Keithley 617 Current Input Adjustment - Page 1
Forum section: electronics / testgear

author: rastro
author_rank: Frequent Contributor
timestamp: May 24, 2014, 10:51:46 pm
content: Just got a Keithley 617 electrometer. The ohm, voltage, and (most) amp functions seem to work. I am having problems with getting a good zero on the lowest pico-amp setting. The amp ranges where checked using a keithley 220. With the exception of the 2-pA range the 617 appears to accurately track the 220 current across all its higher ranges.I preformed the following two procedures from the manual:7.4.8 Input Offset Adjustment: Can adjust R314 (zero check) to +/- 1 count on 2pA range. This is fairly stable after warm up (OK).7.4.9 Input Current Adjustment: Adjusted R348 fully C.C.W (most negative) but still have ~ +0.0650 pA. (Problem) The procedure asks for +/- 15 counts after about a minute settling time. The total adjustment range on R348 is approximately 0.1500 pA with about 30-turns end-to-end.The offset was initially greater than 0.4000 pA after first receiving the unit. I've had the cover off a couple of times (wearing gloves) and cleaned some green residue off of Q308. Reseated connections from the triaxial connector. It's also been running it for a couple of days. I suspect the unit may have been sitting unused for a while and possibly needs to outgas some residual moister as it stays on and warm??? So it seems to have improved. However it's still at the end of the range of adjustment on R348 with out getting within 30 counts of zero.I'm a little hesitant to start cleaning the electrometer PCB with methanol. I want to see if it just needs to run for a few days drive off possible moister from sitting in storage. Hopefully this will help but I doubt it will fix the problem.My gut feeling is the electronics are OK but I'm not familiar with the failure mode of the JFET Q308 - perhaps it developed some leakage. I checked pin 2 & 3 on op-amp U309 it measured 0.20 uVolts (LT1012 datasheet shows 25uV Max offset) so it seems OK. Since there is good input current adjustment range I figure that Q311/R348 is probably OK - it's just getting swamped from some rogue error source.Questions:1) Any further ideas of pursuing this as an electronic component or contamination issue?2) There seems to be some differences between the manual (Fig. 8-1) and my electrometer PCB.a. Note 155 shows some kind of bracket/clip on the metal can of Q308. My PCB does not have this.b. Also I do not see R353 on my PCB. On the schematic it's a 100K that goes from bootstrap common to the Q308 (guard or case???)Could someone with a 617 check to see if this is on their unit?3) On the back of the unit next to the guard switch is an access hole for a pot on the motherboard which was not shown in the manual. Does anyone know what this is for?Thanksrastro

author: rastro
author_rank: Frequent Contributor
timestamp: May 25, 2014, 12:36:02 am
content: On question 3 I think I've answered my own question. R155 looks like it controls over current for voltage output.

author: tautech
author_rank: Super Contributor
timestamp: May 25, 2014, 01:17:36 am
content: Fractions of a pA?Where on earth is this degree of resolution/accuracy needed?