Keithley 260 Nanovolt Source - Page 1
Forum section: electronics / testgear

author: VintageNut
author_rank: Frequent Contributor
timestamp: September 12, 2016, 08:11:12 pm
content: I bought a KE260 from EvilBay recently. There were multiple issues that had to be fixed to fire it up.1. Power cord was missing chunks of insulation. Inside the instrument, the power cord wire insulation crumbled when touched. Replaced the power cord.2. Fuse holder was partially missing. Replaced the entire fuse holder assembly.3. Three power supply caps had leaked onto the circuit board. Removed the board. Removed the caps. Cleaned the board. Replaced the 25V 500 uF caps with 50V 1000 uF Nichicon caps.Fired it up. Checked voltages. All is well functionally.Did the initial voltage adjustments to get the millivolt output set for full scale 1V, 0.1V and 0.01V.When I checked output for dial setting of 0-0-0, there is 2 mV of output. Scanning the schematic, it appears that there may be some significant contact resistance in the lowest decade.Has anyone taken apart one of these low thermal decades and tried to clean and re-assemble it?

author: TimFox
author_rank: Super Contributor
timestamp: September 12, 2016, 10:14:01 pm
content: I have re-calibrated several -hp- and Keithley supplies where the decade dials are a series resistance. Contact resistance on the switches (all decades) will give some resistance at zero dial setting. I was able to improve this with GoldX contact cleaner. Some supplies also have an offset trimpot that can remove the output offset, but you should clean the switches first.

author: VintageNut
author_rank: Frequent Contributor
timestamp: September 12, 2016, 10:42:23 pm
content: Hello TimThis supply is three decades in series. The first decade is 200 ohms ( 20 ohms per step). The second decade is 20 ohms ( 2 ohms per step). The third decade is 1 ohm (0.1 ohm per step). Not sure why the third decade is not 2 ohms total.The output voltage is 1.11 volts at 221 ohms total resistance so the current is 5.023 mA. The offset at dial setting 0-0-0 is about 1.5 mV, so the contact resistance I am looking for is 0.0015 / 0.005023 = 0.299 ohms.I have some De-Oxit on my shelf. Do you think that this would be good to try now rather than order something?

author: VintageNut
author_rank: Frequent Contributor
timestamp: September 13, 2016, 02:05:20 pm
content: I tried alcohol and Q-tips. While the offset became lower, the offset is greater than 60 microvolts and is out of spec.I traced the offset to the lowest decade. The wafer stacked switch has copper rings below the wafers. The lowest bottom copper cannot be accessed unless the switch is dismounted/de-soldered.What I did is to bypass the lowest decade with solid copper wire wrapped around convenient components. This can be unwrapped at a later date to restore the arrangement to factory original and for repair.Before the bypass, the 0-0-0 dial setting offset was above 400 uV. Now with the bypass, the offset is 52 uV and is below the limit of 60 uV and now everything passes calibration.On the second decade switch, the 7 digit is flaky. All of the other digits appear stable and in spec. The 7 position probably requires the cleaning that the lowest decade requires.For now, the instrument is usable for the purpose of checking out and adjusting nanovoltmeters.