley build? The early models have a common problem with electrolyte leaking Nichicon caps which damage the PCB.

author: McBryce
author_rank: Super Contributor
timestamp: August 16, 2015, 08:44:50 pm
content: Built in 2005. I've already opened it up and gave it a full visual, the caps (in fact everything) look brand new.McBryce.

author: sync
author_rank: Frequent Contributor
timestamp: August 16, 2015, 08:52:55 pm
content: I think it should be save. My is from 1994/1995 and had the leaking caps problem.

author: McBryce
author_rank: Super Contributor
timestamp: August 16, 2015, 08:57:37 pm
content: There's no escaping leaking caps. It will happen to mine too, just 10 years later than yours.I'm about to start checking my Tektronix 2465 (original not A or B). It's working fine, but the caps have never been changed. I'm dreading what I might find in thereMcBryce.

author: Macbeth
author_rank: Super Contributor
timestamp: August 16, 2015, 08:58:04 pm
content: Yeah my 2015 does the same - it's the super high gigaohm impedance. I kind of like it, it's just like a screensaver if you think about it - helps stop burn in of the VFD.

author: McBryce
author_rank: Super Contributor
timestamp: August 16, 2015, 09:07:02 pm
content: That's one way of looking at itI suppose I'm just going to have to get used to it. I've already started tipping both probes on my finger between measurements which drops it back down instantly.McBryce.

author: sync
author_rank: Frequent Contributor
timestamp: August 16, 2015, 09:10:22 pm
content: Quote from: McBryce on August 16, 2015, 08:57:37 pmI don't think so. Caps leaking electrolyte are rare. Except these Nichions from that age. I guess it's a design fault. They are not venting through the top. They are leaking at the leads. Directly on the PCB

author: McBryce
author_rank: Super Contributor
timestamp: August 16, 2015, 09:17:08 pm
content: I didn't mean specifically with Keithley, I meant in general. As an Amiga owner I've had a lot of experience with leaking capsMcBryce.