Keithley 6517A Voltage Drift - Page 1
Forum section: electronics / testgear

author: TronCarter
author_rank: Newbie
timestamp: February 23, 2017, 04:29:17 pm
content: Hello everyone. I've tried google searching for information, but am having trouble coming up with anything close, probably because I'm not quite sure what I'm looking for (i.e. keywords).I have a Keithley 6517A on my bench that hasn't been used for several years. With nothing connected except for the power cord, the voltage reading starts to increase by 5mV per second or so. If I connect my BK meter to the preamp output, I also measure the same voltages. So, something is making the voltage drift upwards, but I don't know where to go from there in troubleshooting. I don't have anything connected as inputs, triggers, leads, etc.The ADC board has some numbers written on it in sharpie, which (along with the 2001 date) lead me to believe it has been changed in the past.All of the electrolytics on the analog board (dated 1994) look normal, except for the six large caps labeled C315. They seem a bit "bulgey" on the top, but I don't know if that is normal for this particular cap as they don't have the normal + sign on the top, rather a stampled part number. One of the six is different, so it likely has been changed in the past.Also, the 4 regulators on the analog board seem to get quite warm/hot when this unit is operating. Perhaps this is normal?Does this ring any bells with anyone before I try to figure out the board testing procedures in the service manual? Some of the pins are difficult to get at, so if someone has "seen this a million times before", I'd be happy to forego the testing.Tron

author: Kleinstein
author_rank: Super Contributor
timestamp: February 23, 2017, 05:44:28 pm
content: It is absolutely normal for an open super high impedance input of an voltmeter to drift. To test the drift of such a meter one would have a short at the input.The other test is for input bias current - this uses an low leakage capacitor in the 100 pF range at the input, to get a better defined relation form drift to current.

author: TronCarter
author_rank: Newbie
timestamp: February 23, 2017, 07:42:24 pm
content: OK, I think I understand what you are saying. I shorted the inputs and now read a consistent .5mV. The reason I'm looking at this meter in the first place is that the end user was trying to measure some resistors in the T0hm range and when she did, the values kept drifting significantly, not a little bit, but a lot. I thought it might be related to the perceived voltage drift, but perhaps not. I don't have a 100pF cap. The closest I have is 5800pF.Now I'm testing the V SOURCE HI. Setting the output voltage to any value results in .005 volts or less, measuring between HI and the center pin of INPUT. I believe this might be at least part of my problem. The jumper wire between the HI jack (and LOW) goes to a header right next to those suspect caps.

author: cncjerry
author_rank: Unknown
timestamp: February 23, 2017, 10:05:41 pm
content: First off, for Tohms, you need to use high quality PTFE (I think) triaxial cables and probably an isolation box of some type. Then move across the room and sample it with GPIB or some type of statistical process built into the meter. Find the video on the electrometer experiments and that will give you an idea of the level of isolation you need to have in order to not impact the test at those levels.Recently I bought new triax connectors and special cable that is used in missiles in order to make some acceptable leads for my electrometer. I say acceptable because if you try to buy high quality electrometer cables you can easily spend $150 to $400 or even $1,500 for good leads. As someone mentioned on another thread, some of the clips we use on a normal DMM act as a short on an electrometer. When measuring Tohms, I think that meter of yours uses like 1,100 volts so be careful or you'll get a tingle.